hydrological characteristics of the drainage system beneath a surging glacier a rare combination of natural circumstances permits assessment of current theories on water flow beneath glaciers outburst floods from the subglacial lake grimsvotn in iceland took place before during and after surging of skei ararjokull the glacier beneath which the outburst floods drain the observable drainage patterns associated with these floods show the different nature of the basal water conduit system of the glacier during surge and non surge phases during surge conditions basal water is dispersed slowly across the bed in a distributed drainage system but when the glacier is not surging water is transported rapidly through a system of tunnels

relationship between thermal maturation gradients geothermal gradients and estimates of the thickness of the eroded foreland section southern alberta plains canada abstract analysis of post laramide tertiary uplift and erosion in the southern alberta plains indicates that the greatest erosion occurred in regions where the youngest stratigraphic units are preserved this is consistent with geological structure and the few preserved stratigraphic constraints on erosional history throughout this region erosion is pervasive and the uplift history is commonly constrained by coalification data the analysis of coalification erosion and paleogeothermics are interrelated and knowledge of them is important to understand hydrocarbon generation and tectonic history several independent attempts to analyse coalification erosion and paleogeothermics in this area have given different results the most striking differences are those between estimates derived from the analysis of well profile coalification gradient and those determined using a commonly accepted relationship between the amount of eroded section and the equilibrium moisture content of near surface coals the match between present and paleogeothermal gradient fields is better when the latter technique is used correlation is expected between the late orogenic and present geothermal gradient fields because the hydrodynamically controlled advective heat flow pattern that now dominates geothermal gradients in the mesozoic and tertiary foreland succession was initiated by the growth of topography in the west throughout cordilleran orogeny well profile coalification gradients have significant though commonly unacknowledged uncertainties these uncertainties are propagated in erosion and paleogeothermal gradient calculations ideally the relationship between erosion estimates using well profile coalification gradients and near surface coal properties can be compared unfortunately the uncertainties in well profile coalification gradients prevent this

quartz cemented breccias from the mid atlantic ridge samples of a high salinity hydrothermal upflow zone textural mineralogical chemical and fluid inclusion studies on a suite of quartz rich sulfide bearing greenstone breccias indicate that high salinity high fluid flow systems were recently active on the mid atlantic ridge south of the kane fracture zone these systems involved fluids with temperatures close to 300 c and salinities almost 3 times that of seawater rounded pebblelike fragments enclosed within a matrix of euhedral quartz crystals appear to have been cemented by quartz deposition from a moving fluid which attained velocities in excess of 1 m s homogenization temperatures and freezing point depression effects in fluid inclusions from texturally early and late quartz crystals indicate temporal evolution of fluid temperatures and nacl equivalent salinities from 290 c and 10 wt to 200 c and 4 7 wt during alteration and cementation of the breccias highly variable cobalt concentrations in the evolving hydrothermal solutions are indicated by fine scale oscillatory zoning in euhedral pyrite within the breccias a continuous supply of the high salinity fluids may be generated either by supercritical two phase separation at depths well below the level of inclusion entrapment or by retrograde dissolution of hydroxy chloride phases in the deeper portions of the hydrothermal system either model requires the initial temperature of the deeply circulating fluids to have been distinctly higher than 415 c and the fluid to have been cooled by conduction or by mixing with lower temperature seawater prior to entrapment within the inclusions

a coupled sheet conduit mechanism for jokulhlaup propagation 1 the largest glacier outburst flood jokulhlaup ever recorded in iceland occurred in 1996 and came from subglacial lake grimsvotn in vatnajokull ice cap among other noteworthy features this flood was characterized by an unprecedentedly high lake level prior to flood initiation extremely rapid linear rise in lake discharge delay between the onset of lake drainage and floodwater arrival at the glacier terminus formation of short lived supraglacial fountains and initially unchannelized outbursts of floodwater at the terminus observations suggest that the 1996 flood propagation mechanism was fundamentally different than that of previously observed floods from grimsvotn we advance a new model whereby floodwater initially propagates in a turbulent subglacial sheet which feeds a nascent system of conduits this model is able to explain key observations made of the 1996 jokulhlaup and may shed light on other outburst floods that do not conform to the standard model

consequences of phase separation on the distribution of hydrothermal fluids at ashes vent field axial volcano juan de fuca ridge recent studies of hydrothermal fluid chemistry at the axial seamount hydrothermal emissions study ashes vent field of axial volcano indicate separation of liquid and vapor phases during the migration of fluids to the seafloor the brine phase liquid emanates from discrete sulfide chimneys while fluid which has been through vapor phase separation flows diffusely from fissures in the surrounding basalt one possible mechanism to explain this spatial segregation of the fluid phases is based on the relative permeability terms in the equations of two phase flow a qualitative model for the distribution of fluids observed at ashes is postulated in which brine phase fluids are confined within flow conduits by a surrounding relative permeability barrier while vapor phase fluids flow diffusely into the surrounding host rock

discussion of relationship between thermal maturation gradients geothermal gradients and estimates of the thickness of the eroded foreland section southern alberta plains canada by majorowicz et al 1990

hydraulics of subglacial outburst floods new insights from the spring hutter formulation using a slightly modified form of the spring hutter equations glacial outburst floods are simulated from three classic sites hazard lake yukon canada summit lake british columbia canada and grimsvotn iceland in order to calibrate the hydraulic roughness associated with subglacial conduits previous work has suggested that the manning roughness of the conduits is remarkably high but the new calibration yields substantially lower values that are representative of those for natural streams and rivers the discrepancy can be traced to a poor assumption about the effectiveness of heat transfer at the conduit walls the simulations reveal behaviour that cannot be interred from simplified theories 1 during flood onset water pressure over much of the conduit can exceed the confining pressure of surrounding ice 2 local values of fluid potential gradient can differ substantially from the value averaged over the length of the conduit contradicting an assumption of simple theories 3 as the flood progresses the location of flow constrictions that effectively control the flood magnitude can jump rapidly over large distances 4 predicted water temperature at the conduit outlet exceeds that suggested by measurements of exit water temperature

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

the relationship between intrusion hosted cu mo mineralization and the vms deposits of the archean sturgeon lake mining camp northwestern ontario volcanogenic massive sulfide vms deposits of the archean sturgeon lake district are underlain by the 2734 ma beidelman bay composite tonalite trondhjemite synvolcanic intrusion this 20 by 2 km sill complex is comagmatic with the vms hosting felsic volcaniclastic strata dated at 2735 ma the top of the beidelman bay intrusion is host to weak cu mo porphyry type sulfide mineralization associated with igneous and hydrothermal breccia zones and a plagioclase quartz and quartz plagioclase porphyry dike swarm it was earlier postulated that the presence of intrusion hosted base metal mineralization indicates that the beidelman bay intrusion not only initiated and sustained the subsea floor convective system responsible for the formation of the cauldron hosted vms deposits but may also have supplied metals to the fluid system new u pb zircon analyses of a plagioclase quartz porphyry dike overprinted by the cu mo mineralization resulted in a date of 2720 5 3 0 3 5 ma approximately 14 m y younger than the host biotite trondhjemite phase of the beidelman bay intrusion rare earth and trace element concentrations differ greatly between the porphyry dike and the tonalite and trondhjemite phases of the beidelman bay intrusion but are similar in composition to a 2717 9 1 5 2 9 ma postcauldron rhyolite tuff unit this demonstrates that the porphyry type mineralization hosted within the beidelman bay synvolcanic intrusion is unrelated to the vms hydrothermal system and is most likely related to later more evolved oceanic arc magmatism

development of a brine dominated hydrothermal system at temperatures of 400 500 c in the upper level plutonic sequence troodos ophiolite cyprus abstract microthermometric analyses of quartz hosted fluid inclusions in fresh and epidotized plagiogranites from the troodos ophiolite cyprus indicate that brine rich fluids 46 56 wt nacl equivalent at temperatures 400 500 c were pervasive throughout many of the plagiogranite bodies high salinity fe bearing inclusions along healed microfractures homogenize by halite dissolution at temperatures of 400 500 c liquid dominated low salinity 2 7 wt nacl secondary fluid inclusions are ubiquitous in all plagiogranite and gabbro samples studied homogenization of the low salinity inclusions occurs at uncorrected temperatures of 200 400 c the quartz hosted high salinity fluids represent either phase separation of hydrothermal seawater or an exsolved magmatic aqueous phase at uncorrected temperatures of 400 500 c migration and segregation of brine and vapor phases into fractures near the margins of the crystallizing plagiogranite bodies resulted in preferential entrapment of the fe rich brines in the deep seated high temperature portions of the hydrothermal system subsequent fracturing of the upper plutonic rocks at temperatures of 200 400 c allowed penetration by seawater which pervasively altered the plagiogranites hydration reactions involving formation of secondary minerals and or mixing with phase separated fluids resulted in fluid salinities two times that of seawater

a comparison of the horne volcanogenic massive sulfide deposit and intracauldron deposits of the mine sequence noranda quebec most of the massive sulfide deposits of the rouyn noranda area are associated with rhyolite and andesite flows of the mine sequence and occur within the noranda cauldron a volcanic subsidence structure related to partial evacuation of an underlying magma chamber the flavrian pluton the horne massive sulfide orebodies lie within a sequence of rhyolitic breccias located just south of the inferred margin of the subsidence structure and are separated from the mine sequence by the horne creek fault traditionally the horne deposits and the intracauldron deposits have been interpreted to occur within the same stratigraphic interval even though they share some characteristics such as mineral zoning patterns alteration assemblages and an association with synvolcanic faults the two deposit groups are clearly distinguishable intracauldron massive sulfide deposits are small 5 mt cu zn rich concordant mounds associated with interflow horizons and are closely linked to vent zones where synvolcanic faulting provided hydrothermal fluids with cross stratal structural permeability the three major sulfide deposits at the horne mine are much larger 20 150 mt and formed in a fault bounded sediment filled graben on the flank of a rhyolitic vent complex the orebodies are au cu rich but zn poor and have bowl shaped morphologies which resulted from extensive subsea floor sulfide precipitation whereas the intracauldron sulfide deposits lie within a bimodal flow dominated succession consisting mainly of andesite basalt and high silica rhyolite with volumetrically insignificant pyroclastic rocks the horne orebodies occur in felsic fragmental rocks and rhyolite flows which are part of a stratigraphic sequence which contains virtually no mafic volcanic rocks also compared to all intracauldron rhyolites the horne rhyolites show a fundamental depletion in incompatible trace elements such as the high field strength and rare earth elements which implies that the horne rhyolites may have followed a different magmatic evolutionary path such differences in deposit characteristics their paleoenvironments of formation and the physical volcanology and geochemistry of their host volcanic successions suggest that contrary to previous interpretations the massive sulfide deposits of the intracauldron mine sequence and those of the horne sequence may have formed at different stages in the evolution of the noranda cauldron

synthetic fluid inclusions viii vapor saturated halite solubility in part of the system nacl cacl2 h2o with application to fluid inclusions from oceanic hydrothermal systems abstract halite solubility along part of the vapor saturated liquidus in the system nacl cacl 2 h 2 o has been determined using the synthetic fluid inclusion technique data allow the construction of liquidus isotherms for temperatures up to 500 c and bulk compositions containing 60 wt total salt and as much as 25 wt cacl 2 combined with previous data for the binary system nacl h 2 o and for the ternary system nacl cacl 2 h 2 o in the low salinity low temperature region a preliminary ternary phase diagram can be constructed that remains incomplete only in the cacl 2 rich region results are applied to the interpretation of saline fluid inclusions from quartz veins in oceanic metagabbros and can be applied to many other natural inclusions containing aqueous solutions with nacl and cacl 2 the dominant solutes microthermometric measurements at equilibrium of the melting temperature of ice tm ice and of the dissolution temperature of halite tm halite are sufficient to determine the bulk composition of the nacl cacl 2 h 2 o fluid

seafloor hydrothermal fluids ben nevis area abitibi greenstone belt implications for archean 2 7ga seawater properties abstract a central part of the 2 7 ga abitibi greenstone belt between the destor porcupine and cadillac larder lake structures has been subjected to sub greenschist facies metamorphism t the majority of observed fluid inclusions are two phase and liquid dominated with a vapor bubble volume of 10 microthermometry on 58 samples 1100 fluid inclusions and eutectic temperatures between 45 and 30 c indicate the presence of at least one significant divalent cation in the fluids hence salinity is calculated as wt cacl2 equiv the peak of the salinity distribution of inclusions from vesicles and interstitial and drainage cavity quartz falls between 4 and 10 wt cacl2 equiv a small subset of inclusions yielded salinities between 18 and 22 wt cacl2 equiv most homogenization temperatures fall in an interval between 110 and 160 c a geothermal gradient estimated from the data of 10 15 c km is reasonable and consistent with the interpretation of the area as a regional seafloor recharge zone primary porosity in the modern ocean floor does not show the rapid precipitation of quartz that is indicated for the archean however the global occurrence of silica rich banded iron formations and the lack of silica consuming marine organisms in the precambrian indicates that marine geochemistry and sio2 precipitation mechanisms may have been different during the precambrian the total salinity of a fluid is dependent primarily on its total chlorinity hence water rock interactions with ion exchange cannot be the only reason for the higher salinities encountered in this study indications for boiling or high temperature phase separation are not evident suggesting that these options are not a reason for the observed high salinities our data are in agreement with six other data sets from the 3 75 ga isua greenstone belt w greenland the 3 24 ga pilbara region w australia the 3 2 ga barberton greenstone belt south africa 2 the 2 7 ga matagami area abitibi greenstone belt canada and the proterozoic 2 2 ga ongeluk formation south africa the consistency of fluid inclusion data suggests that archean and early proterozoic seawater may have been more saline and potentially stratified compared to modern seawater

cathodoluminescence characteristics of archean volcanogenic massive sulfide related quartz noranda ben nevis and matagami districts abitibi subprovince canada 1 the cathodoluminescence cl of quartz from ore stockwork veins and interstitial fillings between lava pillows from the 2 7 ga noranda ben nevis and matagami volcanogenic massive sulfide vms districts abitibi greenstone belt has been investigated using the hot cathode technique hc1 lm system to assess the potential of these various sample types to host primary seafloor vms related fluids trapped as inclusions in minerals with primary depositional morphologies the cl responses indicate that the various quartz types are of hydrothermal origin and are therefore a potential host for primary hydrothermal fluid inclusions most notable is a transient t 120 s blue cl characteristic of hydrothermal quartz observed in most samples cl characteristics are similar over 250 km indicating coherent nonrandom behavior furthermore in ore and stockwork material from the matagami and noranda districts respectively cl reveals primary concentric growth zoned quartz that predates pulses of sulfide deposition clear evidence that the quartz is undeformed and directly related to vms mineralization these growth zones are not apparent in transmitted light in addition ore and stockwork quartz commonly show a very unstable t 30 s yellow cl coincident with microfractures and grain boundaries defining areas affected by secondary hydrothermal activity in ore from matagami local zones of nontransient brown cl may reflect strain zones associated with the deformation and recrystallization of the massive sulfide mound and indicate that such modifications can be recognized and are minor in the investigated cases cl clearly reveals pseudo hexagonal apparently zoned structures in sulfide mineralized breccia pipe quartz from the ben nevis area these structures and their host quartz characterized by a very unstable t 20 s bright yellow cl are interpreted as recrystallized quartz that has undergone rapid growth from a strongly supersaturated solution and noncrystalline precursor the cl also clearly reveals colloform crustiform textures indicative of open space filling these textures are not visible optically

chloride depletions and enrichments in seafloor hydrothermal fluids constraints from experimental basalt alteration studies na k ca cl fluid was reacted with diabase at 400 and 425 c 400 bars and fluidrock mass ratio of 0 5 to assess the relative mobility of dissolved cl fluids from the present experiments reveal relatively large time and temperature dependent decreases in cl temperature increase enhances cl removal whereas reaction progress has the opposite effect these changes are not due to boiling and or phase separation at 400 and 425 c 400 bars cl removal from solution is accompanied by ca and na fixation respectively both experiments produced mixed layer chlorite smectite clinozoisite and albite olivine was not detected by x ray diffraction or petrographic analysis of run products from either experiment however we propose that the time dependent changes in dissolved cl result from olivine replacement by an fe hydroxy chloride phase followed by hydration effects quenched and washed alteration products however did not reveal anomalous cl which suggests that the cl bearing phase is characterized by retrograde solubility this is consistent with the relative magnitude of cl fixation observed for experiments at both temperatures and from results of an isobaric cooling experiment the experiments provide evidence for the non conservative behavior of cl during hydrothermal alteration of basalt the data are particularly important in light of the cl depleted nature of some ridge crest hot spring fluids and suggest temperatures of formation for these fluids of approximately 410 c assuming sub seafloor pressure of 400 bars in addition the retrograde solubility of the cl bearing phase responsible for cl fixation during high temperature basalt alteration may help to explain cl enrichment in hot spring fluids which have conductively cooled below 350 c

double diffusive convection in this paper we present a rather personal view of the important developments in double diffusive convection a subject whose evolution has been the result of a close interaction between theoreticians laboratory experimenters and sea going oceano graphers more recently applications in astrophysics engineering and geology have become apparent in the final section we attempt to draw some general conclusions and suggest that further progress will again depend on a close collaboration between fluid dynamicists and other scientists

supercritical two phase separation of hydrothermal fluids in the troodos ophiolite the hot 350 c black smoker springs of the ocean floor are the vents of hydrothermal systems in which cold seawater descends below the ocean floor is heated reacts with the host rock and is transformed into metal bearing brine the salinity of these brines varies between vents from 0 7 to 2 0 times seawater salinity1 4 this variation has been ascribed to crustal hydration5 crystallization of chloride bearing minerals6 or two phase separation of seawater into components of differing salinity4 7 12 here we present fluid inclusion evidence for two phase separation in the ancient troodos sulphide forming systems of cyprus and discuss with the nacl h2o system as a model how this process could explain observed salinity variations in hydrothermal fluids

variations in the thermal output of the subglacial grimsvotn caldera iceland a 69 year record 1922 1991 of heat release has been obtained from the subglacial geothermal system of the grimsvotn caldera within the vatnajokull ice cap the data were derived from in situ measurements of the volume of meltwater accumulated in the caldera lake subtracting climatically induced melting the overall fluctuations in the heat flux are closely related to volcanic activity and are dominated by a main pulse of 11 600 mw caused by a major eruption in 1938 gradually declining to 1600 mw in 1976 1982 heat extracted from the roof of a magma chamber with the aid of hydrothermal convection may have given a basic contribution to the heat flux of 1500 to 2000 mw an upper bound the variable part of the heat flux from 0 to 10 000 mw was released from magma erupted at the base of the glacier and from shallow intrusions the total heat released over the period 1922 to 1991 was 8 1 1 6 1018j equivalent to the energy released by the solidification and cooling of 2 1 0 4 km of basaltic magma the contribution to the total heat flux was 45 max from a magma chamber 35 min from shallow intrusions and 20 from eruptions this implies that magma at the roof of a chamber solidified and cooled at the rate of 1 2 1 6 107 m a 1 or about 1 km over the last 69 years heat release at grimsvotn was probably more intense in the 19th century when volcanic eruptions were more frequent

u pb geochronology of the eastern abitibi subprovince part 2 noranda kirkland lake area u pb zircon ages for 15 volcanic and plutonic units in the noranda and kirkland lake areas help constrain the history of volcanism plutonism sedimentation and deformation in the south central part of the abitibi belt volcanism occurred over an interval of at least 50 ma beginning with the deposition of the volcanic and volcaniclastic units within the pacaud structural complex at 2747 ma following a period of apparent quiescence magmatism resumed at 2730 2725 ma with the eruption of volcanic rocks in the normetal and lac abitibi area from 2715 until about 2698 ma volcanism occurred sporadically throughout much of the area culminating in the eruption of the blake river group from 2703 to 2698 ma several large intrusive bodies yield ages that indicate that they are plutonic equivalents of the blake river group plutons that are considered to have been emplaced during the kenoran orogeny give ages that are only slightly younger than the youngest volcanic units of the blake river group emphasizing

drainage networks lakes and water fluxes beneath the antarctic ice sheet antarctica bedmap2 datasets are used to calculate subglacial hydraulic potential and the area depth and volume of hydraulic potential sinks there are over 32 000 contiguous sinks which can be thought of as predicted lakes patterns of subglacial melt are modelled with a balanced ice flux flow model and water fluxes are cumulated along predicted flow pathways to quantify steady state fluxes from the main basin outlets and from known subglacial lakes the total flux from the continent is 21 km3 a 1 byrd glacier has the greatest basin flux of 2 7 km3 a 1 fluxes from subglacial lakes range from 1 10 4 to 1 5 km3 a 1 lake turnover times are calculated from their volumes and fluxes and have median values of 100 a for known active lakes and 500 a for other lakes recurrence intervals of a 0 25 km3 flood range from 2 months to 2000 a median 130 a for known active lakes and from 2 to 2400 a median 360 a for other lakes thus several lakes that have recently been observed to fill and drain may not do so again for many centuries and several lakes that have not so far been observed to fill and drain have the potential to do so even at annual to decadal timescales

salinity variations in submarine hydrothermal systems by layered double diffusive convection various mechanisms have been proposed to explain the salinity variations in vent fluids of seafloor geothermal systems new experiments reacting diabase and evolved seawater were carried out to reproduce earlier published observations of cl depletions attributed to formation of an ephemeral cl bearing mineral the absence of any cl depletions in the present study suggests that the formation of cl bearing minerals is not sufficiently widespread to account for the observed salinity variations in the vent fluids a re evaluation of both field and laboratory evidence has led to a new model for subseafloor circulation that accounts for salinity variations as well as other chemical and mineralogic observations in place of a simple single pass convection system we propose that the seafloor systems consist of two vertically nested convection cells in which a brine layer at depth heats and drives an overlying seawater cell such layering of salinities a process known in fluid mechanics as double diffusive conve

fracture analysis of a volcanogenic massive sulfide related hydrothermal cracking zone upper bell river complex matagami quebec application of permeability tensor theory located in the matagami mining district of the abitibi greenstone belt quebec the bell river complex is a 5 km thick tholeiitic layered gabbro and or anorthosite body 2724 6 2 5 1 9 mau pb which likely acted as the heat source that drove hydrothermal convection and volcanogenic massive sulfide vms hydrothermal mineralization in the area abundant fractures and veins crosscutting the western lobe of the bell river complex formed over a range of temperatures from 250 to 700 c the 250 to 400 c assemblage quartz epidote sericite chlorite plagioclase is the most widespread and occurs as orthogonal anastomosing and random vein sets vein densities average between 15 and 25 veins per m2 locally reaching as high as 40 to 60 veins per m2 these veins typically 1 to 3 mm wide are interpreted to represent thermal cracking associated with hydrothermal fluid mineralization in the district furthermore they crosscut earlier higher temperature pyroxene plagioclase 600 c and magnetite rich 300 600 c veins 1 10 mm wide densities commonly 0 5 veins per m2 detailed field measurements of quartz epidote vein geometries coupled with permeability tensor theory have produced a first order approximation of the maximum model permeability structure veins unfilled of the hydrothermal cracking zone representative district wide values indicate maximum model bulk permeabilities of 10 10 to 10 8 m2 for the hydrothermal cracking zone similar to permeabilities calculated for the fractured sheeted dike complexes of the semail and troodos ophiolites however a high flow zone located within the central parts of the hydrothermal cracking zone is characterized by a maximum model permeability of 10 7 m2 locally 1 m2 the high flow zone reaches maximum model permeability values as high as 10 6 to 10 5 m2 where two or more veins occur with apertures in excess of 2 cm mapping has shown that the hydrothermal cracking zone is confined to an 350 m thick interval located within a strongly layered gabbro and or anorthosite horizon of the layered zone upper bell river complex the base of this interval is 1 000 m below the top of the bell river complex furthermore in and around the town of matagami the dominant orientation of quartz epidote veins parallels the orientation of the layering 110 15 this parallelism is further reflected in the orientation of the calculated quartz epidote vein permeability tensors 94 30 1 n 71 the parallelism suggests that heterogeneities associated with the layer contacts provided planes of low tensile strength along which the hydrothermal cracks preferentially developed such an interpretation further explains why the hydrothermal cracking zone appears to be restricted to the strongly layered zone there are many more low tensile strength planes zones of quartz epidote veining approximately orthogonal to layering are also present the orthogonal veins are usually less continuous and commonly truncated but not crosscut by layer parallel veins the orthogonal subset of veins is interpreted to represent short pathways that allowed fluids to travel between adjacent layer parallel veins however locally longer layering orthogonal quartz epidote veins showing offsets of 10 to 30 cm normal to the layering of the bell river complex may represent initial conduits that allowed fluids to travel into overlying stratigraphy and through to the paleosea floor although volumetrically small in comparison with the permeability structure of the bulk sea floor the significantly higher permeability of the hydrothermal cracking zone indicates its importance in controlling the flow paths and fluxes of fluids deep within the hydrothermal system

fluid temperature and salinity characteristics of the matagami volcanogenic massive sulfide district quebec fluid inclusions hosted within various lithologic units of the 40 million metric ton mt matagami district abitibi greenstone belt preserve samples of archean volcanogenic massive sulfide vms and postvolcanogenic massive sulfide hydrothermal fluid microthermometric measurements on ore hosted primary two phase liquid vapor inclusions from matagami s south limb deposits indicate that the vms hydrothermal fluid was highly saline 16 2 4 7 wt nacl cacl2 equiv 1 n 230 and of moderate temperature trapping temperature tt 208 32 c 1 n 230 a fluid with these characteristics is capable of transporting 5 10 4 m 30 ppm zn as zncl3 and zncl4 2 chloride complexes however the low temperature of this fluid precluded efficient cu transport 3 ppm which may in part explain the relatively cu poor nature of the matagami deposits calculated densities of this ore fluid as high as 1 10 g cm3 are consistent with a bottom hugging brine model however a subset of the data indicate a fluid less dense than ambient seawater suggesting that buoyant hydrothermal plumes were also present a microthermometrically determined high cacl2 content xnacl 0 55 molar na ca 2 3 1 for the vms ore hosted primary fluid is consistent with an archean hydrothermal fluid more ca rich than modern day seawater quartz epidote veins located in the hydrothermal cracking zone of the bell river complex host primary liquid vapor halite inclusions these inclusions are interpreted to be samples of the deep seated equivalent to the vms ore hosted hydrothermal fluid described above microthermometry indicates that these inclusions trapped a high temperature brine tt 373 44 c 1 n 92 38 2 1 9 wt nacl equiv 1 n 92 we interpret this brine to be a phase separated product of modified model seawater 3 2 wt nacl an exsolved magmatic fluid or a combination thereof deep within the hydrothermal system at 650 to 670 c and a near lithostatic pressure of 90 mpa phase separation and subsequent convection lowered the temperature of the brine prior to its entrapment within the hydrothermal cracking zone the occurrence of high temperature brine overlain by lower temperature salinity fluid suggests a two cell convection model consistent with metal mass balance calculations for the south limb the high salinity of the ore hosted fluid inclusions indicates two possibilities 1 a significant amount of brine was incorporated into the upper cell and mixed with heated seawater during convection 2 archean seawater itself was very saline and of variable salinity with the cooling of the bell river complex lower temperature fluids dominantly of seawater origin circulated deep within the hydrothermal system modified by water rock interaction yet not phase separated these fluids sealed off the remnant permeability of the fracture network of the hydrothermal cracking zone and were locally trapped as secondary liquid vapor fluid inclusions homogenization temperature th 242 17 c 9 1 1 6 wt nacl equiv 1 n 14 hosted within the bell river complex quartz epidote vein material post and or waning stage vms hydrothermal activity is evident from the presence of quartz epidote veins crosscutting wabassee group hanging wall rocks microthermometry on quartz hosted primary liquid vapor fluid inclusions suggests that this activity occurred at relatively low temperatures th 76 177 c n 212 over a wide range of salinity 6 0 32 4 wt nacl cacl2 equiv n 212 and with a high apparent cacl2 content xnacl 0 06 these fluid inclusion data illustrate the importance of subsea floor chemical and physical processes directly related to metal transport and deposition in vms systems in particular phase separation deep within the hydrothermal system is interpreted as a key process for generating saline brines capable of forming significant ore deposits

geochemistry of north cleft segment vent fluids temporal changes in chlorinity and their possible relation to recent volcanism hydrothermal vent fluids from the north cleft segment of the juan de fuca ridge between 44 54 and 45 00 n were sampled in 1988 1990 1991 and 1992 in 1988 chloride depleted relative to seawater diffuse fluids with low dissolved metal concentrations relative to other mid ocean ridge mor fluids were sampled over a distance of 10 km along axis in 1990 and 1991 both high temperature 200 c and diffuse vents were consistently metal and chloride enriched the end member compositions of high temperature vents and nearby diffuse vents were very similar there were small but significant correlated changes in diffuse and high temperature composition from 1990 to 1991 the large scale but temporary venting of low chlorinity fluids over the entire north cleft segment provides a resolution to the apparent mass imbalance implied by observations of continuous venting of chloride enriched fluids it is possible that a volcanic event along the north cleft segment prior to 1987 for which there is firm geologic evidence initially caused a boiling event which resulted in the preferential venting of vapor enriched fluids through 1988 followed by a transition to brine enriched fluids by 1990 high iron low sodium and low sr ca ratios in the high chlorinity fluids suggest that the brine phase has continued to react and approach reequilibration with an alteration mineral assemblage after the phase separation event the absence of chloride depleted fluids from 1990 onward and the systematics of lithium boron and manganese with chloride in the high temperature fluids from north cleft suggest that the evolution toward lower chlorinity at monolith vent from 1990 to 1992 is caused by progressive dilution of a brine with hydrothermal seawater

rapid discharge connects antarctic subglacial lakes a hidden subglacial world consisting of more than 140 lakes lies beneath the antarctic ice sheet it is widely assumed that these lakes are a static ecosystem cut off from each other and the rest of the world far from it according to a 16 month study of surface elevation changes in the region changes indicating a rapid discharge of water from one lake to another were detected and it is possible that the entire subglacial drainage basin is flushed through periodically by massive water transfers observation of surface elevation changes over 16 months in an area of the east antarctic ice sheet overlying subglacial lakes suggests a rapid discharge of one lake to at least two other lakes involving a water volume of 1 8 km3 in conflict with previous expectations that subglacial lakes have long residence times and slow circulations the existence of many subglacial lakes1 provides clear evidence for the widespread presence of water beneath the east antarctic ice sheet but the hydrology beneath this ice mass is poorly understood2 such knowledge is critical to understanding ice flow basal water transfer to the ice margin glacial landform development and subglacial lake habitats here we present ice sheet surface elevation changes in central east antarctica that we interpret to represent rapid discharge from a subglacial lake our observations indicate that during a period of 16 months 1 8 km3 of water was transferred over 290 km to at least two other subglacial lakes while viscous deformation of the ice roof above may moderate discharge the intrinsic instability of such a system3 suggests that discharge events are a common mode of basal drainage4 if large lakes such as lake vostok or lake concordia1 are pressurizing it is possible that substantial discharges could reach the coast5 6 our observations conflict with expectations that subglacial lakes have long residence times and slow circulations2 7 8 and we suggest that entire subglacial drainage basins may be flushed periodically the rapid transfer of water between lakes would result in large scale solute and microbe relocation and drainage system contamination from in situ exploration is therefore a distinct risk

multistage hydrothermal alteration of gabbroic rocks from the failed mathematician ridge oxygen isotopic mineralogical and fluid inclusion data are presented for fresh and hydrothermally altered gabbroic rocks from the failed mathematician ridge the mathematician ridge though initially a fast spreading ridge presently has a deep central rift valley more typical of slow spreading rifts the inner rift walls expose rare plutonic rocks uncomplicated by proximity to deep large offset fracture zones undeformed mathematician ridge gabbros preserve most of the original texture and phase compositions and have been only slightly mineralogically or isotopically modified by post crystallization hydrothermal alteration metagabbros have been subjected to a pervasive ductile deformation and recrystallization to foliated rocks composed of aluminous amphibole low calcium plagioclase and ilmenite oxygen isotope and phase chemical data suggest this event began at temperatures of up to 700 c water rock ratios of less than one and produced modified seawater fluids with isotopic compositions of 3 5 6 0 the amphibolites are cross cut by multiple generation quartz epidote veins forming a network replacement of the amphibole grade mineralogies by greenschist grade mineralogies fluid oxygen isotopic compositions of 2 4 and a two phase separation in the fluid captured in conjugate vapor fluid inclusions are all associated with the formation of this network fluid inclusion data for the quartz indicate initial temperatures of over 500 c these samples may represent rocks from the 1 2 km deep plumbing system of black smoker type venting

the mattagami lake mine archean zn cu sulfide deposit quebec hydrothermal coprecipitation of talc and sulfides in a sea floor brine pool evidence from geochemistry 18 o 16 o and mineral chemistry discussion

an active subglacial water system in west antarctica mapped from space satellite laser altimeter elevation profiles from 2003 to 2006 collected over the lower parts of whillans and mercer ice streams west antarctica reveal 14 regions of temporally varying elevation which we interpret as the surface expression of subglacial water movement vertical motion and spatial extent of two of the largest regions are confirmed by satellite image differencing a major previously unknown subglacial lake near the grounding line of whillans ice stream is observed to drain 2 0 cubic kilometers of water into the ocean over 3 years while elsewhere a similar volume of water is being stored subglacially these observations reveal a wide spread dynamic subglacial water system that may exert an important control on ice flow and mass balance

australian volcanic hosted massive sulfide deposits features styles and genetic models the cambro ordovician period has yielded the major development of volcanic hosted massive sulfide vhms deposits in australia resulting in about 12 million metric tons of contained metal copper lead and zinc concentrated in the mount read volcanics tasmania and the mount windsor volcanics queensland the archean 4 million metric tons of metal and the silurian 3 5 million metric tons of metal constitute the next important episodes while the devonian and permian have produced isolated deposits the deposits range from cu type to zn cu type to zn pb cu type the zn cu type deposits are restricted to the archean and silurian whereas the cu type and zn pb cu type occur sporadically throughout the time span from early archean to permian in general terms the major vhms bearing districts are calc alkaline in character and display a thick basal pile of rhyolitic volcanics 1 3 km thick including lavas epiclastics and subvolcanic intrusions which is overlain by a polymodal sequence containing various proportions of rhyolite dacite andesite basalt and sediments the major deposits are commonly located at the top contact of the rhyolite pile or within the lower part of the overlying polymodal sequence there is a wide range in variability of styles of australian vhms deposits including mounds pipes sheets layered deposits stacked deposits stockwork and disseminated deposits distal reworked deposits and cyclic layered deposits although these various styles show a range of features there is a consistent theme across the spectrum of metal zonation alteration mineralogy alteration chemistry sulfur isotopes macrotextures microtextures and host volcanic relationships which strongly suggests that they all belong to the one genetic group of ore deposit the classic mound style deposits such as hellyer have a series of key features subsets of which are represented in the other deposit styles the mound deposits are considered to form from deposition of metal sulfides on the sea floor immediately around the hydrothermal vent growth of the mound occurs by upward replacement of sulfide assemblages stable at higher fluid temperatures leading to zone refining and lead zinc silver gold enrichment in the outer and upper parts of the mound departure from the classic mound style of deposit is related principally to three key factors the chemistry of the ore fluid salinity temperature f02 and ah2s the nature permeability and chemistry of the volcanic pile and the sea hoor environment sea hoor topography and seawater depth these factors control the aspect ratio of the deposit the extent of stringer zone development the degree of subsea floor replacement mineralization the nature and spatial development of footwall alteration and the development and style of related distal mineralization sulfur isotope studies of australian vhms deposits indicate that reduced seawater sulfate is a major source of sulfur in the deposit whereas lead and strontium isotope results are compatible with the metals being derived by seawater convection and leaching from the volcanic pile and basement rocks this conclusion is supported by the application of various leaching models which demonstrate the availability of an adequate source of both base and precious metals in the footwall volcanic sequences the input of metals directly from volcanic magma chambers is not precluded by the available data and one likely scenario involves the contribution of gold and copper from a magmatic vapor plume rising to mix with convective seawater fluids which contribute lead zinc silver and gold leached from the footwall volcanics and basement rocks the relative importance of the magmatic input compared with the seawater convective input may help to explain the spectrum of deposit styles arid their spatial relationship to volcanic centers or adjacent sedimentary basins

fluid chemistry of archean seafloor hydrothermal vents implications for the composition of circa 3 2 ga seawater seafloor hydrothermal vents of mid archean age ca 3230 ma have been identified and mapped in the barberton greenstone belt south africa and are known as the ironstone pods fluid inclusion homogenization temperature data when combined with gas chromatographic data provide a minimum calculated water depth for the pods of 982 m ironstone pod hydrothermal fluid endmember concentrations mg 0 of various dissolved components derived from bulk fluid inclusion crush leach experiments include cl 730 mmol l br 2 59 i 0 058 na 822 nh4 11 4 k 21 5 ca 42 6 and sr 0 15 this hydrothermal fluid also contains up to 1 07 mol co2 0 03 mol n2 0 02 mol ch4 262 ppm cos and minor amounts of c2 c4 hydrocarbons hydrothermal endmember ca sr and nh4 in particular and to a lesser degree k i and co2 commonly plot on or very close to modern vent fluid trends by contrast endmember na and br concentrations are distinct higher from modern vent fluids high i and nh4 concentrations are consistent with contributions from sediments and or organic matter calculated 18oh2o values for the pod hydrothermal endmember fluid define a narrow range from 0 9 to 1 6 very similar to that of modern vent fluid values 0 4 2 1 a best estimate for the ironstone pod seawater endmember composition is cl 920 mmol l br 2 25 so4 2 3 i 0 037 na 789 nh4 5 1 k 18 9 mg 50 9 ca 232 and sr 4 52 barberton seawater components are commonly within an order of magnitude of modern seawater values with the exception of significantly higher 1 nh4 ca and sr in the inclusions sulfate concentrations are minimum estimates for barberton seawater fluid inclusion samples containing the greatest amount of seawater component have higher n2 up to 0 1 mol and low co2 when compared to samples dominated by the hydrothermal endmember fluid barberton ambient seawater is considered to have been an evaporative brine of nacl cacl2 composition during the time of pod deposition ironstone pod fluid inclusion seawater endmember br cl and i cl values of 2 45 10 3 and 40 2 10 6 respectively are within error of bulk earth 2 38 10 3 and 190 10 6 and are consistent with the chemistry of 3 23 ga barberton seawater being buffered by the mantle

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

chemical controls on the composition of vent fluids at 13 11 n and 21 n east pacific rise six vent fields sampled at 13 11 n east pacific rise epr in may 1984 exhibit large interfield variations and a much wider range of chemical compositions than previously observed at 21 n measured ph at 25 c are acidic ranging from 3 1 to 3 7 sodium and chloride vary from 40 lower to 30 higher than seawater iron concentrations range from 2 to 10 mmol kg compared with 0 7 2 5 mmol kg at 21 n other sulfide forming metals cu zn cd and pb are generally lower at 11 13 n than at 21 n reliable temperature measurements were obtained at only two of the six vents and both were 350 5 c the vent fields at 21 n epr were resampled in august 1985 thus extending to almost 6 years the time period over which they have been monitored previous expeditions were made in november 1979 and november 1981 campbell et al this issue have shown that the chemistry of the hydrothermal fluids from these fields has been very stable over the period of repeated observation equilibrium calculations for the fluids from the fields at 13 n and 21 n using a greatly improved thermodynamic data base are described in this report they indicate that the chemistry is rock buffered and that the stability of these systems over time is a result of equilibrium control with respect to a greenschist type mineral assemblage at depth calculated high temperature ph of the fluids range from 4 1 to 4 7 with those from 13 11 n at the more acidic end of the range calculated affinities show that the fluids are close to or at saturation with respect to quartz albite muscovite smectite chlorite epidote and pyrrhotite the computations imply that lower temperature vents such as national geographic smoker ngs 273 c may have their silica concentrations controlled by equilibrium with respect to a phase other than quartz a comparison of fluid chemistry between ngs and vent 5 at 11 n suggests that the latter vent may also have a temperature 300 c while the fluid chemistry of a given field is compositionally stable the compositions themselves are unique for a particular field and cover a wide range between fields the processes controlling this variability probably include the depth of reaction i e pressure which is difficult to treat thermodynamically at present in addition increasing depth in the crust may be accompanied by decreasing permeability and hence a probable decrease in the water rock ratio it may be difficult to separate these two effects if the age differences between the fields at present unknown are of the order of decades then their temporal histories may have an important influence on their particular chemistries

breaking the seal at grimsvotn iceland of several problems associa ted wit h theoretica l expla nation s of the jokul hla ups which emerge from the outlet gla cie r skeioarar jokull of the ice ca p vatna jokull in sou theast iceland the mechani sm of flood init iation is one that has hi therto defied expl anat ion we provide such an expl ana tion based on a car efu l analysis of the cla ssica l nye ro thlisbe rger model near the subglacial lake grimsvotn the hyd raulic potenti al gradie nt is towards the lake and there is theref ore a catchme nt boun dary under the ice whose location dep ends on the subglacial mel twater drai nage charac terist ics as the con dit ions for a flood approa ch we show that the wat er divi de migra tes towards the lake while at the sa me time the lake pre ssure in creases when the hydra ulic potential gradient to wards the lake is low and the refillin g rate is sl ow the se al will break whe n the cat chment boun dary reaches the lake while the la ke level is st ill below flotati on pressu re whereas if refillin g is rapid flo tation can be achie ved bef ore a flood is initia ted this theory can thus explain why the seal is normally broken when the lake le vel at grimsvotn is still some 60 m below flotation level in addit ion we are abl e to explain why thej okulhla up following the 1996 eruption did not occur until flo tation level was achie ved and we show how the cyeli cit y and magnitude ofjoku lhla ups can be explained wit hin this theory

explanation for many of the unusual features of the massive sulfide deposits of the iberian pyrite belt newly published fluid inclusion data from quartz in stockwork veins beneath seven massive sulfide lenses in the iberian pyrite belt suggest that the lenses were formed from fluids that on reaching the sea reversed buoyancy and ponded in basins sulfides quenched in the resulting brine pool would have settled to form a sulfide mud this process provides a relatively efficient trapping mechanism for metal in the fluids and effectively excludes ambient seawater accounting for the deposits tending to have the characteristics of large size sheet like form absence of relict chimney structures and a mineral content characterized by pyrite arsenopyrite and absence or scarcity of barite marcasite and fe oxides if total s was less than total metals in the stockwork fluids some or all of the more soluble zn and pb could have been swept from the basin at the overflow accounting for the variable but generally low zn and pb contents of the ores the lack of sedimentary source for the high salinities implicates magmatic intrusions possibly similar to those related to sn w mineralization

the critical point and two phase boundary of seawater 200 500 c abstract the two phase boundary of seawater was determined by isothermal decompression of fully condensed seawater in the range of 200 500 c the pressure at which phase separation occurred for each isotherm was determined by a comparison of the refractive index of fluid removed from the top and bottom of the reaction vessel the critical point was determined to be in the range of 403 406 c 285 302 bar and was located by the inflection in the two phase boundary and by the relative volume of fluid and vapor as a function of temperature the two phase boundary of 3 2 nacl solution was found to coincide exactly with that of seawater over the range tested in the present study the boundary for both is described by a single seventh order polynomial equation the two phase boundary defines the maximum temperature of seawater circulating at depth in the oceanic crust thus the boundary puts a limit of about 390 c for seawater circulating near the seafloor at active ocean ridges 2 5 km water depth and about 465 c at the top of a magma chamber occurring at 2 km below the seafloor

interactions between magma chambers and hydrothermal systems oceanic and ophiolitic constraints a compilation of published exit temperatures and salinities for seafloor hydrothermal fluids along with fluid inclusion data on oceanic and ophiolitic rocks shows that ridge axes hydrothermal systems are generally characterized by maximum temperatures around 300 360 c thus clearly being in the one phase field they overlie a lower system marked by the generation of highly saline brines at high temperatures the brines may have originated either when seawater approached the top of the magma chamber or more probably as exsolution products of the differentiated melts that typify the roof of oceanic magma chambers in ophiolites the brines typically occur within the transition zone between the sheeted dike complex and the plutonic sequence i e on top of magma chambers this transition zone relatively well known from ophiolite studies oman troodos bay of islands josephine is characterized by mutually intrusive relationships between gabbro plagiogranite and dikes and xenoliths of altered diabase within plagiogranite and gabbro in fast to medium spreading ophiolites oman it is the locus of a major downward decrease in the density of hydrothermal veins and alteration the vein system is strongly anisotropic and shows a well marked preferred vertical along strike orientation the underlying cumulates are almost unaltered and exhibit only a diffuse net of amphibole veins while the overlying sheeted dike complex is densely veined and pervasively altered this zone is marked by major gradients in the physical parameters over short distances temperature several degrees celsius per meter pressure several hundred kilopascals transition from brittle to ductile in steady state systems direct interactions between the magma chamber and the hydrothermal system are restricted to this zone characterized by exsolution of magmatic volatiles into the hydrothermal system and the incorporation of hydrothermal fluids into the magma chamber by digestion of altered roof rocks complications to this steady state model resulting from a non steady state magma chamber e g josephine ophiolite are mainly the development of late stage faulting and more pronounced retrogressive alteration

phase relations and adiabats in boiling seafloor geothermal systems abstract observations of large salinity variations and vent temperatures in the range of 380 400 c suggest that boiling or two phase separation may be occurring in some seafloor geothermal systems consideration of flow rates and the relatively small differences in density between vapors and liquids at the supercritical pressures at depth in these systems suggests that boiling is occurring under closed system conditions salinity and temperature of boiling vents can be used to estimate the pressure temperature point in the subsurface at which liquid seawater first reached the two phase boundary data are reviewed to construct phase diagrams of coexisting brines and vapors in the two phase region at pressures corresponding to those of the seafloor geothermal systems a method is developed for calculating the enthalpy and entropy of the coexisting mixtures and results are used to construct adiabats from the seafloor to the p t two phase boundary results for seafloor vents discharging at 2300 m below sea level indicate that a 385 c vent is composed of a brine 7 nacl equivalent in equilibrium with a vapor 0 1 nacl brine constitutes 45 by weight of the mixture and the fluid first boiled at approximately 1 km below the seafloor at 415 c 330 bar a 400 c vent is primarily vapor 88 wt 0 044 nacl with a small amount of brine 26 nacl and first boiled at 2 9 km below the seafloor at 500 c 520 bar these results show that adiabatic decompression in the two phase region results in dramatic cooling of the fluid mixture when there is a large fraction of vapor

magnetite rich calc silicate alteration in relation to synvolcanic intrusion at the ansil volcanogenic massive sulfide deposit rouyn noranda quebec canada the ansil cu au volcanogenic massive sulfide deposit is located within an archean age cauldron infill sequence that contains the well known noranda base metal mining district the deposit is unusual in that 17 of the massive pyrrhotite chalcopyrite orebody is replaced by semi massive to massive magnetite temporally associated with the magnetite formation are several calc silicate mineral assemblages within the massive sulfide lens and the underlying sulfide stockwork vein system coarse grained andradite hedenbergite and ferroactinolite ilvaite alteration facies formed in the immediate footwall to the massive magnetite sulfide lens whereas an epidote albite pyrite rich mineral assemblage overprints the margins of the chlorite rich stockwork zone the epidote rich facies is in turn overprinted by a retrograde chlorite magnetite calcite mineral assemblage and the andradite hedenbergite is overprinted first by ferroactinolite ilvaite followed by semi massive to massive magnetite the footwall sulfide and magnetite rich alteration facies are truncated by a late phase of the flavrian synvolcanic tonalite trondhjemite complex early phases of this intrusive complex are affected to varying degrees by calc silicate rich mineral assemblages that are commonly confined to miarolitic cavities pipe vesicles and veins the vein trends parallel the orientation of synvolcanic faults that controlled volcanism and hydrothermal fluid migration in the overlying cauldron succession the magnetite rich calc silicate alteration facies are compositionally similar to those of volcanic hosted ca fe rich skarn systems typical of oceanic arc terranes tonalite trondhjemite phases of the flavrian complex intruded to within 400 m of the base of the earlier formed ansil deposit the low al trondhjemites generated relatively oxidized acidic ca fe rich magmatic hydrothermal fluids either through interaction with convecting seawater or by assimilation of previously altered rocks these fluids migrated upsection along synvolcanic faults that controlled the formation of the original volcanogenic massive sulfide deposit this is one of the few documented examples of intense metasomatism of a vms orebody by magmatic hydrothermal fluids exsolved from a relatively primitive composite sub seafloor intrusion

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

exsolution of immiscible vapor and liquid phases from a crystallizing silicate melt implications for chlorine and metal transport abstract the compositional variation of a cl bearing aqueous fluid exsolving from a crystallizing silicate melt is evaluated for the system haplogranitic melt na k c1 h 2 o with attention to the effect of vaporliquid immiscibility on the aqueous fluid above critical pressures for the aqueous fluid p 1 5 kb at 800 c where the fluid is single phase the cl concentration in the aqueous fluid c cl aq gradually decreases with crystallization of a silicate melt under rayleigh fractionation as demonstrated by previous studies however the variation of ca is quite different below critical pressures where vapor liquid immiscibility occurs in particular at low pressures p cl h 2 o ratio in a melt is small and ca will increase with crystallization a high cl h 2 o ratio at low pressure results in liquid exsolution and decreasing ca with crystallization eventually both vapor and liquid phases will exsolve simultaneously regardless of the initial exsolving phase during this simultaneous exsolution the composition of the exsolving vapor and liquid phases the mass ratio of these phases and the cl concentration of the silicate melt will all remain constant comparison of the model and published experimental data indicates that the simultaneous exsolution of vapor and liquid may occur from a normal rhyolite crystallized at pressures below the critical pressure for the aqueous fluid therefore simultaneous exsolution of the immiscible aqueous phases is likely to be a common phenomenon in magmatic hydrothermal systems at conditions for the vapor liquid immiscibility model calculations at 0 6 and 1 2 kb indicate that major amounts of cl in the system will be distributed to the liquid phase particularly at the lower pressure this suggests that the majority of chloridecomplexed metals exsolving from a magma may be transported in the liquid phase under shallow crustal conditions

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

postmagmatic thermal activity in the abitibi greenstone belt noranda and matagami districts evidence from whole rock pb isotope data the pb isotope compositions of the footwall sequences of the noranda and matagami volcanogenic massive sulfide deposits in the abitibi greenstone belt have been examined at noranda whole rock pb isotope data for the flavrian northwest rusty ridge and amulet formations and the subvolcanic flavrian intrusion scatter about a line that corresponds to a pb pb age of 2638 or 14 ma pb pb ages calculated for the individual units are all within error of this age but are significantly younger than the 2700 or 2 ma u pb zircon age for the flavrian intrusion the pb pb array passes above a tightly constrained field for the galena pb from ore deposits within the mine sequence the younger pb pb ages and the lack of collinearity with the sulfide ores reflect rehomogenization of the pb isotope compositions in these rocks approximately 60 m y after their formation at matagami whole rock pb isotope data fall along two arrays the first which includes the porphyritic metarhyolite of the watson lake group from the footwall and the key tuffite corresponds to a pb pb age of 2584 super 19 sub 20 ma the second array includes the bell river igneous complex and the overlying wabassee group basalts and corresponds to an age of 2647 or 14 ma these pb pb ages are approximately 75 and 140 m y younger than corresponding zircon u pb ages of 2725 or 2 and 2725 super 3 sub 2 ma both arrays pass above a tightly constrained field for galena pb from several matagami ore deposits and are probably the result of differences in the initial pb isotope compositions of the respective rocks the apparent initial pb values most likely represent heterogeneities in the mantle source regions for these rocks the younger pb pb ages for the two arrays indicate pb isotope homogenization in these rocks approximately 100 m y after their formation the cause of the different ages for the two arrays remains controversial but may be the result of either incomplete isotopic homogenization of pb or an earlier closure of the u pb systems in the rocks constituting the older array the pb homogenization implied by the young pb pb ages may be the result of the pervasive circulation of hydrothermal solutions that may have been derived from the dewatering of the lower parts of a newly formed thickened archean crust such large scale processes apparently occurred 50 to 150 m y after this crust was formed and assembled

the economic significance of the bell river complex abitibi subprovince quebec the exposed part of the bell river complex on the northern edge of the abitibi subprovince represents the upper portion of a layered complex with a massive gabbroic main zone being overlain by a magnetite rich gabbroic anorthositic layered zone and a granophyre the cumulate rocks display cryptic variation in mineral and whole rock compositions towards a more evolved signature with stratigraphic height there is a decrease in an content of plagioclase from an73 et an51 and in mg of augite from mg 75 to mg 56 an increase in la yb ratio of whole rocks from 0 5 to 2 9 and a decrease in cr from 770 to 5 ppm and ni from 315 to 45 ppm contents of whole rocks concentrations of platinum group elements pge are mostly below detection limit compositional comparison with the south african bushveld complex shows that the lowest exposed levels of the complex are equivalent to the critical main zone boundary that is in the immediate hanging wall of the pge reefs based on recent geophysical studies showing that the bell river complex is approximately 5 km thick we consider the presence of a hidden ultramafic zone a distinct possibility and propose a site for an exploration drilling programme a comparison of compositional data from the bell river complex and associated basalts and rhyolites of the watson lake and wabassee groups suggests that the three suites are cogenetic based on the fact that the numerous important volcanogenic massive sulphide vms deposits in the region are located around the intrusion and the similar age of the intrusion and the volcanic rocks we support earlier workers in suggesting that the intrusion and the vms deposits are genetically linked that is that the bell river complex provided the heat source that drove the hydrothermal fluids responsible for forming the vms deposits la partie expose du complexe de be11 river sur la bordure septentrionale de la sous province de l abitibi represente la portion superieure d un complexe stratifie incluant une zone principale formee d une masse gabbroique recouverte par une zone stratifiee gabbroique anorthositique riche en magnetite et une unite de granophyre les compositions obtenues sur roches totales et sur mineraux des lithologies b cumulats presentent des variations relativement subtiles neanmoins elles portent la signature d une differenciation magmatique plus prononcee en gravissant les niveaux stratigraphiques on observe une diminution de la composante an du plagioclase de an73 et an51 et du mg de l augite de mg 75 a mg 56 un accroissement pour roches totales du rapport la yb de 0 5 a 2 9 et une decroissance pour roches totales des teneurs en cr de 770 a 5 ppm et en ni de 315 a 45 ppm les concentrations des elements du groupe du platine egp sont quasiment sous leur limite de detection la comparaison de ces compositions avec celles du complexe du bushveld d afrique du sud montre que les niveaux exposes a la base du complexe sont equivalents a la frontiere des zones critique principale c est a dire du toit immediat des filons porteurs des egp les etudes geophysiques recentes ont revele que le complexe de bell river a une epaisseur d environ 5 km et a partir de cette donnee nous inferons l existence d une zone ultramafique cachee et nous proposons un site pour entreprendre un programme de forage d exploration une comparaison des compositions obtenues pour le complexe de bell river avec les basaltes et rhyolites associes des groupes de watson lake et de wabassee suggere que les trois suites sont cogenetiques sachant que de nombreux depots majeurs de sulfures massifs volcanogeniques smv dans la region sont localises tout autour d une intrusion et que l intrusion et les roches volcaniques sont sensiblement du meme ge par consequent nous adherons aux interpretations anterieures suggerant que i intrusion et les depots des smv sont lies genetiquement c est a dire que le complexe de bell river represente la source de chaleur qui activait la circulation des fluides hydrothermaux responsables de la formation des depots de smv

u pb geochronology of the eastern abitibi subprovince part 1 chibougamau matagami joutel region u pb zircon titanite and baddeleyite ages are reported for 20 volcanic and plutonic rock units in the chibougamau matagami and joutel areas of the northern abitibi belt pretectonic magmatism occurred in three main pulses volcanic rocks of the obatagamau formation in the chibougamau area were erupted at 2759 ma following a possible hiatus in igneous activity volcanism accompanied by the emplacement of layered mafic intrusions occurred in the chibougamau matagami and joutel areas in the period of 2730 2722 ma in the chibougamau area a slightly younger period of plutonism between 2720 and 2712 ma is thought to be comagmatic with felsic volcanic rocks in the uppermost part of the roy group syntectonic intrusive rocks in the northern abitibi belt range in age from 2701 to 2693 ma these ages are broadly comparable to ages obtained for syntectonic plutons elsewhere in the abitibi belt suggesting that tectonism associated with the kenoran orogeny occurred at approximately the same time throughout

the relationship between flow and permeability field in seafloor hydrothermal systems the permeability of oceanic crust is spatially variable and probably anisotropic as well using realistic permeability fields for young oceanic crust we have performed numerical simulations of finite amplitude steady and unsteady convective fluid flow in layered and or anisotropic porous media heated from below to investigate particular patterns of fluid flow and temperature in mid ocean ridge hydrothermal systems on the flanks of mid ocean ridges permeability measurements in deep sea boreholes suggest that only the top few hundred meters of oceanic crust is permeable given this permeability structure and assuming some minimum permeability and layer thickness our models predict that convection occurs in the form of numerous cells with aspect ratios of order unity within this permeable layer such convection results in fluid flux and diagenetic reactions within the permeable layer with a negligible effect on heat flow at the seafloor in agreement with field observations estimates of oceanic crust permeability based on hydrothermal veins in ophiolites and measurements in deep sea boreholes suggest that pillow basalts lava flows are much more permeable than underlying sheeted dikes this is particularly true at the ridge crest where voids have not yet collapsed and filled given this permeability structure our study suggest that a small percentage of the fluid entering the ridge crest circulates through the sheeted dikes before exiting the system at high temperatures in very focused discharge zones in our models these discharge zones narrow considerably at the interface between the sheeted dikes and the pillow basalts lava flows much of the fluid entering the system however never circulates below the pillow basalts lava flows and exits the seafloor at low temperatures in general discharge zones are more focused than recharge zones these results are consistent with observations of narrow and focused discharge zones in ophiolites and localized high temperature venting amid widespread low temperature flow on the ridge crest the spacing of upflow zones at the surface is strongly controlled by convective flow in the bottom permeable layer in our models near field effects dominate over far field effects in systems with lateral variations in permeability and no large scale flow develops between widely spaced areas of contrasting permeability the time to steady state and evolution of convective flow in porous media vary considerably with initial conditions our simulations suggest that the time to steady state is relatively long and that it is possible that hydrothermal convection at the ridge axis never reaches a steady state flow pattern in the sense that the variations in the system boundary conditions such as basal heat flux may occur on a time scale less than the response time of the hydrothermal system it is possible however that these systems may be quasi steady for significant periods of time

numerical analysis of rapid water transfer beneath antarctica we use a simple energy conservation model and a model based on rothlisberger s theory for steady state water flow in a subglacial conduit to model water movement between lakes in the adventure subglacial trench region of east antarctica during a 1996 98 jokulhlaup using available field evidence to constrain the models suggests that water flow would likely be accommodated in a tunnel with a cross sectional area of 36 m 2 and a value for k the reciprocal of manning s roughness parameter larger than the 12 5 m 1 3 s 1 previously calculated we also use nye s theory for time dependent conduit water flow to model the temporal evolution of conduit discharge cross sectional area water pressure and lake draining and filling during the flood we initially assume one source and one sink lake we perform sensitivity tests on the input parameter set matching modeled source and sink lake depth changes with measured surface elevation data using a simple function for vertical ice deformation in which surface deformation scales linearly to the lake depth change we find the scaling factor is of the order 4 10 3 of the ice thickness the most likely value of k lies in the range 55 68 m 1 3 s 1 and the ratio of source to sink lake radii is approximately 1 1 4 finally we experiment using nye s theory to model water movement between one source and three sink lakes the model fails to produce the observed patterns of water movement as indicated by the surface deformation data

petrology of philippine geothermal systems and the application of alteration mineralogy to their assessment abstract philippine geothermal systems occur in the vicinity of large holocene calc alkaline volcanic complexes wells drilled in these areas encountered multiple intrusions the latest dikes are the subsurface manifestations of the youngest heat source commonly at least two hydrothermal regimes are juxtaposed in a single area with the latest being in equilibrium with the present temperature and chemical regime alteration by neutral ph water is pervasive and abundant a contact metamorphic aureole also occurs near intrusives alteration due to acid sulfate fluids is generally confined to permeable structures neutral ph alteration is divided into four zones on the basis of key clay minerals and two subzones are defined by calc silicates these are the smectite ambient to 180 c transition 180 230 c illite 230 320 c and biotite 270 340 c zones subzones are defined by epidote 250 340 c and amphibole 280 340 c the four main zones of acid alteration are kaolinite ambient to 120 c dickite kaolinite 120 200 c dickite pyrophyllite 200 250 c and pyrophyllite illite 230 320 c where relict high temperature alteration reaches the surface the area being drilled is usually the outflow zone of the present system these hydrothermal mineral assemblages are used 1 as geothermometers 2 to assist in determining the depth at which the production casing will be set during drilling 3 to estimate fluid ph and other chemical parameters 4 to predict possible corrosion and scaling tendencies of the fluids 5 as a measure of permeability and possible cold water influx into wells 6 as a guide to field hydrology and 7 to estimate roughly the thickness of the eroded overburden

calcium rich brines and other hydrothermal fluids in fluid inclusions from plutonic rocks oceanographer transform mid atlantic ridge abstract fluid inclusions in metagabbro plagiogranite and epidosite from the oceanographer transform mid atlantic ridge were investigated using microthermometric measurements and laser raman spectroscopy inclusions in quartz reveal the presence of a high salinity average 40 wt nacl cacl 2 high temperature 340 c calcium rich na ca about 1 by weight fluid in metagabbro and plagiogranite at room temperature these fluids bear a halite daughter crystal and may also contain small crystals of calcite titanite and hematite a low density vapor rich fluid bearing a small amount of co 2 is closely associated with the briney fluids in one sample all samples contain a population of high temperature 150 to 400 c aqueous liquid dominated fluid inclusions commonly of secondary origin having seawaterlike salinity 3 to 8 wt nacl equivalent we conclude that the calcium rich brines are produced by a combination of two processes fluid phase separation and segregation of brines from vapor prior to trapping to generate the high salinities and extensive fluid mafic rock interaction at low water rock ratio to promote the calcium enrichment later introduction along microfractures of low salinity modified seawater at high water rock ratio is recorded by the secondary fluid inclusions the chemical complexity and association with co 2 may indicate that there is a magmatic component to the brines in addition to a seawater source differences between the oceanographer transform brines and those reported in previous studies of oceanic and ophiolite samples are attributed to variations in the degree of water rock interaction the host rock lithology and the influence of a magmatic fluid component

geochemistry of hydrothermal fluids from axial seamount hydrothermal emissions study vent field juan de fuca ridge subseafloor boiling and subsequent fluid rock interaction hydrothermal fluids collected from the ashes vent field in 1986 1987 and 1988 exhibit a very wide range of chemical composition over a small area 60 m in diameter compositions range from a 300 c gas enriched 285 mmol kg co2 low chlorinity 33 of seawater fluid to a 328 c relatively gas depleted 50 mmol kg co2 high chlorinity 116 of seawater fluid the entire range of measured compositions at ashes is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust followed by partial segregation of the vapor and brine phases other mechanisms proposed to produce chlorinity variations in hydrothermal fluids precipitation dissolution of a chloride bearing mineral or crustal hydration cannot produce the covariation of chlorinity and gas content observed at ashes there is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation in which gases are partitioned according to henry s law and all salt remains in the liquid phase significant enrichments in silica lithium and boron in the low chlorinity fluids over levels predicted by the model are attributed to fluid rock interaction in the upflow zone depletions in iron and calcium suggest that these elements have been removed by iron sulfide and anhydrite precipitation at some time in the history of the low chlorinity fluids the distribution of low and high chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability the relatively shallow depth of the seafloor 1540 m and the observed chemistry of ashes fluids are consistent with phase separation in the sub critical or near critical region

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

hydrothermal alteration of basalt by seawater under seawater dominated conditions fresh mid ocean ridge basalt glass and diabase have been reacted with seawater at 150 300 c 500 bar and water rock mass ratios of 50 62 and 125 using experimental apparatus which allowed on line sampling of solution to monitor reaction progress these experiments characterize reaction under what we have called seawater dominated conditions of hydrothermal alteration in an experiment at 300 c basalt glass undergoing alteration removed nearly all mg2 from an amount of seawater 50 times its own mass in the process the glass was converted entirely to mixed layer smectite chlorite anhydrite and minor hematite removal of mg from seawater occurred as a mg oh 2 component incorporated into the secondary clay this produced a precipitous drop in solution ph early in the experiment accompanied by a dramatic increase in the concentrations of fe mn and zn in solution as mg removal neared completion and the glass was hydrolyzed ph rose again and heavy metal concentrations dropped at water rock ratios of 62 and 125 and 150 300 c the mineral assemblage produced was similar to that at a water rock ratio of 50 solution chemistry however contrasted with the earlier experiment in that mg concentrations in solution were greater and ph lower this caused significant leaching of heavy metals at 300 c nearly all of the na ca cu zn and co2 and most of the k ba sr and mn were leached from the silicates h2s al si and possibly co were also significantly mobilized whereas v cr and ni were not little or no seawater sulfate was reduced although submarine hot spring solutions sampled to date along mid ocean ridges clearly come from rock dominated hydrothermal systems evidence from ocean floor metabasalts and from heat flow studies indicates that seawater dominated conditions of alteration prevail at least locally both in axial hightemperature systems and in ridge flank systems at lower temperatures

a simple holistic hypothesis for the self destruction of ice sheets abstract ice sheets are the only components of earth s climate system that can self destruct this paper presents the quantitative force balance for bottom up modeling of ice sheets as first presented qualitatively in this journal as a way to quantify ice bed uncoupling leading to self destruction of ice sheets hughes 2009a rapid changes in sea level and climate can result if a large ice sheet self destructs quickly as did the former laurentide ice sheet of north america between 8100 and 7900 bp thereby terminating the last cycle of quaternary glaciation ice streams discharge up to 90 percent of ice from past and present ice sheets a hypothesis is presented in which self destruction of an ice sheet begins when ubiquitous ice bed decoupling quantified as a floating fraction of ice proceeds along ice streams this causes ice streams to surge and reduce thickness by some 90 percent and height above sea level by up to 99 percent for floating ice so the ice sheet undergoes gravitational collapse ice collapsing over marine embayments becomes floating ice shelves that may then disintegrate rapidly this floods the world ocean with icebergs that reduce the ocean to atmosphere heat exchange thereby triggering climate change calving bays migrate up low stagnating ice streams and carve out the accumulation zone of the collapsed ice sheet which prevents its recovery decreases earth s albedo and terminates the glaciation cycle this sequence of events may coincide with a proposed life cycle of ice streams that drain the ice sheet a first order treatment of these life cycles is presented that depends on the longitudinal force balance along the flowbands of ice streams and gives a first approximation to ice bed uncoupling at snapshots during gravitational collapse into ice shelves that disintegrate thereby removing the ice sheet the stability of the antarctic ice sheet is assessed using this bottom up approach

advances in modelling subglacial lakes and their interaction with the antarctic ice sheet subglacial lakes have long been considered hydraulically isolated water bodies underneath ice sheets this view changed radically with the advent of repeat pass satellite altimetry and the discover

salinity of oceanic hydrothermal fluids a fluid inclusion study abstract an extensive microthermometric study of quartz epidote plagioclase anhydrite and sphalerite hosted fluid inclusions from ophiolitic semail oman and trinity california ophiolites and oceanic east pacific rise hydrothermal vents gorringe bank odp leg 111 hole 504b crust has been carried out in order to constrain a model accounting for wide salinity variations measured in the oceanic hydrothermal fluids recorded salinities in fluid inclusions vary between 0 3 and 52 wt nacl eq however more than 60 of the mean standard deviation salinities of the samples are within the range 3 2 0 3wt nacl eq microthermometric error and the mean salinity of all fluid inclusions without the brines is 4 0 wt nacl eq with a standard deviation of 1 6 wt nacl eq whereas most samples display slightly higher salinities than seawater several samples exhibit very high salinities more than two times that of seawater these high salinities are restricted to the plagiogranites semail and trinity ophiolites which mark the top of the fossil magma chamber in the transition zone between the plutonic sequence and the sheeted dyke complex the fluid inclusion population studied in the plagiogranites is characterized by the occurrence of four major fluid inclusion families 1 low to medium salinity liquid vapor fluid inclusions which homogenize into the liquid phase 2 low salinity liquid vapor fluid inclusions with pseudocritical homogenization 3 low to medium salinity liquid vapor fluid inclusions which homogenize into the vapor phase and 4 high salinity liquid vapor halite fluid inclusions which homogenize into the liquid phase by halite dissolution and exhibit salinities as high as 52 wt nacl eq these fluid inclusion families are interpreted as resulting from phase separation occurring in hydrothermal or magmatic fluids within the transition zone between the hydrothermal system and the magma chamber at temperatures higher than 500 c very low salinities less than half that of seawater have been found only in one dioritic sample from the gorringe bank mineralogical and petrological data from the semail ophiolite shows the absence of cl rich minerals furthermore hydration of the crust although very important cannot account for the large salinity variations thus the large salinity variations measured are interpreted as resulting from phase separation and generation of brine rich fluids at the top of the magma chamber this phase separation and subsequent phase segregation and brine accumulation may result in a double diffusive convection of the fluids in the hydrothermal system

reply to a discussion of relationship between thermal maturation gradients geothermal gradients and estimates of the thickness of the eroded foreland section southern alberta plains canada by majorowicz et al 1990

melting behavior of fluid inclusions in laboratory grown halite crystals in the systems naclh2o naclkclh2o naclmgcl2h2o and naclcacl2h2o abstract the chemical composition of aqueous fluid inclusions in crystals of halite can be accurately determined from observed melting behaviors of ice hydrohalite and sylvite some fluid inclusion melting behaviors observed in laboratory grown halite crystals systems nacl h 2 o nacl kcl h 2 o nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o differ from predicted stable equilibrium relations in the nacl h 2 o and nacl kcl h 2 o systems observed first melt temperatures are up to 15 c below the equilibrium eutectic temperatures of 21 2 and 22 9 c respectively the final melting temperature of ice in the presence of hydrohalite and the final melting temperature of hydrohalite are reproducible and match predicted melting temperatures the limit of detection of sylvite daughter crystals in the nacl kcl h 2 o system is approximately 5 wt 1 molal kcl final melting temperatures of sylvite match published equilibrium data to within 0 3 c in the nacl kcl h 2 o system at halite saturation m kcl can be determined from the final sylvite dissolution temperature or from the final melting temperature of hydrohalite fluid inclusions in the nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o systems that form stable salt hydrates mgcl 2 12h 2 o and cacl 2 6h 2 o during freezing first melt within 3 c of predicted eutectic temperatures 37 and 52 c however fluid inclusions with mgcl 2 or cacl 2 may also start melting at temperatures as low as 80 c such low first melt temperatures indicate the presence of metastable salt hydrates presumably mgcl 2 8h 2 o mgcl 2 6h 2 o or cacl 2 4h 2 o the formation of metastable phases during freezing of fluid inclusions can lead to misinterpretation of the chemical composition of fluid inclusions in natural samples this is especially true for fluid inclusions with first melt temperatures below 37 c which may be erroneously interpreted as being rich in cacl 2 the final melting of ice in the presence of hydrohalite may vary by more than 15 c in fluid inclusions of different size but identical bulk composition and occurs at lower temperatures than predicted in fluid inclusions from the nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o systems however the final melting temperature of ice in inclusions which fail to nucleate hydrohalite and the final melting temperature of hydrohalite are reproducible to within 0 1 c and can be used to determine mgcl 2 and cacl 2 molalities

synthetic fluid inclusions in natural quartz ii application to pvt studies synthetic fluid inclusions have been used to determine volumetric properties of h2o and a 20 wt kcl solution from 300 700 c and 1 3 kb comparison of results obtained for h2o with previously published data indicates that the synthetic fluid inclusion technique provides pvt data that are within a few percent of those obtained from conventional pvt studies this technique thus provides a fast relatively simple means of determining pvt properties of many fluids of geologic interest

response of two phase fluids to fracture configurations within submarine hydrothermal systems submarine hydrothermal systems at mid ocean ridges evolve within a region of brittle deformation overlying a region of ductile deformation the interplay between brittle and ductile modes of deformation and the geometry of fracture systems within the brittle zone can exert a significant influence on fluid flow especially under conditions leading to aqueous two phase separation within the brittle region fractures vary between large conduits directly open to the seafloor and small veins and microcracks with constricted or indirect connections to the seafloor in the ductile region fluids may be trapped under near lithostatic pressure while fluids within throughgoing veins in the brittle region will experience hydrostatic pressure in a narrow transition zone between these two extremes a steep pore fluid pressure gradient must exist in the brittle region itself fluid pressures will vary between lithostatic and hydrostatic as fluid occupies interstices varying from isolated pores through tortuous microcracks to channels directly connected to the main upflow conduits a hydrostatic gradient modified by the decreased density of hot fluid and the effects of the flow itself should be applicable in major conduits open to the seafloor fluctuating pressure conditions in the transition zone may induce phase changes in a fluid remaining at constant depth as a result of the large volume increase that accompanies two phase separation a two phase fluid must either accelerate upward or exert an increased pressure on its surroundings possibly leading to enhanced fracturing the fracture configuration will also affect the behavior of the mixed fluid after phase separation a two phase fluid experiencing rapid turbulent flow in a large open conduit is more likely to remain a well mixed suspension of vapor and liquid whereas a two phase mixture in a constricted branching series of channels moving more slowly in a laminar flow regime is more likely to undergo physical isolation of the phases leading to venting of a vapor phase and retention of a saline brine phase at depth we propose a possible configuration for the fracture pattern within a submarine hydrothermal system consisting of a feeder zone comprised of anastomozing microcracks and veinlets progressively increasing in diameter and decreasing in abundance feeding into a main upflow stockwork zone in which flow directly to the surface takes place in relatively large unconstricted conduits fluid movement within a feeder zone may be modified by capillary flow tortuous channelways disconnected fractures blocking of cracks by mineral deposition and opening of new fractures

stratigraphic and structural aspects of the volcanic rocks of the matagami mining camp quebec implications for the norita ore deposit the matagami mining camp is an important base metal producer in the canadian shield the volcanic stratigraphy of the area is folded into the galinee anticline which is an east west trending and west plunging regional fold the south flank consists of a low strain do main characterized by a southeast trending volcanic sequence dipping moderately at 45 degrees to the south the stratigraphic succession is well defined with the watson lake group at the base comprising a lower dacitic unit and an upper quartz porphyritic rhyodacite unit all but one of the deposits lie along the key tuffite unit located at the top of the watson lake group the north flank of the anticline is affected by a series of east west and northwest trending high angle reverse shears which defines the lac garon high strain zone the volcanic sequence within this zone is displaced into four overlapping sigmoidal segments in which stratigraphic reconstruction is possible the norita massive sulfide deposit located in the center of the north flank has been deformed and faulted its internal metal zonation and its underlying stringer zone were flattened and rotated its present geometry suggests that it was deformed during north side up high angle reverse movement this sense of strain is compatible with the displacement of the marker units inside the lac garon high strain zone and the morphological modifications are believed to be consistent with the movement within this zone it is suggested that the norita massive sulfide deposit is a deformed equivalent of the less deformed south flank massive sulfide deposits and in contrast with an earlier suggestion of a distal origin shares the same genetic traits as the south flank deposits

numerical studies of ice flow over subglacial geothermal heat sources at grimsvotn iceland using full stokes equations 1 the importance of studying geothermal heat sources under glaciers lies in 1 the potential for jokulhlaups from such sites 2 the potential for enhanced sliding of ice masses caused by meltwater lubrication of the ice bedrock interface and 3 the potential to gain a deeper understanding of the interaction between the geothermal system and the overlying ice we study the temporal evolution of an elongated depression on the eastern side of the geothermally active grimsvotn caldera through numerical 2 dimensional forward modeling solving the full stokes equation the 100 150 m deep depression formed gradually between 1998 and 2004 the model results show that heat flux estimates based on depression volume are strongly dependent on the value of the rate factor a in glen s flow law if flow of ice into the depression is not taken into account heat flux underestimates of 15 75 occur corresponding to rate factors of 10 68 x 10 16 s 1 kpa 3 the estimated heat flux at the study site was 260 390 w m 2 with the best estimate being 280 w m 2 obtained from the best estimate of a 23 10 16 s 1 kpa 3 the total power of the modeled site was 250 300 mw about one tenth of the total heat output of grimsvotn this heat flux is of comparable magnitude to that of other powerful subglacial geothermal areas finally as the maximum modeled strain rates 1 x 10 7 s 1 are within the range of the applicability of glen s flow law this suggests that the full stokes approach should be valid for the study of many subglacial geothermal areas

initial results from a distributed physically based model of glacier hydrology this paper describes the development and testing of a distributed physically based model of glacier hydrology the model is used to investigate the behaviour of the hydrological system of haut glacier d arolla valais switzerland the model has an hourly time step and three main components a surface energy balance submodel a surface flow routing submodel and a subglacial hydrology submodel the energy balance submodel is used to calculate meltwater production over the entire glacier surface the surface routing submodel routes meltwater over the glacier surface from where it is produced to where it either enters the subglacial hydrological system via moulins or runs off the glacier surface the subglacial hydrology submodel calculates water flow in a network of conduits which can evolve over the course of a melt season simulation in response to changing meltwater inputs the main model inputs are a digital elevation model of the glacier surface and its surrounding topography start of season snow depth distribution data and meteorological data model performance is evaluated by comparing predictions with field measurements of proglacial stream discharge subglacial water pressure measured in a borehole drilled to the glacier bed and water velocities inferred from dye tracer tests the model performs best in comparison with the measured proglacial stream discharges but some of the substantial features of the other two records are also reproduced in particular the model results show the high amplitude water pressure cycles observed in the borehole in the mid melt season and the complex velocity discharge hysteresis cycles observed in dye tracer tests the results show that to model outflow hydrographs from glacierized catchments effectively it is necessary to simulate spatial and temporal variations in surface melt rates the delaying effect of the surface snowpack and the configuration of the subglacial drainage system itself the model s ability to predict detailed spatial and temporal patterns of subglacial water pressures and velocities should make it a valuable tool for aiding the understanding of glacier dynamics and hydrochemistry 1998 john wiley sons ltd

processes of brine generation and circulation in the oceanic crust fluid inclusion evidence from the troodos ophiolite cyprus detailed temporal thermal and compositional data on aqueous fluid inclusions from a suite of plutonic and diabase samples from the troodos ophiolite cyprus provide the first documentation that generation of high temperature brines may be common at depth in the oceanic crust anastomosing arrays of fluid inclusions in rocks of the upper intrusive sequence record episodic fracturing events the earliest fracturing event at temperatures 450 600 c resulted in entrapment of brine rich aqueous fluids with salinities of 36 61 wt nacl equivalent homogenization of the brine inclusions by haute dissolution the virtual absence of vaporrich fluid inclusions throughout the upper level plutonic sequence and the restriction of brine inclusions to the most evolved plutonic rocks suggests that exsolution of brines off of the late stage gabbro and plagiogranite melts played a significant role in generating the quartz hosted high salinity inclusions cooling of the fluids during pulses of fluid migration associated with episodic fracturing events resulted in entrapment of the brines in the deep seated high temperature portion of the hydrothermal system in localized areas the high temperature brines nacl kcl cacl2 caused extreme alteration of the plagiogranite bodies and in the formation of podiform epidosites arrays of low temperature low salinity fluid inclusions which in some samples crosscut fractures dominated by brine inclusions indicate downward propagation of a cracking front subsequent to collapse of the high temperature magmatic system resulting in penetration of seawaterlike fluids into the plutonic sequence at temperatures 200 400 c hydration reactions under greenschist facies conditions or limited mixing with brine rich fluids may have resulted in salinity variations from 70 below to 200 above seawater concentrations temperatures and compositions of the low salinity inclusions are similar to those found in stockwork systems beneath troodos ore deposits and to those of fluids exiting active submarine hydrothermal vents at mid ocean ridge spreading centers the low temperature fracture networks may represent an extensive deep seated feeder system which coalesced to form zones of concentrated hydrothermal upflow

differential thermal stresses in the earth summary rocks have a significant thermal expansion coefficient and a high modulus of elasticity small thermal changes can therefore generate very substantial stresses a convenient idealized example is the mechanically free cooling thermal boundary layer in a material that suddenly turns from plastic to rigid in cooling through a rigidus temperature this simplified case can be solved analytically for both a linear and an error function temperature profile a compressed layer develops near the surface with a logarithmic singularity of stress at the boundary at a depth of 0 2 of the elastic thickness the stress changes to tensile and peaks at 15 per cent of the confined shrinkage stress in the middle of the rigid layer the situation is similar to the stresses in one half of a piece of tempered glass like the tempered glass both the cooling lithosphere and a plated boundary to a magma chamber are subject to breakage when cracks are induced in the tensile layer positive pore magma pressure is required to overcome confining pressure at depth in the lithosphere but cracking could become catastrophic in the plated boundary to a near surface magma chamber