the role of magma injection in localizing black smoker activity the mechanisms for localization of black smoker systems at mid ocean ridges remain to be fully understood seismic data for a segment of the juan de fuca ridge with long lived black smoker vents reveal ongoing magma recharge into the crustal magma chamber thereby providing an explanation for the localization

a two dimensional model of overpressure development and gas accumulation in venture field eastern canada 1 we used a basin scale model to reconstruct the development of overpressures in a sedimentary basin the area of application is the venture gas field nova scotia canada it is in an overpressured zone in which sedimentation rates have been relatively low since 90 ma and where large horizontal stresses have been evidenced we investigated the importance of overpressure mechanisms using the institut francais du petrole temispack model which integrates compaction hydraulic fracturing fluid flow heat transfer and the formation and migration of hydrocarbons we first constructed a reference model using geological geochemical and physical data second we studied sensitivity of the model to several input parameters and from this deduced additional constraints the modeled permeability along faults and of shaly beds interlayered in the reservoirs must be very low to enable both the local and regional distributions of overpressure and gas to be reproduced several runs of the reference model studying the relative importance of possible overpressuring mechanisms and the relative contribution of plausible source rocks to gas accumulations showed that gas generation and accumulation and lateral compression contribute little to overpressuring which is mainly accounted for by compaction disequilibrium in spite of low sedimentation rates faults and shaly beds play an important role in maintaining the overpressure gas generation in the reservoir units or close to them does not contribute significantly to the field gas accumulations gas sources are likely to be found in the underlying formations

are time and temperature the only constraints to the simulation of organic matter maturation abstract confined pyrolysis hydrous pyrolysis and high pressure hydrous pyrolysis were performed with various samples toarcian shale and kerogen woodford shale and kerogen manakam coal and under different experimental conditions in order to test the effects of pyrolysis variables on the simulation of maturation of organic matter results show that i experimental time and temperature are not exchangeable parameters as suggested by first order kinetic law models ii increasing confining pressure 300 1300 bars in gold cell experiments performed on woodford samples has a limited influence on the reduction rate of the oil potential the tsom total soluble organic matter yield and the structure of the polars iii the presence of excess liquid water 0 100 weight in confined pyrolysis experiments does not significantly influence organic matter maturation however comparisons with classic hydrous pyrolysis 200 weight water could indicate major differences in timing and composition of the tsom bitumen expelled fraction as well as of the solid residue it appears that excess water is not necessary to simulate organic matter maturation when the system is sufficiently confined hydrous pyrolysis conditions low partial pressure of products delay woodford kerogen conversion in addition results concerning the mahakam coal suggest that the structure of the organic matter plays an important role concerning the water effect iv increasing water pressure in hydrous conditions 220 1300 bars drastically lowers and delays woodford kerogen maturation quantitative analysis of the water produced during confined pyrolysis and comparison of the extent of aromatization in hydrous and confined pyrolysis suggests that two competing hydrogen transfer mechanisms occur during organic matter pyrolysis this study shows that additional parameters must be defined when comparing the results of the experimental simulation of organic matter maturation

estimations of heat transfer from grotto s north tower a neptune observatory case study abstract the overall heat transfer through an active hydrothermal sulfide edifice in particular the north tower of grotto has been estimated at 80 mw or greater based on the following in situ measurements and assumptions 1 the heat transfer by diffuse flow is estimated at 33 380 mw based on extrapolating the acoustically mapped area to all sides of the north tower visible area 30 m 2 extrapolated area 100 m 2 and using the range of available spot measurements of temperature 6 23 c and vertical velocity 0 07 0 28 m s the lower number 33 mw is more likely but there is insufficient knowledge of the temporal and spatial variability of diffuse flow to be certain 2 the heat transfer by focused flow is estimated at 30 70 mw based on summing the estimated individual rates of heat transfer for 4 out of 7 documented black smokers and flanges 3 conductive heat transfer out of the mound is unknown but is likely to be much less than the advective heat transfer additionally the plume transport of heat is estimated at 20 40 mw based on the direct measurement of temperature within the plume at 5 25 m above the top of the edifice despite uncertainties the lower estimate of plume versus smoker heat transfer suggests that heat transfer is dominantly by diffuse flow furthermore not all plumes from individual smokers may merge even for so small an area as the north tower of grotto

numerical modeling of phase separation at main endeavour field juan de fuca ridge 1 before being disrupted by a magmatic event in 1999 the vent temperatures and salinities along the axis of the main endeavour field on the juan de fuca ridge exhibited a quasi steady spatial gradient in which the southern vent fluids were hotter and less saline than the northern vent fluids we present 2 d numerical models of two phase flow in a nacl h2o system to understand these gradients we consider homogenous permeability models with a range of bottom boundary temperature distributions and heterogeneous permeability models by imposing layer 2a extrusives with a constant bottom boundary temperature distribution the aim is to understand the impact of both bottom boundary temperature and layer 2a permeability on hydrothermal fluids and to determine what combination of these controlling factors could cause the observed trend we find that variations in bottom boundary temperature alone cannot explain the span of surface temperatures and salinities measured at the main endeavour field heterogeneous permeability within layer 2a that has higher overall permeability in the northern part of the vent field than the southern part can reproduce the observed north to south temperature gradient but such a permeability distribution cannot reproduce the observed salinity gradient we conclude that both deep seated heterogeneous permeability perhaps localized by a fault zone and a heterogeneous layer 2a are required to produce the trend of temperatures and salinities in vent fluids at the main endeavour field prior to the 1999 event

experimental heating and kinetic models of source rocks comparison of different methods abstract closed system isothermal micro pyrolysis has been carried out for nine upper jurassic source rocks from the norwegian shelf supplemented by hydrous pyrolysis experiments and open system pyromat kinetic determinations in all investigated cases open system kinetics is able to model the residual potential derived from s2 but not the respective low temperature closed system isothermal experiments the relatively detailed micro pyrolysis generation patterns obtained allow us to differentiate between primary generation and secondary cracking of different carbon number classes of hydrocarbons formation of non hydrocarbons or hydrocarbon sinks in residues can be inferred from these patterns comparison of results from low and high temperature experiments suggests that the potential of a source rock may vary according to heating history discrepancies between high and low temperature experiments may be explained in terms of competing reaction pathways these may constitute a main source of uncertainty when extrapolating bulk kinetic models into geological conditions

crustal magnetization reveals subsurface structure of juan de fuca ridge hydrothermal vent fields a near bottom geophysical survey on the endeavour segment of the northern juan de fuca ridge shows that regions of well defined low crustal magnetization are strongly correlated with both active and extinct submarine hydrothermal vent sites in particular at the main endeavour field we find discrete magnetization lows associated with each cluster of vents magnetization lows are directly centered beneath the vent clusters and have diameters of 100 m which implies a near vertical narrow pipe like source region located directly beneath the surface expression of the vent edifices lows are also separated from each other by only 200 m which further implies highly focused zones magnetization lows are also associated with inactive and extinct vent areas which indicates that alteration of the magnetic minerals in the crust rather than necessarily temporary thermal demagnetization is the primary process responsible for the low magnetization these narrow pipe like bodies are highly characteristic of alteration pipes found in ophiolites and are indicative of hydrothermal fluid up flow zones thus each magnetization low may define an individual upwelling zone with distinct subsurface plumbing and thermal structure the crustal magnetization patterns provide important constraints on the geometry of the subsurface plumbing beneath these hydrothermal vent systems at the main endeavour field magnetization lows are distributed along the trend of the rift valley in a semiregular pattern with a spacing of 200 m arguing that upward flow may be partitioned into regularly spaced intervals along the axis of the rift valley

hydrothermal cooling of midocean ridge axes do measured and modeled heat fluxes agree abstract accurate estimates of near axis hydrothermal cooling are keys to understanding many geological chemical and biological processes at midocean ridges at present however field observations h heat flux vent field are few and imprecise and model results h m heat flux km of axis vary widely depending on assumptions about crustal formation crustal composition and the cooling efficiency and spatial and temporal extent of hydrothermal circulation in this paper i compare all published vent field scale measurements of h to a range of published h m for slow 40 mm yr and fast 40 mm yr ridges to assess the present state of knowledge about the magnitude of near axis hydrothermal cooling h has been evaluated 28 times either by point measurements using inventories of individual sources 362 552 mw n 12 measurements or by integrated measurements using water column observations 876 1011 mw n 16 comparable estimates of mean vent field expected h at any spreading rate can be derived by merging h m from various models with the predicted global inventory of active vent sites these estimates range from 1 1 5 gw field and are approximately constant for all spreading rates this range agrees with the best constrained results of h from slow ridge vent fields 1669 1354 mw but is considerably higher than those from fast ridges 245 170 mw inferences about the spatial and temporal variability of h can be gained by defining the parameter l h h m equivalent to the axis length needed to continuously power a hydrothermal field the resultant large l values 13 333 km for slow ridges 1 32 km for fast demand punctuated cooling where for short periods h h m the percentage of time that a given vent field might be active can be speculatively estimated by dividing l by the length of axis the field cools examples of this percentage range from a low of 50 for some fast ridge fields

a vitrinite reflectance kinetic model incorporating overpressure retardation abstract the models currently used to predict maturation and hydrocarbon generation in petroleum basins use time and temperature as controlling parameters a small number of studies have indicated that overpressure can retard maturation although some of the published information from experimental investigations appears to be conflicting careful examination of the experimental conditions shows that both vitrinite reflectance and hydrocarbon generation are retarded by overpressures overpressure appears to result in the retention of volatiles within the molecular structure which prevents the molecular reorganisation necessary to produce higher reflectance values using a database containing vitrinite reflectance from both overpressured and normally pressured wells a kinetic model presro has been developed in which overpressure modifies the arrhenius a factor the model has been tested on two different basins south china sea and sable exhibiting a range of time temperature overpressure characteristics the results suggest that the maturity and timing of the onset of hydrocarbon generation differ significantly from those predicted by current models

hydrothermal seismicity beneath the summit of lucky strike volcano mid atlantic ridge abstract we present two years july 2007 july 2009 of earthquake locations from a local seismological network on the lucky strike volcano mid atlantic ridge whose summit hosts one of the most active and largest deep sea hydrothermal fields known two clusters of small m l

parameters of subsurface brines and hydrothermal processes 12 15 months after the 1999 magmatic event at the main endeavor field as inferred from in situ time series measurements of chloride and temperature 1 phase separation at mid ocean ridge hydrothermal systems is a widely distributed frequently occurring process it is highly sensitive to dynamic subsurface pressure and temperature conditions and has a significant impact on chloride concentrations of hydrothermal fluid although vapor phases produced by phase separation are observed in a variety of hydrothermal settings observation of conjugate brines that must be produced concurrently are more rare and the fate of subsurface brines is still not well understood here we use an array of in situ chloride sensors deployed at the main endeavor field on the juan de fuca ridge in the wake of a magmatic event to monitor the behavior of hydrothermal vent effluent with high temporal resolution our results provide evidence of near critical chloride depleted fluids within the crust up to 486 m beneath the seafloor we estimate that a brine reservoir stored beneath the main endeavor field is constrained to temperatures between 131 c and 326 c for chloride concentrations ranging from 6 to 25 wt nacl respectively our data suggest that subsurface fluids and circulation pathways vary widely over spatial scales on the order of meters under these circumstances simultaneous measurements of multiple fluid parameters at a high temporal resolution are essential to understanding subsurface hydrothermal processes associated with magmatic events

experimental study on the pressure dependence of vitrinite maturation we conducted the first systematic experiments over a pressure range of 0 5 to 20 0 kbar at temperatures of 200 to 350 c and run durations of 2 4 and 7 days to investigate the influence of pressure on vitrinite reflectance the data indicate that applied pressure lowers vitrinite reflectance the suppression appears to be stronger under wet than under dry conditions in addition high pressure favors liptinite preservation at temperatures 300 c whereas at the same temperature range yet at low pressure liptinite is decomposed our data fit an empirical rate equation which expresses rr as a function of pressure temperature and time the derived equation satisfactorily describes our experiments and enables extrapolations to geological timescales values of rr calculated for a pressure of 0 5 kbar are remarkably consistant with previous numerical pressure independent models a literature review shows that previous experimental data and field observations are inconsistent regarding the effect of pressure on vitrinite maturation this and some previous studies suggest that vitrinite maturation is suppressed by increased pressure whereas other workers regarded pressure as having little influence on the rate of coalification we suggest that original virinite composition experimental design and natural environment tectonic regime play important roles regarding the effects of pressure on vitrinite maturation interpretation of geothermal histories of sedimentary rocks based on vitrinite reflectance data must therefore include evaluation of the pressure otherwise vitrinite reflectance data from high pressure terranes should be interpreted with caution if used as a quantitative paleogeothermometer

seismic structure of the endeavour segment juan de fuca ridge correlations with seismicity and hydrothermal activity 1 multichannel seismic reflection data collected in july 2002 at the endeavour segment juan de fuca ridge show a midcrustal reflector underlying all of the known high temperature hydrothermal vent fields in this area on the basis of the character and geometry of this reflection its similarity to events at other spreading centers and its polarity we identify this as a reflection from one or more crustal magma bodies rather than from a hydrothermal cracking front interface the endeavour magma chamber reflector is found under the central topographically shallow section of the segment at two way traveltime twtt values of 0 9 1 4 s 2 1 3 3 km below the seafloor it extends approximately 24 km along axis and is shallowest beneath the center of the segment and deepens toward the segment ends on cross axis lines the axial magma chamber amc reflector is only 0 4 1 2 km wide and appears to dip 8 36 to the east while a magma chamber underlies all known endeavour high temperature hydrothermal vent fields amc depth is not a dominant factor in determining vent fluid properties the stacked and migrated seismic lines also show a strong layer 2a event at twtt values of 0 30 0 09 s 380 120 m below the seafloor on the along axis line and 0 38 0 09 s 500 110 m on the cross axis lines a weak moho reflection is observed in a few locations at twtt values of 1 9 2 4 s below the seafloor by projecting hypocenters of well located microseismicity in this region onto the seismic sections we find that most axial earthquakes are concentrated just above the magma chamber and distributed diffusely within this zone indicating thermal related cracking the presence of a partially molten crustal magma chamber argues against prior hypotheses that hydrothermal heat extraction at this intermediate spreading ridge is primarily driven by propagation of a cracking front down into a frozen magma chamber and indicates that magmatic heat plays a significant role in the hydrothermal system morphological and hydrothermal differences between the intermediate spreading endeavour and fast spreading ridges are attributable to the greater depth of the endeavour amc and the corresponding possibility of axial faulting

the focus thermal study around the spreading center of southwestern okinawa trough abstract voluminous hydrothermal systems are corresponding to the heat flow anomaly discovered from previous geophysical surveys in the southern okinawa trough sot which is an area with a high sedimentation rate supplied by the east china sea shelf and the island of taiwan our latest fifteen field measurements around the geolin mound glm 24 36 6 n 122 53 e are located at a sedimentary basin west of the yonaguni rift which is nearby the southwestern tip of a ne sw trending volcano trail in the sot the heat flow results illustrate a concentric decrease from 31 477 to 180 mw m2 in a 1 km radius around the glm evidently proving the focus fluid migration along the major fluid conduit the ultrahigh temperature observations revealed the existence of magmatic components in shallow sediments combining the subsurface profiles with the temperature fields around the glm a 0 16 km2 acoustic blanking area of the g1 fluid conduit was recognized from the chirp sonar image and numerous interpreted igneous reflections with fluids related features from seismic profiles as a result we proposed a conceptual model which simultaneously illustrates the hydrothermal circulation pattern from the previous general surveys and the enormous thermal anomaly from current detailed measurements the sill to sill feeding sedimentary basin of the glm is capable of demonstrating the enormous thermal anomaly around the spreading center of the sot and appropriately represents the potential hydrothermal discharge and recharge by heat flow distribution around the glm

hydrothermal sulfide accumulation along the endeavour segment juan de fuca ridge highlights high resolution auv bathymetry is used to identify hydrothermal sulfides on the seafloor 1 2 mt of massive sulfide is identified along 24 km of mid ocean ridge length massive sulfide accumulated at a rate of 400 t yr efficiency of sulfide deposition is 5 of total mobilized metals and reduced sulfur current global seafloor sulfide estimates likely underestimate amount present by about a factor of four abstract hydrothermal sulfide deposits that form on the seafloor are often located by the detection of hydrothermal plumes in the water column followed by exploration with deep towed cameras side scan sonar imaging and finally by visual surveys using remotely operated vehicle or occupied submersible hydrothermal plume detection however is ineffective for finding hydrothermally inactive sulfide deposits which may represent a significant amount of the total sulfide accumulation on the seafloor even in hydrothermally active settings here we present results from recent high resolution autonomous underwater vehicle based mapping of the hydrothermally active endeavour segment of the juan de fuca ridge in the northeast pacific ocean analysis of the ridge bathymetry resulted in the location of 581 individual sulfide deposits along 24 km of ridge length hydrothermal deposits were distinguished from volcanic and tectonic features based on the characteristics of their surface morphology such as shape and slope angles volume calculations for each deposit results in a total volume of 372 500 m3 of hydrothermal sulfide sulfate silica material for an equivalent mass of 1 2 mt of hydrothermal material on the seafloor within the ridge s axial valley assuming a density of 3 1 g cm3 much of this total volume is from previously undocumented inactive deposits outside the main active vent fields based on minimum ages of sulfide deposition the deposits accumulated at a maximum rate of 400 t yr with a depositional efficiency proportion of hydrothermal material that accumulates on the seafloor to the total amount hydrothermally mobilized and transported to the seafloor of 5 the calculated sulfide tonnage represents a four fold increase over previous sulfide estimates for the endeavour segment that were based largely on accumulations from within the active fields these results suggest that recent global seafloor sulfide resource estimates which were based mostly on the sizes and distribution of hydrothermally active deposits may be similarly underestimating the amount of sulfide along the global submarine neovolcanic zones

quantitative estimate of heat flow from a mid ocean ridge axial valley raven field juan de fuca ridge observations and inferences a systematic heat flow survey using thermal blankets within the endeavour segment of the juan de fuca ridge axial valley provides quantitative estimates of the magnitude and distribution of conductive heat flow at a mid ocean ridge with the goal of testing current models of hydrothermal circulation present within newly formed oceanic crust thermal blankets were deployed covering an area of 700 by 450 m in the raven hydrothermal vent field area located 400 m north of the main endeavour hydrothermal field a total of 176 successful blanket deployment sites measured heat flow values that ranged from 0 to 31 w m 2 approximately 53 of the sites recorded values lower than 100 mw m 2 suggesting large areas of seawater recharge and advective extraction of lithospheric heat high heat flow values were concentrated around relatively small hot spots integration of heat flow values over the raven survey area gives an estimate of conductive heat output of 0 3 mw an average of 0 95 w m 2 over the survey area fluid circulation cell dimensions and scaling equations allow calculation of a rayleigh number of approximately 700 in layer 2a the close proximity of high and low heat flow areas coupled with previous estimates of surficial seafloor permeability argues for the presence of small scale hydrothermal fluid circulation cells within the high porosity uppermost crustal layer of the axial seafloor

a turbulent convection model with an observational context for a deep sea hydrothermal plume in a time variable cross flow 1 a turbulent convection model for a hydrothermal fluid discharging into a tidally modulated stratified cross flow is used to investigate time variable conditions in plumes such as the one rising from dante a sulfide mound at 2175 m depth on the endeavour segment of the juan de fuca ridge that plume is the consequence of the coalescence of 10 or more small individual plumes from chimneys discharging hot salt diminished fluid into the near bottom ocean at dante the discharge encounters ambient horizontal currents with speeds oscillating from near zero to a maximum of 7 cm s 1 speeds which can bend a plume more than 45 from the vertical model results are compatible with field measurements of the plume footprint size and vertical velocity both 20 m above the source when earlier estimates for dante s heat flux of 50 mw drive the convection the small scale short period variability of velocities and properties distributions observed in the field is mimicked in model results plumes pool above a source during periods of weak cross flows but stream away from the source with more diluted concentrations and lower rise heights at other times plume distributions at identical cross flow speeds differ whether the flow is accelerating or decelerating small changes in background hydrographic profiles create differences in rise heights comparable to those caused by large changes in source buoyancy flux if put into an entrainment context results suggest an entrainment coefficient eff that varies from 0 11 to 0 025 with increasing height 2 76 m above the source

age dependent porosity of young upper oceanic crust insights from seafloor gravity studies of recent volcanic eruptions 1 an october 1997 alvin program collected a series of seafloor gravity stations over the february 1996 volcanic eruption site on the northern gorda ridge 46 41 n 126 47 w these measurements yielded estimates of the density and porosity of mid ocean ridge crust in the first few decades following formation the estimated crustal density for this pillow eruption of 2270 260 kg m3 is very low but in good agreement with the estimated densities of other recent seafloor volcanic flows rock matrix densities measured from hand samples retrieved from the study area give a bulk porosity for this flow of 34 16 this data when included with similar estimates of upper crustal porosity argue for a rapid evolution of upper crustal porosities from very high values 30 for recent eruptions to much lower values 10 15 where the seafloor is older than 0 5 million years these variations of density and porosity are of a magnitude consistent with previously observed changes in seismic compressional wave velocity and permeability of ocean crust

hydrothermal circulation within the endeavour segment juan de fuca ridge areas of the seafloor at mid ocean ridges where hydrothermal vents discharge are easily recognized by the dramatic biological physical and chemical processes that characterize such sites locations where seawater flows into the seafloor to recharge hydrothermal cells within the crustal reservoir are by contrast almost invisible but can be indirectly identified by a systematic grid of conductive heat flow measurements an array of conductive heat flow stations in the endeavour axial valley of the juan de fuca ridge has identified recharge zones that appear to represent a nested system of fluid circulation paths at the scale of an axial rift valley conductive heat flow data indicate a general cross valley fluid flow where seawater enters the shallow subsurface crustal reservoir at the eastern wall of the endeavour axial valley and undergoes a kilometer of horizontal transit beneath the valley floor finally exiting as warm hydrothermal fluid discharge on the western valley bounding wall recharge zones also have been identified as located within an annular ring of very cold seafloor around the large main endeavour hydrothermal field with seawater inflow occurring within faults that surround the fluid discharge sites these conductive heat flow data are consistent with previous models where high temperature fluid circulation cells beneath large hydrothermal vent fields may be composed of narrow vertical cylinders subsurface fluid circulation on the endeavour segment occurs at various crustal depths in three distinct modes 1 general east to west flow across the entire valley floor 2 in narrow cylinders that penetrate deeply to high temperature heat sources and 3 supplying low temperature diffuse vents where seawater is entrained into the shallow uppermost crust by the adjacent high temperature cylindrical systems the systematic array of conductive heat flow measurements over the axial valley floor averaged 150 mw m2 suggesting that only about 3 of the total energy flux of ocean crustal formation is removed by conductive heat transfer with the remainder being dissipated to overlying seawater by fluid advection

near axis heat flow measurements on the northern juan de fuca ridge implications for fluid circulation in oceanic crust present models of the cooling of oceanic crust suggest that convection of hydrothermal fluid is a major component of the process in axial regions abundant faults and open fissures are associated with the venting of high temperature hydrothermal fluid in older crust where the insulating sediment cover is thick previous studies have shown that basement topography is the dominant forcing factor for within crust fluid circulation in the intermediate region where young crust is lightly sedimented heat flow data are difficult to obtain with traditional techniques to determine whether topography or permeability is the dominant process controlling fluid circulation in the near axis region we conducted a profile of heat flow measurements using the submersible alvin on the endeavour segment of the juan de fuca ridge our data indicate that topographic forcing is responsible for the long wavelength variations with high heat flow at the ridge summits and low values in the inter ridge valleys the locations of the extreme values of heat flow taken within the context of subsurface faulting are consistent with a model where a ridge valley topographic pair comprises a single circulation cell this model predicts that the source area for the high temperature axial vents may be in the flanking inter ridge valleys

high resolution near bottom vector magnetic anomalies over raven hydrothermal field endeavour segment juan de fuca ridge high resolution near bottom vector magnetic data were collected by remotely operated vehicle jason over the raven hydrothermal vent field 47 57 3 n 129 5 75 w located north of main endeavour vent field on the endeavour segment of the juan de fuca ridge the survey was part of a comprehensive heat flow study of the raven site using innovative thermal blanket technology to map the heat flux and crustal fluid pathways around a solitary hydrothermal vent field raven hydrothermal activity is presently located along the western axial valley wall while additional inactive hydrothermal deposits are found to the nw on the upper rift valley wall magnetic inversion results show discrete areas of reduced magnetization associated with both active and inactive hydrothermal vent deposits that also show high conductive heat flow higher spatial variability in the heat flow patterns compared to the magnetization is consistent with the heat flow reflecting the currently active but ephemeral thermal environment of fluid flow while crustal magnetization is representative of the static time averaged effect of hydrothermal alteration a general nw to se trend in reduced magnetization across the raven area correlates closely with the distribution of hydrothermal deposits and heat flux patterns and suggests that the fluid circulation system at depth is likely controlled by local crustal structure and magma chamber geometry magnetic gradient tensor components computed from vector magnetic data improve the resolution of the magnetic anomaly source and indicate that the hydrothermally altered zone directly beneath the raven site is approximately 15 106 m3 in volume

mobility of au and related elements during the hydrothermal alteration of the oceanic crust implications for the sources of metals in vms deposits volcanogenic massive sulphide vms deposits are commonly enriched in cu zn and pb and can also be variably enriched in au as sb se and te the behaviour of these elements during hydrothermal alteration of the oceanic crust is not well known ocean drilling program odp hole 1256d penetrates a complete in situ section of the upper oceanic crust providing a unique sample suite to investigate the behaviour of metals during hydrothermal alteration a representative suite of samples was analysed for au as sb se and te using low detection limit methods and a mass balance of metal mobility has been carried out through comparison with a fresh mid oceanic ridge basalt morb glass database the mass balance shows that au as se sb s cu zn and pb are depleted in the sheeted dyke and plutonic complexes by 46 12 27 5 2 5 0 5 27 6 8 4 0 7 9 6 1 6 7 9 0 5 and 44 6 respectively arsenic and sb are enriched in the volcanic section due to seawater derived fluid circulation calculations suggest that large quantities of metal are mobilised from the oceanic crust but only a small proportion is eventually trapped as vms mineralisation the quantity of au mobilised and the ratio of au to base metals are similar to those of mafic vms and ten times enrichment of au would be needed to form a au rich vms the cu rich affinity of mafic vms deposits could be explained by base metal fractionation both in the upper sheeted dykes and during vms deposit formation

mathematical modeling of diffuse flow in seafloor hydrothermal systems the potential extent of the subsurface biosphere at mid ocean ridges abstract we describe a variety of one and two dimensional mathematical modeling approaches to characterizing diffuse flow circulation at mid ocean ridge hydrothermal systems the goal is to estimate the potential extent of the sub seafloor microbial biosphere based on subsurface contours of the 120 c isotherm as determined from the various models the models suggest that the sub seafloor depth for microbial life may range from less than 1 m in some places to the thickness of crustal layer 2a of 500 m in others this depth depends primarily on how diffuse flow is driven the 120 c isotherm tends to be much deeper if diffuse flow is induced as boundary layer flow near high temperature plumes than if it results from conductive cooling or mixing near the seafloor because the heat flow alone may not allow identification of the flow regime in the subsurface we highlight the use of chemical tracers as an additional constraint that sheds light into the flow and reaction patterns associated with vents we use thermodynamic modeling which connects the temperature of the diffuse fluid to its chemical composition as the temperature composition relationships differ for mixing versus conductive heating and cooling the fluid geochemistry can shed light on subsurface transport using methane as an example the geochemical models indicate subsurface microbial methane production and consumption in different regions of the vent field near epr 9 50 n

dissolved carbon species in associated diffuse and focused flow hydrothermal vents at the main endeavour field juan de fuca ridge phase equilibria and kinetic constraints 1 despite significant advances in the understanding of hydrothermal processes at mid ocean ridges the linkage between focused flow and diffusively flowing vent fluids has remained elusive here we report the distribution of dissolved carbon species in fluids issuing from associated low moderate and high temperature vent systems collected in 2005 at the main endeavour field mef juan de fuca ridge excess co aq measured in the moderate temperature diffuse flow fluids indicates redox disequilibria between co aq and co2 aq suggesting hydrothermal circulation of short residence time in near seafloor mixing zones according to geochemical modeling the chemical variability of mef fluids is consistent with a combination of conductive cooling reaction transport and subsequent seawater mixing of a single source fluid the similar distribution of c1 c3 alkanes observed in diffuse and focused flow fluids likely indicates a minimal fingerprint of biological and methanogenic metabolism on organic fluxes consistent with a possible short lived hydrothermal near seafloor circulation accordingly dissolved carbon species in low temperature vent fluids can serve as geochemical proxies to distinguish the extent of compositional change in subsurface microbial habitats in the mef hydrothermal fluids and elsewhere part of this reflects conditions unique to the near surface diffuse flow environment but the flux of components from deeper seated hydrothermal processes is important as well in comparison with earlier measurements 1999 and 2000 of high temperature vent fluids at mef the 2005 data set exhibits lower li cl ratios and h2 aq concentrations indicative of higher fluid rock mass ratios and more oxidizing and or lower temperature conditions respectively

continuous near bottom gravity measurements made with a bgm 3 gravimeter in dsv alvin on the east pacific rise crest near 9 31 n and 9 50 n a bell bgm 3 gravimeter has been used to collect continuous underway near bottom 3 to 10 m altitude gravity measurements from the deep diving submersible dsv alvin during surveys on the east pacific rise epr crest near 9 31 n and 9 50 n closely spaced 20 to 30 m gravity measurements were made along transects up to 8 km long in both regions repeatability of measurements made at the same location on different dives is 0 3 mgal along track spatial resolution of anomalies is 130 160 m with the limiting factors being precision and sampling rate of the pressure gauge depth data used to calculate vertical accelerations of the submersible the average upper crustal density of the ridge crest determined from the relationship between depth and free water gravity anomalies varies greatly between 9 31 n and 9 50 n average upper crustal densities of 2410 kg m 3 for the 9 50 n area and 2690 kg m 3 for the 9 31 n area were calculated the different densities are not due to differing geometry of the layer 2a 2b boundary or a regional cross axis gravity gradient differences in porosity of the shallow crustal rocks or a difference in the proportion of low density extrusives to higher density dikes and sills within layer 2a in these two areas are the likely causes of the different upper crustal densities bouguer gravity anomalies near the epr axis are primarily small amplitude 0 5 2 mgal are a few hundred meters across and appear to be lineated parallel to the axis larger amplitude bouguer anomalies of up to 4 mgal were found at a few locations across the crestal plateau and are associated with pillow ridges composed of lavas which are clearly younger than the surrounding seafloor these ridges have distinct chemical compositions compared to lavas from the axial summit collapse trough asct at the same latitude probable sources of the 0 5 to 2 mgal anomalies observed on the summit plateau include areas of collapsed and fissured terrain and dike swarms feeding melt through layer 2a to the surface a grid survey of the ridge axis near 9 50 n shows bouguer anomalies lineated along the axis suggesting that dike swarms do contribute to the observed bouguer anomalies the along axis continuity of the gravity anomalies is disrupted at a 75 m offset of the asct suggesting that shallow feeders of lava to the surface may be segmented on a finer scale than the deeper crustal magmatic system this initial study confirms the ability to conduct high resolution near bottom continuous gravity measurements from alvin it also provides important information on how the shallow crustal structure of a fast spreading mid ocean ridge develops and how it varies with the surface morphology

direct measurements of oceanic crustal density at the northern juan de fuca ridge density and porosity of oceanic crust are critically important to understanding the circulation of hydrothermal fluid at mid ocean ridges to determine these parameters directly we completed a fixed on bottom gravity survey on the endeavour segment of the juan de fuca ridge which is a well studied spreading center that is both hydrothermally and tectonically active a bell aerospace gravity meter mounted within the submersible alvin was used to make gravity measurements from the floor of the axial valley to the summit of the west axial ridge analysis of these data gave a density of 2240 50 kg m 3 for the upper 130 m of crust comparison of the outcrop density with that of recovered rock samples resulted in a calculated porosity of 38 4 forward modeling of the seafloor gravity data indicated that both the valley floor and bounding walls have unusually low density crust that is flanked by higher density crust

can high temperature high heat flux hydrothermal vent fields be explained by thermal convection in the lower crust along fast spreading mid ocean ridges we present numerical models to explore possible couplings along the axis of fast spreading ridges between hydrothermal convection in the upper crust and magmatic flow in the lower crust in an end member category of models corresponding to effective viscosities m lower than 1013 pa s in a melt rich lower crustal along axis corridor and permeability k not exceeding 10 16 m2 in the upper crust the hot melt rich gabbroic lower crust convects as a viscous fluid with convection rolls parallel to the ridge axis in these models we show that the magmatic hydrothermal interface settles at realistic depths for fast ridges i e 1 2 km below seafloor convection cells in both horizons are strongly coupled and kilometer wide hydrothermal upflows plumes spaced by 8 10 km arise on top of the magmatic upflows such magmatic hydrothermal convective couplings may explain the distribution of vent fields along the east epr and south east pacific rise sepr the lower crustal plumes deliver melt locally at the top of the magmatic horizon possibly explaining the observed distribution of melt rich regions pockets in the axial melt lenses of epr and sepr crystallization of this melt provides the necessary latent heat to sustain permanent 100 mw vents fields our models also contribute to current discussions on how the lower crust forms at fast ridges they provide a possible mechanism for focused transport of melt rich crystal mushes from moho level to the axial melt lens where they further crystallize feed eruptions and are transported both along and off axis to produce the lower crust

fluid flow pathways in actively deforming sediments the role of pore fluid pressures and volume change abstract the manner in which sediments respond to burial and shear depends largely on the ease with which fluids can escape rapid increase of overburden pressure or shear prevents efficient dewatering and can lead to the generation of excess pore fluid pressures a principal control on the development of overpressure and hence mechanical strength is the sediments permeability observations from modern plate margin accretionary prisms indicate that many faults are associated with both overpressuring and efficient fluid flow that operates in pulses indicative of permeability that is constantly changing we present the results of laboratory experiments exploring how the permeability of sediment retrieved from the barbados accretionary prism and laboratory analogues varies with progressive strain volume changes are linked to permeability fluctuations and the drainage capacity of the material we investigate the importance of the previous stress history imparted on the sediment and quantify the extent to which different deformation structures can acts as conduits for fluid flow under conditions of fluctuating effective stress material that has been initially consolidated and then subjected to reduction of effective stress either by raised fluid pressure or reduction of load inducing overconsolidation dilates during shear zone formation and displays an increase in permeability during strain shear fabrics can localise flow further where the principal mode of deformation is intensely localised and brittle and effective stress approaches zero conversely normally consolidated sediments deform by bulk volume loss without generating discrete fabrics and hence do not enhance flow under reduced effective stress conditions permeability can increase during strain by up to one order of magnitude when deformation results in sudden porosity collapse and pore water expulsion microstructural analysis and permeability testing before during and after shear have helped constrain the principal factors that control the evolution of flow conduits in fine grained low permeability sediments

organic geochemistry of core samples from an ultradeep hot well 300 c 7 km south texas cores of lower cretaceous rocks from a depth of 6400 8 to 7544 6 m at present day temperatures of 262 296 c have high concentrations of c15 hydrocarbons bitumen coefficients range from 105 to 367 mg g and c15 extractable bitumen ranges from 500 to 2200 ppm some generation potential remains associated with the kerogen of these rocks in addition to exhibiting the above organic geochemical properties characteristic of the zone of intense hydrocarbon generation these rocks also have organic geochemical properties attributed to the zone of hydrocarbon extinction or greenschist metamorphism these characteristics are high vitrinite reflectance r0 values 4 4 4 8 low h c ratios 0 30 0 58 high saturate aromatic hydrocarbon ratios 7 05 20 6 high hydrocarbon nso ratios 2 65 4 66 and high transformation index ratios s1 s1 s2 0 61 0 87 the data from this and other wells we have studied show that high concentrations of c15 hydrocarbons are thermally stable to high temperatures at least 300 c in abnormally pressured semi closed systems over geologic time concepts prevelant among petroleum organic geochemists concerning the thermal fate of hydrocarbons with subsequent graphite formation and greenschist metamorphism are in sharp contradiction to these data conventional concepts of the distribution of heavy hydrocarbons with increasing temperature and depth apparently require further review and revision

sulfide geochronology along the endeavour segment of the juan de fuca ridge 1 forty nine hydrothermal sulfide sulfate rock samples from the endeavour segment of the juan de fuca ridge northeastern pacific ocean were dated by measuring the decay of 226ra half life of 1600 years in hydrothermal barite to provide a history of hydrothermal venting at the site over the past 6000 years this dating method is effective for samples ranging in age from 200 to 20 000 years old and effectively bridges an age gap between shorter and longer lived u series dating techniques for hydrothermal deposits results show that hydrothermal venting at the active high rise sasquatch and main endeavour fields began at least 850 1450 and 2300 years ago respectively barite ages of other inactive deposits on the axial valley floor are between 1200 and 2200 years old indicating past widespread hydrothermal venting outside of the currently active vent fields samples from the half graben on the eastern slope of the axial valley range in age from 1700 to 2925 years and a single sample from outside the axial valley near the westernmost valley fault scarp is 5850 205 years old the spatial relationship between hydrothermal venting and normal faulting suggests a temporal relationship with progressive younging of sulfide deposits from the edges of the axial valley toward the center of the rift these relationships are consistent with the inward migration of normal faulting toward the center of the valley over time and a minimum age of onset of hydrothermal activity in this region of 5850 years

overpressure retardation of organic matter maturation a kinetic model and its application abstract a kinetic model t p ro has been developed to address the issue that overpressure increases the activation energies during organic matter maturation and that the retardation of organic matter maturation by overpressure is due to the fact that organic matter is hindered from progressive molecular ordering under overpressured conditions the mean effective stress i e mean stress minus pore pressure is much lower and the pore pressure is usually higher than under hydrostatic pressure conditions as a result the progressive molecular ordering of the vitrinite such as the aromatization is retarded and the vitrinite reflectance can hardly be increased the reflectance values predicted by t p ro model as it has been applied to the well ld3011 at yinggehai basin in south china sea are similar to the measured ones the kinetic model predicts that overpressure can cause the oil window floor and the peak oil generation to shift to greater burial depth in the case of yinggehai basin the local geological conditions are prone to generate light hydrocarbons and natural gas instead of oil hence they constitute the predominant products in this high temperature heating rate basin

conductive heat loss in recent eruptions at mid ocean ridges a new technique for measuring conductive heat flow from unsedimented volcanic rocks on the sea floor has been tested on two new eruption sites in the ne pacific this technique consists of isolating the surficial rocks from sea water using water saturated urethane foam as an insulating thermal blanket the thermal gradient transferred from the outcrop to the thermal blanket is a quantitative measurement of the conductive heat flow that takes place in unsedimented volcanic areas we deployed two thermal blankets at 13 sites on the 1993 and 1996 juan de fuca gorda ridge flows and found 1 a factor of 10 decrease in heat flow over a period of 12 months on the 1993 coaxial flow 2 a value of 6950 mw m 2 on the 8 month old gorda flow and 3 measurements of heat flow versus age since eruption indicate that newly extruded volcanic units are quite permeable to fluid circulation and cool rapidly by convection in only a few years these new heat flux data confirm that the extrusive volcanic layer is not the primary heat source for long lived high temperature hydrothermal systems which must instead rely on a more isolated thermal reservoir within the lower crustal rocks

the influence of pressure on petroleum generation and maturation as suggested by aqueous pyrolysis because fluid pressures are transient in sedimentary basins over geologic time the effect of increasing fluid pressure on organic matter metamorphism is difficult to determine and conflicting opinions exist concerning its influence properly performed aqueous pyrolysis experiments can closely simulate hydrocarbon generation and maturation in nature and thus offer an excellent way to study the influence of pressure such experiments carried out on the retort phosphatic shale member of the lower permian phosphoria formation type ii s organic matter at different constant temperatures demonstrated that increasing pressure significantly retards all aspects of organic matter metamorphism including hydrocarbon generation maturation and thermal destruction this conclusion results from detailed quantitative and qualitative analyses of all products from hydrocarbon generation from the c1 to c4 hydrocarbon gases to the asphaltenes and also from analyses of the reacted rocks we have documented that our aqueous pyrolysis experiments closely simulated natural hydrocarbon generation and maturation thus the data taken as a function of pressure have relevance to the influence of normal and abnormal fluid pressures as related to 1 depths and temperatures of mainstage hydrocarbon generation 2 the thermal destruction of deposits of gas or light oil or their preservation to unexpectedly high maturation ranks and 3 the persistence of measurable to moderate concentrations of c15 hydrocarbons in fine grained rocks even to ultra high maturation ranks

hydrothermal recharge and discharge across 50 km guided by seamounts on a young ridge flank hydrothermal circulation within the sea floor through lithosphere older than one million years myr is responsible for 30 of the energy released from plate cooling and for 70 of the global heat flow anomaly the difference between observed thermal output and that predicted by conductive cooling models 1 2 hydrothermal fluids remove significant amounts of heat from the oceanic lithosphere for plates typically up to about 65 myr old3 4 but in view of the relatively impermeable sediments that cover most ridge flanks5 it has been difficult to explain how these fluids transport heat from the crust to the ocean here we present results of swath mapping heat flow geochemistry and seismic surveys from the young eastern flank of the juan de fuca ridge which show that isolated basement outcrops penetrating through thick sediments guide hydrothermal discharge and recharge between sites separated by more than 50 km our analyses reveal distinct thermal patterns at the sea floor adjacent to recharging and discharging outcrops we find that such a circulation through basement outcrops can be sustained in a setting of pressure differences and crustal properties as reported in independent observations and modelling studies

surficial permeability of the axial valley seafloor endeavour segment juan de fuca ridge 1 hydrothermal systems at mid ocean spreading centers play a fundamental role in earth s geothermal budget one underexamined facet of marine hydrothermal systems is the role that permeability of the uppermost seafloor veneer plays in the distribution of hydrothermal fluid as both the initial and final vertical gateway for subsurface fluid circulation uppermost seafloor permeability may influence the local spatial distribution of hydrothermal flow a method of deriving a photomosaic from seafloor video was developed and utilized to estimate relative surface permeability in an active hydrothermal area on the endeavour segment of the juan de fuca ridge the mosaic resolves seafloor geology of the axial valley seafloor at submeter resolution over an area greater than 1 km2 results indicate that the valley walls and basal talus slope are topographically rugged and unsedimented providing minimal resistance to fluid transmission elsewhere the axial valley floor is capped by an unbroken blanket of low permeability sediment resisting fluid exchange with the subsurface reservoir active fluid emission sites were restricted to the high permeability zone at the base of the western wall a series of inactive fossil hydrothermal structures form a linear trend along the western bounding wall oriented orthogonal to the spreading axis high temperature vent locations appear to have migrated over 100 m along ridge strike over the decade between surveys while initially an expression of subsurface faulting this spatial pattern suggests that increases in seafloor permeability from sedimentation may be at least a secondary contributing factor in regulating fluid flow across the seafloor interface

upper crustal densities derived from sea floor gravity measurements northern juan de fuca ridge a transect of sea floor gravity stations has been analyzed to determine upper crustal densities on the endeavour segment of the northern juan de fuca ridge data were obtained using alvin along a corridor perpendicular to the axis of spreading over crustal ages from 0 to 800 000 years calculated elevation factors from the gravity data show an abrupt increase in density with age distance for the upper 200 m of crust this density change is interpreted as a systematic reduction in bulk porosity of the upper crustal section from 23 for the axial ridge to 10 for the off axis flanking ridges the porosity decrease is attributed to the collapse and filling of large scale voids as the abyssal hills move out of the crustal formation zone forward modeling of a plausible density structure for the near axis region agrees with the observed anomaly data only if the model includes narrow along strike low density regions adjacent to both inner and outer flanks of the abyssal hills the required low density zones could be regions of systematic upper crustal fracturing and faulting that were mapped by submersible observers and side scan sonar images and whose presence was suggested by the distribution of heat flow data in the same area

tectonic influence on vitrinite reflectance abstract vitrinite reflectance is a popular indicator of coal rank and index to thermal maturation it has been used by numerous workers to model numerically the thermal development of sedimentary basins buntebarth and stegena 1986 vitrinite reflectance is generally acknowledged by most workers to increase with temperature and time the thermal variation of vitrinite reflectance is reasonably well explained as a response to a first order chemical reaction described by the arrhenius equation a hypothesis has not been advanced to describe its variation with time most workers assume that pressure has little influence on vitrinite reflectance despite observations of hower and davis 1981 that 1 pressure increases the anisotropy of vitrinite reflectance and 2 pressure suppresses maximum vitrinite reflectance at low pressures less than about 200 to 300 mpa hower and davis 1981 fig 18 the aim of this paper is to propose a hypothesis of vitrinite reflectance variation that is dependent on temperature pressure and time and is fully explained by known physical processes vitrinite reflectance can be considered as a measure of the number of photons that have been vertically reflected from locations in the vitrinite molecular structure the number of these vertical reflection locations vrls increase with temperature and the work of hower and davis 1981 suggests that the number of vrls decrease with pressure under normal burial conditions is slight and is masked by the strong dependence of vitrinite reflectance on burial temperature the suppression of vrls with increasing pressure at low pressure is possibly due to both intracrystalline molecular plastic slip and rotation of rigid inequant grains molecules as discussed by levine and davis 1984 further i propose that these suppressed vrls slowly reappear according to either a plastic creep or diffusion law i shall assume a diffusion law for the present model for pressures greater than about 300 mpa vitrinite reflectance increases with increasing pressure and this behavior is probably controlled by nucleation and growth of favorably oriented grains molecules as suggested by levine and davis 1984 therefore a numerical model of vitrinite reflectance behavior with temperature pressure and time can be proposed on the above principles empirical observations lead to the conclusion that the vitrinite reflectance 1 increases with temperature 2 decreases with pressure low pressure 3 increases with high pressure and 4 increases with time i assume the vitrinite reflectance r is described by the relationship r temperature factor nucleation factor diffusion factor the temperature factor is described by the arrhenius equation the nucleation factor is assumed to be a constant at low pressures and the diffusion factor is the suppression effect plus its slow recovery with time the quantitative statement of this relation is r exp a t a b sin 3 14 p s exp t c where t is temperature in degrees celsius p is pressure in mpa t is time a a b and c are constants and s is the pressure where the diffusion factor becomes negligible about 300 mpa the above equation is valid for pressure in the range to s the equation has been calibrated with the empirical observations of hower and davis 1981 fig 18 and various other workers buntebarth and stegena 1986 and the values of the constants are assumed to a 232 ma a 7 0 b 0 6a c 200 ma and s 300 mpa this model provides a logical and physically meaningful dependence of vitrinite reflectance on time table 1 shows vitrinite reflectance versus time as calculated using models proposed by hood et al and middleton and falvey see buntebarth and stegena 1986 wood 1988 and the present hypothesis calculations were made assuming a rapid burial to a temperature of 110 degrees celsius and a pressure of 75 mpa about 3 km the values of vitrinite reflectance determined by the present hypothesis compare favorably with those determined by other methods in conclusion the present hypothesis for the influence of tectonic processes temperature and pressure on vitrinite reflectance includes the empirical observation of reflectance suppression at low pressures and assumes the process is governed by a diffusion law the diffusion law provides a physical basis for the observed time dependence of vitrinite reflectance the present model is only a preliminary and simplistic formulation and further work is required to elucidate details such as the temperature dependence of the diffusion term which appears in the emperical observations of hower and davis 1981 fig 18

sedimentary processes and crustal cycling on venus sediment exists on the venus surface it is observed in venera images between outcrops and boulders of sedimentary rocks sediment is produced by pyroclastic volcanism and chemical weathering chemical weathering is driven by an enhanced activity of water and an elevated surface temperature sediment is transported by wind action and lithified by cementation and diagenesis cementation may be by carbonate or silica cement diagenesis may be by products of chemical weathering acting as cement or by compaction and recrystallization of sediment into a texture with interlocking grains sediment may be transported from the top of sialic continents such as ishtar to the modal plains where it is deposited lithified and integrated into the local crust as new layers are added the bottom of the crust melts and is in part returned to the mantle a steady state chemical exchange might exist by this mechanism of crustal cycling that links atmosphere continents modal plains and mantle

a novel heat flux study of a geothermally active lake lake rotomahana new zealand abstract a new technique for measuring conductive heat flux in a lake was adapted from the marine environment to allow for multiple measurements to be made in areas where bottom sediment cover is sparse or even absent this thermal blanket technique pioneered in the deep ocean for use in volcanic mid ocean rift environments was recently used in the geothermally active lake rotomahana new zealand heat flow from the lake floor propagates into the 0 5 m diameter blanket and establishes a thermal gradient across the known blanket thickness and thereby provides an estimate of the conductive heat flux of the underlying terrain this approach allows conductive heat flux to be measured over a spatially dense set of stations in a relatively short period of time we used 10 blankets and deployed them for 1 day each to complete 110 stations over an 11 day program in the 6 3 km lake results show that lake rotomahana has a total conductive heat flux of about 47 mw averaging 6 w m2 over the geothermally active lake the western half of the lake has two main areas of high heat flux 1 a high heat flux area averaging 21 3 w m2 along the western shoreline which is likely the location of the pre existing geothermal system that fed the famous pink terraces mostly destroyed during the 1886 eruption 2 a region southwest of patiti island with a heat flux averaging 13 1 w m2 that appears to be related to the explosive rift that formed the lake in the 1886 tarawera eruption a small rise in bottom water temperature over the survey period of 0 01 c day suggests the total thermal output of the lake is 112 132 mw and when compared to the conductive heat output suggests that 18 42 of the total thermal energy is by conductive heat transfer