low temperature silica rich gold mineralization in mafic vms systems evidence from the troodos ophiolite cyprus the troodos ophiolite cyprus hosts the on land analogue for actively forming mafic cu rich or cyprus type vms deposits in addition to high temperature 350 c sulfide mound related mineralization other fossil seafloor mineralizing systems are known to operate including those characterized by an enrichment in au and abundant silicification in this study the mineralogy and geochemistry of four au and silica rich localities in troodos are considered and these include kokkinovounaros mathiatis south touronjia and alpen rose we present whole rock geochemical and mineralogical data characterizing the distribution of au in the hydrothermal systems of troodos samples from silica rich localities have two distinct sample mineralogies supergene samples that contain predominantly goethite hematite and jarosite and hypogene samples that contain quartz amorphous silica and minor hematite hypogene samples from mathiatis south and kokkinovounaros are enriched in au with a median concentration of 1 5 ppm relative to supergene samples that contain 0 1 ppm n 107 this indicates that au enrichment occurred on the seafloor and is not solely related to supergene weathering of silica rich mineralization we suggest that silica rich au mineralization in troodos formed during the migration of newly formed crust off axis or as white smokers proximal to known vms deposits in silica rich mineralization that is located distally from known massive sulfide deposits au was probably remobilized from shallow crustal reservoirs during the low temperature fluid flow 100 300 c in the lower extrusive sequence as the crust cooled and migrated off axis based on modern seafloor analogues we propose a revised model for the troodos hydrothermal system that explains the distribution of au in silica rich mineralization that formed off axis and postdates vms formation and in zones proximal to vms deposits that formed simultaneously as white smoker type vents we suggest that silica rich mineralized zones formed an intermediary between high temperature 350 c on axis vms deposits and the low temperature silicification of umbers 100 c

three dimensional p and s velocity structure in the coalinga region california the coalinga earthquake sequence of 1983 provided a unique opportunity to perform a three dimensional velocity and hypocenter inversion in an area of complex three dimensional structure dominated by folding and blind thrusts additionally since other varied geological and geophysical studies have been completed in this area the three dimensional inversion solution could be compared to other interpretations inversion of 7696 p and 1511 s first arrivals from earthquakes and 696 p first arrivals from refraction shots produced a three dimensional velocity model with grid spacing of 1 2 km in the hypocentral area the overall shape and location of velocity features correspond well to the mapped surface geology the three dimensional inversion yields details of folds where the resolution is good and the general shape where resolution is lower the amounts of structural relief inferred for the local folds are similar to values inferred from geologic data the three dimensional velocity solution has several distinctive features there is a linear high velocity body 6 1 6 5 km s about 25 km long from 6 to 8 km depth that may represent a fragment of coast range ophiolite a shallow low velocity zone lvz which extends for 20 km along the fold axis at about 6 km depth and correlates with lvzs observed in both the refraction and the reflection data may indicate high pore pressure caused by lateral compressive strain deeper lvzs occur within inferred franciscan material and are characterized by horizontal or southwest dipping zones of varied thickness 4 8 km wide and 5 10 km long these lvzs may represent multiple thrust faults their locations and geometry are consistent with thrust faults inferred with seismic reflection data the three dimensional velocity solution compares well to prior two dimensional seismic reflection and refraction models and observed gravity the shape of the inferred sedimentary section agrees well with the reflections from the cenozoic strata compared to the refraction model the three dimensional solution has similar velocities and similar locations of velocity features but is more detailed in the hypocentral zone where it uses more data the gravity computed from the three dimensional velocities is similar to the observed gravity in both shape and amplitude both a simple one dimensional initial model and a complex initial model derived from the refraction interpretation were tried a simple starting model gave the best results the s velocity solution has different resolution than the p velocity solution because it uses a different set of stations and it has lower resolution because it uses fewer arrival times while the general patterns of velocity variation are similar for both vp and vs the vs solution tends to have more smearing of velocity features and can have somewhat different locations of velocity features

identification of sulfur sources and isotopic equilibria in submarine hot springs using multiple sulfur isotopes abstract multiple sulfur isotopes were measured in metal sulfide deposits elemental sulfur and aqueous hydrogen sulfide to constrain sulfur sources and the isotopic systematics of precipitation in seafloor hydrothermal vents areas studied include the eastern manus basin and lau basin back arc spreading centers and the unsedimented basalt hosted southern east pacific rise sepr and sediment hosted guaymas basin mid ocean ridge spreading centers chalcopyrite and dissolved hydrogen sulfide h 2 s 34 s values range from 5 5 to 5 6 in manus basin samples 2 4 to 6 1 in lau basin samples and 3 7 to 5 7 in sepr samples values of 34 s for cubic cubanite and h 2 s range from 1 4 to 4 7 in guaymas basin samples multiple sulfur isotope systematics in fluid mineral pairs from the sepr and lau basin show that crustal host rock and thermochemical reduction of seawater derived dissolved sulfate so 4 are the primary sources of sulfur in mid ocean ridge and some back arc systems at pacmanus and susu knolls hydrothermal systems in the eastern manus basin a significant contribution of sulfur is derived from disproportionation of magmatic sulfur dioxide so 2 while the remaining sulfur is derived from crustal host rocks and so 4 reduction at the sedimented guaymas basin hydrothermal system sulfur sources include crustal host rock reduced seawater so 4 and biogenic sulfide vent fluid flow through fresher less mature sediment supplies an increased quantity of reactant organic compounds that may reduce 34 s enriched so 4 while fluid interaction with more highly altered sediments results in h 2 s characterized by a small but isotopically significant input of 34 s depleted biogenic sulfides near zero 33 s values in all samples implicate the abiotic processes of so 4 reduction and leaching of host rock as the major contributors to sulfur content at a high temperature unsedimented mid ocean ridge and at a back arc system 33 s values indicate that so 2 disproportionation is an additional process that contributes sulfur to a different back arc system and to acid spring type hydrothermal fluid circulation at the sedimented guaymus basin near zero 33 s values are also observed despite negative 34 s values that indicate inputs of biogenic pyrite for some samples in contrast with previous studies reporting isotope disequilibrium between h 2 s and chalcopyrite the 34 s values of chalcopyrite sampled from the inner 1 2 mm of a chimney wall are within 1 of 34 s values for h 2 s in the paired vent fluid suggesting equilibrium fluid mineral sulfur isotope exchange at 300 400 c isotopic equilibrium between hydrothermal fluid h 2 s and precipitating chalcopyrite implies that sulfur isotopes in the chalcopyrite lining across a chimney wall may accurately record past hydrothermal activity

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

precious and base metal geochemistry and mineralogy of the grasvally norite pyroxenite anorthosite gnpa member northern bushveld complex south africa implications for a multistage emplacement the grasvally norite pyroxenite anorthosite gnpa member within the northern limb of the bushveld complex is a mineralized layered package of mafic cumulates developed to the south of the town of mokopane at a similar stratigraphic position to the platreef the concentration of platinum group elements pge in base metal sulfides bms has been determined by laser ablation inductively coupled plasma mass spectrometry these data coupled with whole rock pge concentrations and a detailed account of the platinum group mineralogy pgm provide an insight into the distribution of pge and chalcophile elements within the gnpa member during both primary magmatic and secondary hydrothermal alteration processes within the most unaltered sulfides containing pyrrhotite pentlandite and chalcopyrite only the majority of ipge rh and some pd occur in solid solution within pyrrhotite and pentlandite with an associated pt as and pd bi te dominated pgm assemblage these observations in conjunction with the presence of good correlations between all bulk pge and base metals throughout the gnpa member indicate the presence and subsequent fractionation of a single pge rich sulfide liquid which has not been significantly altered in places the primary sulfides have been replaced to varying degrees by a low temperature assemblage of pyrite millerite and chalcopyrite these sulfides are associated with a pgm assemblage characterized by the presence of pd antimonides and pd arsenides which are indicative of hydrothermal assemblages the presence of appreciable quantities of ipge pd and rh within pyrite and to a lesser extent millerite suggests these phases directly inherited pge contents from the pyrrhotite and pentlandite that they replaced the replacement of both the sulfides and pgm occurred in situ thus preserving the originally strong spatial association between pgm and bms but altering the mineralogy precious metal geochemistry indicates that fluid redistribution of pge is minimal with only pd au and cu being partially remobilized and decoupled from bms this is also indicated by the lower concentrations of pd evident in both pyrite and millerite compared with the pentlandite being replaced the observations that the gnpa member was mineralized prior to intrusion of the main zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal s we therefore believe that our observations are more compatible with a multistage emplacement model where preformed pge rich sulfides were emplaced into the gnpa member such a model explains the development and distribution of a single sulfide liquid throughout the entire 400 800 m thick succession it is therefore envisaged that the gnpa member formed in a similar manner to its nearest analogue the platreef notable differences however in pge tenors indicate that the ore forming process may have differed slightly within the staging chambers that supplied the platreef and gnpa member

sulfur cycling at the mid atlantic ridge a multiple sulfur isotope approach abstract the role of sulfur in two hydrothermal vent systems the logatchev hydrothermal field at 14 45 n 44 58 w and several different vent sites along the southern mid atlantic ridge smar between 4 48 s and 9 33 s and between 12 22 w and 13 12 w is examined by utilizing multiple sulfur isotope and sulfur concentration data isotope compositions for sulfide minerals and vent h 2 s from different smar sites range from 1 5 to 8 9 in 34 s and from 0 001 to 0 051 in 33 s these data indicate mixing of mantle sulfur with sulfur from seawater sulfate combined 34 s and 33 s systematics reveal that vent sulfide from smar is characterized by a sulfur contribution from seawater sulfate between 25 and 33 this higher contribution compared with epr sulfide indicates increased seawater sulfate reduction at mar because of a deeper seated magma chamber and longer fluid upflow path length and points to fundamental differences with respect to subsurface structures and fluid evolution at slow and fast spreading mid ocean ridges additionally isotope data uncover non equilibrium isotopic exchange between dissolved sulfide and sulfate in an anhydrite bearing zone below the vent systems at fluid temperatures between 335 and 400 c 34 s values between 0 2 to 8 8 for dissolved and precipitated sulfide from logatchev point to the same mixing process between mantle sulfur and sulfur from seawater sulfate as at smar 34 s values between 24 5 and 6 5 and 33 s values between 0 001 and 0 125 for sulfide bearing sediments and mafic ultramafic host rocks from drill cores taken in the region of logatchev indicate a clear contribution of biogenic sulfides formed via bacterial sulfate reduction basalts and basaltic glass from smar sites with 33 s 0 008 reveal lower 33 s lower values than suggested on the basis of previously published isotopic measurements of terrestrial materials we conclude that the combined use of both 34 s and 33 s provides a more detailed picture of the sulfur cycling in hydrothermal systems at the mid atlantic ridge and uncovers systematic differences to hydrothermal sites at different mid ocean ridge sites multiple sulfur isotope measurements allow identification of incomplete isotope exchange in addition to isotope mixing as a second important factor influencing the isotopic composition of dissolved sulfide during fluid upflow furthermore based on 33 s we are able to clearly distinguish biogenic from hydrothermal sulfides in sediments even when 34 s were identical

spreading evolution of the troodos ophiolite cyprus some recent developments particularly the recognition of transform tectonized and nontransform tectonized fabrics close to the southern troodos transform fault provide a framework for a new evolutionary model of the troodos ophiolite this model proposes that most of the ophiolite formed at the solea axis against a dextrally slipping southern troodos transform spreading was largely accommodated by magmatic accretion the ridge then jumped to the east a last phase of amagmatic stretching driving normal faulting above a detachment horizon in the solea graben a ridge jump boundary separates the old crust formed at the solea axis from the new crust generated at the outside corner of the new ridge with a dextrally slipping continuation of the southern troodos transform the final episode involved extension of the crust generated at the new ridge to form the larnaca graben in the eastern part of the ophiolite

metal fluxes during magmatic degassing in the oceanic crust sulfide mineralisation at odp site 786b izu bonin forearc volcanogenic massive sulfide deposits are enriched in metals that are either derived from hydrothermal alteration of the basement rocks or supplied by exsolution of metal rich volatiles during magmatic differentiation the extent to which each process contributes to metal enrichment in these deposits varies between different tectonic settings ocean drilling program hole 786b recovered 800 m of upper oceanic crust from a supra subduction zone setting and includes a 30 m thick mineralised zone in situ s isotopic compositions of pyrite decrease from 5 9 2 9 in the upper mineralised zone down to 3 3 2 1 in the extensively altered central mineralisation zone potentially indicating strong magmatic fluid input in this area whole rock data and in situ trace element analyses in sulfide minerals show enrichment of ag as au bi mo s se sb and te in the mineralised zone evaluation of metal behaviour during magmatic differentiation and primary metal fertility of basement rocks suggests that degassing melt is the main source for the high au se and s enrichment observed in the mineralised zone magmatic volatile exsolution occurred late during the magmatic differentiation 2 wt mgo concomitant with oxide crystallisation and metal depletion in the melt comparison of ocean drilling program hole 786b with volcanogenic massive sulfide deposits hosted by boninitic volcanic successions such as in the semail ophiolite the newfoundland appalachians and the flin flon belt suggests that magmatic fluid exsolution could be a common mechanism for au enrichment in bimodal mafic volcanogenic massive sulfide deposits

mineralogy sulphur isotope geochemistry and the development of sulphide structures at the broken spur hydrothermal vent site 29 10 n mid atlantic ridge a large collection of hydrothermal sulphides from the broken spur hydrothermal vent site including representative samples of mound sulphide materials has been characterized using optical mineralogy and sulphur isotope analysis young mound sulphides from broken spur have a pyrrhotite dominated mineralogy unusual for bare ridge vent systems however pyrrhotite is metastable and is ultimately converted to iron disulphides mature sulphides are indurated recrystallized and contain abundant quartz sulphide mound materials are developed by three major processes i coalescing of chimney structures ii accumulation of talus from mass wasting and iii precipitation and growth in response to hydrothermal flow progressive maturation of mound materials is by modification of primary textures development of mineralogical zoning and replacement of metastable phases sulphur isotope analysis of 35 mineral separates returned 34 s values of 0 5 to 3 2 these values are similar to those previously measured for broken spur and snakepit but are distinctly 32 s enriched compared to the tag active mound and some pacific sites seawater entrainment and sulphate reduction within the subsurface feeder zone below broken spur mounds do not appear to be important processes at broken spur in contrast to the tag active mound

trace element systematics of pyrite from submarine hydrothermal vents abstract submarine hydrothermal sulfide ores occur at mid ocean ridges intra oceanic arcs and in back arc basins associated with host rocks of highly variable composition pyrite is the dominant sulfide mineral in most samples presented within this study and thus the trace element composition of pyrite may reflect the different metal sources and precipitation processes here we report on a systematic study of minor and trace element contents in pyrite from active and inactive submarine hydrothermal vent fields at different plate tectonic settings including the indian and mid atlantic ridges the tonga kermadec intra oceanic arc the lau back arc and the central okinawa trough our results show that the trace element contents of pyrite from different locations vary significantly but for most elements without any systematic correlation to the concentration in the magmatic host rocks only as appears to be depleted in pyrite from ultramafic hosted vent systems that are affected by serpentinization these as depleted pyrites preferentially host au 0 micro or nano particles bismuth is enriched in pyrite from hydrothermal systems that contain a sedimentary component and micro inclusions of sphalerite are either due to fluid sediment interaction or phase separated fluid venting pyrites from individual locations have highly variable concentrations of elements like au co cu se mo ag and sb that are most likely related to fluid evolution and changes in fluid composition sub seafloor hydrothermal fluid seawater mixing influences the distribution of au co cu se and mo in pyrite elements like au ag sb and pb often have a characteristic affinity to as while cd correlates closely with zn a magmatic volatile contribution to the hine hina hydrothermal system may result in the precipitation of cu enriched pyrite our results show that the concentrations of most trace metals in pyrite are a function of the physicochemical parameters of the fluid phase rather than a reflection of the magmatic host rock composition

sulfur isotope evidence for microbial sulfate reduction in altered oceanic basalts at odp site 801 the subsurface biosphere in the basaltic ocean crust is potentially of major importance in affecting chemical exchange between the ocean and lithosphere alteration of the oceanic crust commonly yields secondary pyrite that is depleted in 34s relative to igneous sulfides although these 34s depleted sulfur isotope ratios may point to signatures of biological fractionation previous interpretations of sulfur isotope fractionation in altered volcanic rocks have relied on abiotic fractionation processes between intermediate sulfur species formed during basalt alteration here we report results for multiple s isotope 32s 33s 34s compositions of altered basalts at odp site 801 in the western pacific and provide evidence for microbial sulfate reduction within the volcanic oceanic crust in situ ion microprobe analyses of secondary pyrite in basement rocks show a large range of 34s values between 45 and 1 whereas bulk rock 34s analyses yield a more restricted range of 15 8 to 0 9 these low and variable 34s values together with bulk rock s concentrations ranging from 0 02 up to 1 28 are consistent with loss of magmatic primary mono sulfide and addition of secondary sulfide via microbial sulfate reduction high precision multiple sulfur isotope 32s 33s 34s analyses suggest that secondary sulfides exhibit mass dependent equilibrium fractionation relative to seawater sulfate in both 33s and 34s values these relationships are explained by bacterial sulfate reduction proceeding at very low metabolic rates the determination of the s isotope composition of bulk altered oceanic crust demonstrates that s based metabolic activity of subsurface life in oceanic basalt is widespread and can affect the global s budget at the crust seawater interface

spreading structure of the troodos ophiolite cyprus orientations of dikes within the sheeted dike complex of the troodos ophiolite reveal primary spreading structure produced at a complex ridge transform intersection three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes faults flatten at depth into a detachment within the upper parts of the plutonic complex large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids we suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north trending present coordinates slow spreading ridge crest a simple model for ridge migration indicates eastward jumps of 8 13 km changes in ridge orientation are suggested by changes in trends of dikes and graben axes comparison of the pattern of dikes near the arakapas fault zone with the structure of active ridge transform intersections suggests that the fault is a right offset sinistral transform in contrast to earlier models in which a ridge to the west of the exposed troodos complex was proposed

three dimensional shear velocity anisotropic model of piton de la fournaise volcano la reunion island from ambient seismic noise we cross correlate 4 years of seismic noise from the seismic network of piton de la fournaise volcano la reunion island to measure the group velocity dispersion curves of rayleigh and love waves we average measurements from vertical and radial components to obtain 577 rayleigh wave dispersion curves the transverse components provided 395 love wave dispersion curves we regionalize the group velocities measurements into 2 d velocity maps between 0 4 and 8 s finally we locally inverted these maps for a pseudo 3 d anisotropic shear velocity model down to 3 km below the sea level using a neighborhood algorithm the 3 d isotropic shear wave model shows three distinct high velocity anomalies surrounded by a low velocity ring the anomaly located below the present plaine des sables could be related to an old intrusive body at the location of the former volcanic center before it migrated toward its present location the second high velocity body located below the summit of the volcano likely corresponds to the actual preferential dyke intrusion zone as highlighted by the seismicity the third high velocity anomaly located below the grandes pentes and the grand brule areas and is an imprint of the solidified magma chamber of the dismantled les alizes volcano radial anisotropy shows two main anomalies positive anisotropy above sea level highlighting the recent edifice of piton de la fournaise with an accumulation of horizontal lava flows and the second one below the sea level with a negative anisotropy corresponding to the ancient edifice of piton de la fournaise dominated by intrusions of vertical dykes

trace element systematics and ore forming processes in mafic vms deposits evidence from the troodos ophiolite cyprus abstract the volcanogenic massive sulfide vms deposits in the troodos ophiolite cyprus are ancient analogues for modern day seafloor massive sulfide mineralisation formed in a supra subduction zone environment in this study we present the first detailed in situ study of trace elements in sulfides from twenty vms deposits hosted in the troodos ophiolite to better understand factors that influence the distribution enrichment and incorporation of trace elements in different sulfide minerals on a mineral scale trace elements exhibit systematic variations between pyrite chalcopyrite and sphalerite pyrite preferentially incorporates as sb au and te whilst chalcopyrite is enriched in co and se sphalerite is trace element poor with the exception of ag and cd selenium averages 278 ppm n 150 in chalcopyrite but only 42 ppm n 1322 in pyrite bismuth and te in pyrite show a weak positive correlation r2 0 35 in some vms deposits possibly linked with the occurrence of bi telluride inclusions trace element concentrations also vary between colloform and euhedral pyrite with an enrichment of au as sb cu and zn in colloform compared to euhedral pyrite time resolved laser ablation profiles reveal that the trace element distribution on a mineral scale is not uniform and varies with crystallographic effects fluctuating physicochemical fluid conditions such as temperature ph fs2 fo2 and ligand availability during sulfide precipitation incorporation mechanisms in sulfides differ between elements in pyrite ag as se and pb are hosted in solid solution or as nanoscale inclusions whilst au sb and te may form micro scale inclusions on a regional scale 20 km the distribution of trace elements exhibits systematic variations between three major structural domains namely the solea mitsero and larnaca grabens the vms deposits of the magmatic tectonic solea graben are enriched in se co te au and cu relative to mitsero which is a purely extensional feature therefore we hypothesise that a variable magmatic volatile influx related to a magma volume b migration of the magmatic hydrothermal crack front and associated brine liberation or c a variation in protolith metal concentration are responsible for regional scale variations in vms geochemistry this is suggested to be intrinsically linked to the spreading architecture of troodos

seismic scattering and absorption mapping of debris flows feeding paths and tectonic units at mount st helens volcano abstract frequency dependent peak delay times and coda quality factors have been used jointly to separate seismic absorption from scattering quantitatively in earth media at regional and continental scale to this end we measure and map these two quantities at mount st helens volcano the results show that we can locate and characterize volcanic and geological structures using their unique contribution to seismic attenuation at 3 hz a single high scattering and high absorption anomaly outlines the debris flows that followed the 1980 explosive eruption as deduced by comparison with remote sensing imagery the flows overlay a nnw sse interface separating rocks of significant varying properties down to 2 4 km and coinciding with the st helens seismic zone high scattering and high absorption anomalies corresponding to known locations of magma emplacement follow this signature under the volcano showing the important interconnections between its feeding systems and the regional tectonic boundaries with frequency increasing from 6 to 18 hz the nnw sse tectonic feeding trends rotate around an axis centered on the volcano in the direction of the regional scale magmatic arc sw ne while the aseismic high scattering region wsw of the volcano shows no evidence of high absorption the regions of highest scattering and absorption are consistently located at all frequencies under either the eastern or the south eastern flank of the volcanic edifice from the comparison with the available geological and geophysical information we infer that these anomalies mark both the location and the trend of the main feeding systems at depths greater than 4 km

tectonic deformation by rotating parallel faults the bookshelf mechanism abstract in various tectonic environments extension of crustal parts and shear distortion has been accommodated by the rotation of an array of parallel faults because of its kinematic resemblance to the tilting of a row of books on a shelf the tectonic process may be referred to as a bookshelf mechanism its most important manifestions are cross faulting between normal faults or parallel wrench faults the extension of deltaic slope deposits and all forms of tilted block tectonics kinematically there are two basic modes of the mechanism a the dilational mode where rotating faults tend to open up and possibly become paths for fluid migration and b the domino style where reclining faults remain closed in both cases formation and rotation of the faults may take place in one tectonic event fault displacements may therefore often remain small and difficult to detect from the mechanical point of view bookshelf operations that accommodate shearing between parallel and predominantly parallel moving rock boundaries bookshelf mechanisms proper have to be differentiated from those operations responding to an imposed crustal extension or shortening after a review of geological evidence the process is addressed from a geomechanical point of view in order to determine the geological operating conditions and controlling parameters the crucial mechanical factor in the bookshelf mechanism is the component of normal stress that acts in the direction of the imposed displacement component parallel to the driving boundary plates in contrast to the usual assumption the driving shear stress on the boundary plates not only depends on the normal stress on the plates but it also varies with the value of the normal stress that acts inside the sheared material in the direction of the plate parallel displacement component it is the relationship between the two normal stresses that also determines the type of rotating parallel faults large scale rotation of parallel normal faults in domino style tilted block tectonics is primarily associated with the extension of ductile substrata from mechanical arguments and sand box experiments it has been inferred how the process and in particular the dip direction of the parallel faults is controlled by the way substratal extension progresses by means of the direction of a substratal squeeze flow the presence of a surface slope the strength anisotropy associated with fore set beds and by the configuration of competent boundaries of the extending overburden

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

magma system recharge of mount st helens from precise relative hypocenter location of microearthquakes 1 four hundred forty seven well recorded earthquakes at mount st helens during the late 1990s are relocated with high precision using a combination of a cross correlation technique and then a procedure to obtain a one dimensional velocity structure solution together with station corrections the resulting high resolution pattern of the spatial temporal evolution of the earthquake activity along with focal mechanisms of events provide additional evidence for the presence of a magma reservoir in the depth range 5 5 10 km connected to the surface by a thin vertical conduit moreover the distribution of the events below 5 5 km yielded a much clearer view of the structures that generate the seismicity at that depth some of the deeper events occur along a sequence of faults under the influence of a radial stress field however a large number of the events occur on a nne ssw striking steeply dipping fault with slip consistent with magma being periodically injected into a truncated dike on the northwest side of this fault

systematic variations of trace element and sulfur isotope compositions in pyrite with stratigraphic depth in the skouriotissa volcanic hosted massive sulfide deposit troodos ophiolite cyprus the troodos ophiolite represents one of the best preserved fossil analogs of modern oceanic crust and includes numerous volcanic hosted massive sulfide deposits the skouriotissa deposit can be separated into a stockwork ore zone and an overlying massive sulfide lens that is covered by metalliferous sediments representing the former sulfide seawater interface pyrite is the dominant sulfide mineral within these ores the trace element composition of pyrite varies systematically with stratigraphic depth down to 150 mbsf probably reflecting fluid temperature variations and effects of phase separation co ni se te bi and cu metal remobilization due to hydrothermal zone refining zn sb and pb and fluid seawater mixing at the seafloor mo represent further important processes controlling the pyrite chemistry massive sulfide hosted sphalerite and euhedral pyrite probably formed from hot fluids 400 c while the occurrence of colloform pyrite indicates lower precipitation temperatures 400 c similar 18o quartz fluid equilibration temperatures 400 c in the stockwork zone suggest that the skouriotissa fluids did not cool significantly during the final 150 m of fluid ascent to the seafloor the 34s composition of deep stockwork pyrite 1 4 suggests that an isotopically light magmatic volatile phase 0 was added to the hydrothermal system of skouriotissa during further fluid ascent about 38 of cretaceous seawater 34s 18 19 was added leading to the precipitation of stockwork pyrite with positive 34s values 6 1 in addition the chemical and textural similarities between skouriotissa and modern seafloor vent systems and massive sulfide deposits suggest that the skouriotissa hydrothermal system has a modern analog

hydrothermal fluids associated with seafloor mineralization at two southern kermadec arc volcanoes offshore new zealand massive sulfide samples have been recovered from brothers and rumble ii west volcanoes of the southern kermadec frontal arc northeast of new zealand both are caldera forming volcanoes with brothers largely dacitic and rumble ii west basaltic to basaltic andesite in composition the sulfide samples are considered to be pieces of chimneys from near the base of the caldera walls and are dominated by cu fe ba pb chalcopyrite pyrite barite galena and to a lesser extent zn fe ba pb sphalerite pyrite barite galena mineralization gold is present in the samples up to 6 1 ppm paragenetic relationships suggest that the two hydrothermal systems have heated up and later cooled down oxidation of the sulfides is common and suggests that parts of the deposits are relatively old fluid inclusion microthermometric data show salinities fall predominantly in the range 3 0 to 3 4 wt nacl equivalent similar to seawater values 3 2 wt about 15 of the brothers salinity data range to both lower 2 2 wt and higher 3 9 wt than seawater values homogenization temperatures for type i inclusions in barite range between 175 and 322 c for the brothers samples and between 205 and 268 c for one rumble ii west sample a dramatic decrease in temperature and or change in the oxidation state of the hydrothermal fluid are considered the main mechanisms responsible for gold deposition at the kermadec vent sites this was achieved by mixing the hydrothermal fluid with ambient seawater within what are inferred to have been 300 c white smoker chimneys consistent with s isotope results fluid inclusion volatiles are dominated by h2o 99 81 99 98 mol followed by co2 0 03 0 17 and lesser amounts of n2 0 004 0 023 ch4 0 002 0 026 and trace cos and c2 c3 hydrocarbons fluid inclusions hosted by barite contain noticeably lesser amounts of gas than those within sulfides consistent with mixing the source of the relatively high concentrations of ch4 is enigmatic although may be related to the thermal degradation of organic rich sediments intercalated within the volcanic pile addition of a pressure correction and the effect of dissolved gases show that the hydrothermal fluids did not boil at the chimney depths however the range of measured salinities in the brothers field combined with slightly positive trends in fluid inclusion co2 ch4 versus co2 n2 plots suggest sub seafloor phase separation may have occurred in the hydrothermal system prior to venting

the magma system of mount st helens non linear high resolution p wave tomography abstract high resolution three dimensional images of p wave velocity anomalies below mt st helens washington were derived using tomographic inversion the model is a 27 5 21 20 km target volume parameterized by blocks 0 5 km per side the area included 39 stations and 5454 local events leading to 35 475 rays used in the inversion to diminish the effects of noisy data the laplacian was constrained to be small within horizontal layers providing smoothing of the model non linear effects were compensated for by iterating three dimensional ray tracing using pseudo bending between inversions and relocating earthquakes relative to the updated three dimensional model the structural differences between the linear and non linear inversions appear to be insignificant although the amplitudes of the anomalies are larger in the non linear models results indicate a low velocity anomaly 7 approximately 1 km in lateral extent from 1 5 to 3 km depths between 3 and 6 km depth the anomaly appears to spread out below 6 km depth the low velocity feature changes to a higher velocity perturbation with lower velocity perturbations flanking around the perimeter of the volcano the higher velocity material which correlates with the higher seismicity at that depth is interpreted as being a plug capping the low velocity magma chamber which begins below 9 km depth

local crustal extension at mount st helens washington seismicity and the orientation of fault planes from focal mechanisms indicate that mount st helens is located at a dextral offset along the st helens seismic zone shz earthquake swarms occurring in this offset are related to volcanic eruptions because motion on the shz is in a right lateral strike slip sense this dextral offset creates extension within a volume of the crust between the offset fault segments this offset geometry is similar to that of geothermal areas along the san andreas fault system we apply a model derived from these geothermal areas to mount st helens and find that the major differences between mount st helens and the geothermal areas can be related to the ratio of the width of the offset between fault segments l to the seismogenic depth h at mount st helens this ratio is 1 whereas in the geothermal areas the ratio is 1 we propose that when l h 1 as at mount st helens the regional minimum principal stress does not completely dominate the small volume under extension and as a consequence the opening geometry is poory established compared to the oblique crustal spreading that characterizes the geothermal areas where l h 1 late quaternary volcanic vents near mount st helens strike northeast similar to the strike of a set of pre quaternary faults and intrusive rocks that are mapped north of the volcano in addition the deepest earthquakes occurring within the extensional volume are aligned along a northeast striking fault since these northeast striking features are aligned approximately prependicular to the regional minimum principal stress we infer that the spatial position of mount st helens is controlled by the junction of the right stepping offset of the shz with the older set of fractures and that these fractures are favorably aligned with respect to the contemporary regional tectonic stress directions for the transport of magma through the brittle crust the sense of fault motions predicted by our model for local crustal extension is consistent with an apparent component of right lateral shear measured from geodetic lines around mount st helens during june and july 1980

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

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

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

geology sulfide geochemistry and supercritical venting at the beebe hydrothermal vent field cayman trough the beebe vent field bvf is the world s deepest known hydrothermal system at 4960m below sea level located on the mid cayman spreading centre caribbean the bvf hosts high temperature 401 c black smoker vents that build cu zn and au rich sulphide mounds and chimneys the bvf is highly gold rich with au values up to 93 ppm and an average au ag ratio of 0 15 gold precipitation is directly associated with diffuse flow through beehive chimneys significant mass wasting of sulphide material at the bvf accompanied by changes in metal content results in metaliferous talus and sediment deposits situated on very thin 2 3km thick oceanic crust at an ultraslow spreading centre the hydrothermal system circulates fluids to a depth of 1 8km in a basement that is likely to include a mixture of both mafic and ultramafic lithologies we suggest hydrothermal interaction with chalcophile bearing sulphides in the mantle rocks together with precipitation of au in beehive chimney structures has resulted in the formation of a au rich volcanogenic massive sulphide vms deposit with its spatial distribution of deposit materials and metal contents the bvf represents a modern day analogue for basalt hosted au rich vms systems this article is protected by copyright all rights reserved

tomographic imaging of local earthquake delay times for three dimensional velocity variation in western washington tomographic inversion is applied to delay times from local earthquakes to image three dimensional velocity variations in the puget sound region of western washington the 37 500 square km region is represented by nearly cubic blocks of 5 km per side p wave arrival time observations from 4 387 crustal earthquakes with depths of 0 to 40 km were used as sources producing 36 865 rays covering the target region a conjugate gradient method lsqr is used to invert the large sparse system of equations to diminish the effects of noisy data the laplacian is constrained to be zero within horizontal layers providing smoothing of the model the resolution is estimated by calculating impulse responses at blocks of interest and estimates of standard errors are calculated by the jackknife statistical procedure results of the inversion are correlated with some known geologic features and independent geophysical measurements high p wave velocities along the eastern flank of the olympic peninsula are interpreted to reflect the subsurface extension of crescent terrane low velocities beneath the puget sound further to the east are inferred to reflect thick sediment accumulations the crescent terrane appears to extend beneath puget sound consistent with its interpretation as a major accretionary unit in themore southern puget sound basin high velocity anomalies at depths of 10 20 km are interpreted as crescent terrane and are correlated with a region of low seismicity near mt ranier high velocity anomalies may reflect buried plutons less

models of ground deformation from vertical volcanic conduits with application to eruptions of mount st helens and mount etna we present simple analytical models of ground deformation from inflation of vertical volcanic conduits or pipes we compare three cases corresponding to 1 a pressurized pipe with closed end at the top that resists the internal pressure 2 a pressurized pipe with top open for which the internal deformation is mainly dislocation of the cylindrical walls and 3 a pipe shaped region that dilates for the closed pipe we use eshelby s inclusion theory to model deformation from a thin pressurized elongated prolate ellipsoid in a full space which we express in terms of double forces to satisfy surface boundary conditions we superimpose image solutions by using double forces derived from mindlin s half space solution for the point force for the open pipe we apply the volterra integral to dislocation across a cylindrical surface and generalize it to the half space using mindlin s point force solution these two solutions show marked differences from the line of dilatation solution ratios of maximum horizontal deformation to maximum vertical deformation for the pipe models are significantly greater than values for the center or line of dilatation models this and the more gradual fall off of the deformation with distance may be used as diagnostics for discriminating between pipe like sources and dilatational sources on volcanoes where both components of the deformation field are available as examples we compare the closed pipe model with deformation associated with dome building on mount st helens volcano and tilt predicted by the open pipe model with tilt measured during a period of explosive eruptive activity on mount etna sicily in 1995 comparison between model values and the measurements suggests that the effective elastic moduli of the volcanic cones are very low

magma accumulation rates and thermal histories of plutons of the sierra nevada batholith ca zircon u pb geochronology results indicate that the john muir intrusive suite of the central sierra nevada batholith california was assembled over a period of at least 12 ma between 96 and 84 ma bulk mineral thermochronology u pb zircon and titanite 40ar 39ar hornblende and biotite of rocks from multiple plutons comprising the muir suite indicates rapid cooling through titanite and hornblende closure following intrusion and subsequent slow cooling through biotite closure assembly of intrusive suites in the sierra nevada and elsewhere over millions of years favors growth by incremental intrusion estimated long term pluton assembly rates for the john muir intrusive suite are on the order of 0 001 km3 a 1 which is inconsistent with the rapid magma fluxes that are necessary to form large volume magma chambers capable of producing caldera forming eruptions if large shallow crustal magma chambers do not typically develop during assembly of large zoned intrusive suites it is doubtful that the intrusive suites represent cumulates left behind following caldera forming eruptions

vp structure of mount st helens washington usa imaged with local earthquake tomography we present a new p wave velocity model for mount st helens using local earthquake data recorded by the pacific northwest seismograph stations and cascades volcano observatory since the 18 may 1980 eruption these data were augmented with records from a dense array of 19 temporary stations deployed during the second half of 2005 because the distribution of earthquakes in the study area is concentrated beneath the volcano and within two nearly linear trends we used a graded inversion scheme to compute a coarse grid model that focused on the regional structure followed by a fine grid inversion to improve spatial resolution directly beneath the volcanic edifice the coarse grid model results are largely consistent with earlier geophysical studies of the area we find high velocity anomalies nw and ne of the edifice that correspond with igneous intrusions and a prominent low velocity zone nnw of the edifice that corresponds with the linear zone of high seismicity known as the st helens seismic zone this low velocity zone may continue past mount st helens to the south at depths below 5 km directly beneath the edifice the fine grid model images a low velocity zone between about 2 and 3 5 km below sea level that may correspond to a shallow magma storage zone and although the model resolution is poor below about 6 km we found low velocities that correspond with the aseismic zone between about 5 5 and 8 km that has previously been modeled as the location of a large magma storage volume

distribution mineralogy and geochemistry of selenium in felsic volcanic hosted massive sulfide deposits of the finlayson lake district yukon territory canada three significant base metal deposits were recently discovered in the finlayson lake district yukon territory canada however widely varying selenium contents e g 1 100 ppm avg se at wolverine 200 ppm avg se at kudz ze kayah and 7 ppm avg se at gp4f have hindered their rapid exploitation observations and data from the deposits in the finlayson lake district show a strong positive correlation between high temperature 300 c copper rich sulfide assemblages at the base of volcanic hosted massive sulfide vhms and the selenium content of all sulfide minerals thermodynamic calculations for selenide and sulfide minerals indicate significantly higher temperatures of formation and lower solubilities of selenide versus sulfide minerals which is consistent with petrographic observations and can explain the distribution of selenium in zone refined vhms and volcanic sediment hosted massive sulfide deposits volumetric mass balance calculations indicate that some selenium could be sourced from contemporaneous volatile degassing of seo 4 its rapid reduction to h 2 se and subsequent entrainment into hydrothermal ore fluid however estimates for the kudz ze kayah and wolverine deposits suggest an additional large se reservoir volumetric mass balance estimates indicate that contemporaneous volcanic rocks and black shales are likely selenium reservoirs for the kudz ze kayah and wolverine deposits respectively

variations in the frequency magnitude distribution with depth in two volcanic areas mount st helens washington and mt spurr alaska the frequency magnitude distribution of earthquakes characterized using the b value is examined as a function of space beneath mount st helens 1988 1996 and mt spurr 1991 1995 at mount st helens two volumes of anomalously high b b 1 3 can be observed at depths of 2 6 3 6 km below the crater floor and below 6 4 km these anomalies coincide with 1 the depth of vesiculation of ascending magma and 2 the suggested location of a magma chamber at mount st helens study of mt spurr reveals an area of high b value b 1 3 at a depth of about 2 3 4 5 km below the crater floor of the active vent crater peak we propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b value at shallow depths alternatively interaction of magma with groundwater may have increased pore pressure and lowered the effective stress the deeper anomaly at mount st helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber our results indicate that detailed mapping of the frequency magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers

tomographic inversion for three dimensional velocity structure at mount st helens using earthquake data tomographic inversion is applied to 17 659 p phase observations at 21 stations from 2023 earthquakes in the vicinity of mount st helens to study the three dimensional velocity structure block size for the inversion is 2 km horizontally and 2 km or more vertically locations of hypocenters are assumed known and are based on a reference one dimensional layered velocity structure a conjugate gradient technique lsqr is used to invert the large sparse system of equations augmented by regularization with a laplacian roughening matrix resolution is estimated by computing the impulse response of the inversion for various critical locations and uncertainties of the estimates are determined by a jackknife approach the results of the inversion show a remarkable correlation with known geological and geophysical features the spirit lake and spud mt plutons are characterized by high velocity regions extending to approximately 9 km depth the st helens seismic zone a band of diffuse seismicity extending nnw from the volcano is evident as a prominent low velocity lineation the change in character of the velocity anomalies south of st helens corresponds well with the near cessation of seismic activity there a low velocity anomaly beneath the crater from 6 to 16 km depths may represent modern magma accumulations

the internal structure of an active sea floor massive sulphide deposit the hydrothermal circulation of sea water through permeable ocean crust results in rock water interactions that lead to the formation of massive sulphide deposits these are the modern analogues of many ancient ophiolite hosted deposits1 4 such as those exposed in cyprus here we report results obtained from drilling a series of holes into an actively forming sulphide deposit on the mid atlantic ridge a complex assemblage of sulphide anhydrite silica breccias provides striking evidence that such hydrothermal mounds do not grow simply by the accumulation of sulphides on the sea floor indeed the deposit grows largely as an in situ breccia pile as successive episodes of hydrothermal activity each form new hydrothermal precipitates and cement earlier deposits during inactive periods the collapse of sulphide chimneys dissolution of anhydrite and disruption by faulting cause brecciation of the deposit the abundance of anhydrite beneath the present region of focused hydrothermal venting reflects the high temperatures 150 c currently maintained within the mound and implies substantial entrainment of cold sea water into the interior of the deposit these observations demonstrate the important role of anhydrite in the growth of massive sulphide deposits despite its absence in those preserved on land

eruption dynamics at mount st helens imaged from broadband seismic waveforms interaction of the shallow magmatic and hydrothermal systems 1 the current eruption at mount st helens is characterized by dome building and shallow repetitive long period lp earthquakes waveform cross correlation reveals remarkable similarity for a majority of the earthquakes over periods of several weeks stacked spectra of these events display multiple peaks between 0 5 and 2 hz that are common to most stations lower amplitude very long period vlp events commonly accompany the lp events we model the source mechanisms of lp and vlp events in the 0 5 4 s and 8 40 s bands respectively using data recorded in july 2005 with a 19 station temporary broadband network the source mechanism of the lp events includes 1 a volumetric component modeled as resonance of a gently nnw dipping steam filled crack located directly beneath the actively extruding part of the new dome and within 100 m of the crater floor and 2 a vertical single force attributed to movement of the overlying dome the vlp source which also includes volumetric and single force components is 250 m deeper and nnw of the lp source at the sw edge of the 1980s lava dome the volumetric component points to the compression and expansion of a shallow magma filled sill which is subparallel to the hydrothermal crack imaged at the lp source coupled with a smaller component of expansion and compression of a dike the single force components are due to mass advection in the magma conduit the location geometry and timing of the sources suggest the vlp and lp events are caused by perturbations of a common crack system

subsurface processes at the lucky strike hydrothermal field mid atlantic ridge evidence from sulfur selenium and iron isotopes at lucky strike near the azores triple junction the seafloor setting of the hydrothermal field in a caldera system with abundant low permeability layers of cemented breccia provides a unique opportunity to study the influence of subsurface geological conditions on the hydrothermal fluid evolution coupled analyses of s isotopes performed in conjunction with se and fe isotopes have been applied for the first time to the study of seafloor hydrothermal systems these data provide a tool for resolving the different abiotic and potential biotic near surface hydrothermal reactions the 34s between 1 5 and 4 6 and se values between 213 and 1640 ppm of chalcopyrite suggest a high temperature end member hydrothermal fluid with a dual source of sulfur sulfur that was leached from basaltic rocks and sulfur derived from the reduction of seawater sulfate in contrast pyrite and marcasite generally have lower 34s within the range of magmatic values 0 1 and are characterized by low concentrations of se 50 ppm for 82se 76se ratios the 82se values range from basaltic values of near 1 5 to 7 the large range and highly negative values of hydrothermal deposits observed cannot be explained by simple mixing between se leached from igneous rock and se derived from seawater we interpret the se isotope signature to be a result of leaching and mixing of a fractionated se source located beneath hydrothermal chimneys in the hydrothermal fluid at lucky strike we consider two sources for s and se 1 the end member hydrothermal fluid with basaltic se isotopic values 1 5 and typical s isotope hydrothermal values of 1 5 2 a fractionated source hosted in subsurface environment with negative 34s values probably from bacterial reduction of seawater sulfate and negative 82se values possibly derived from inorganic reduction of se oxyanions fluid trapped in the subsurface environment is conductively cooled and has restricted mixing and provide favorable conditions for subsurface microbial activity which is potentially recorded by s isotopes fe isotope systematic reveals that se rich high temperature samples have 57fe values close to basaltic values 0 whereas se depleted samples precipitated at medium to low temperature are systematically lighter 57fe values between 1 to 3 an important implication of our finding is that light fe isotope composition down to 3 2 may be explained entirely by abiotic fractionation in which a reservoir effect during sulfide precipitation was able to produce highly fractionated compositions

multiple sources of selenium in ancient seafloor hydrothermal systems compositional and se s and pb isotopic evidence from volcanic hosted and volcanic sediment hosted massive sulfide deposits of the finlayson lake district yukon canada volcanic hosted massive sulfide vhms and volcanic sediment hosted massive sulfide vshms i e hosted by both volcanic and sedimentary rocks deposits in the finlayson lake district yukon canada provide a unique opportunity to study the influence of seafloor and sub seafloor hydrothermal processes on the formation of se poor gp4f vhms deposit 7 ppm se average intermediate kudz ze kayah kzk vhms deposit 200 ppm se average and se enriched wolverine vshms deposit 1100 ppm se average mineralization all three deposits are hosted by mid paleozoic 360 346 ma felsic volcanic rocks but only the wolverine deposit has voluminous coeval carbonaceous argillites black shales in the host rock package here we report the first application of se isotope analyses to ancient seafloor mineralization and use these data in conjunction with pb and s isotope analyses to better understand the source s and depositional process es of se within vhms and vshms systems the wide range of 82se 10 2 to 1 3 relative to nist 3149 34s 2 0 to 12 8 cdt and elevated se contents up to 5865 ppm within the wolverine deposit contrast with the narrower range of 82se 3 8 to 0 5 34s 9 8 to 13 0 and lower se contents 200 ppm average of the kzk deposit the wolverine and kzk deposits have similar sulfide depositional histories i e deposition at the seafloor with concomitant zone refining the se in the kzk deposit is magmatic leaching or degassing in origin whereas the wolverine deposit requires an additional large isotopically negative se source i e 15 82se the negative 82se values for the wolverine deposit are at the extreme light end for measured terrestrial samples and the lightest observed for hypogene sulfide minerals but are within calculated equilibrium values of 82se relative to nist 3149 30 at 25 c between seo4 and se2 we propose that the most negative se isotope values at the wolverine deposit record the 82se of the se source and that the wide range in 82se values results from the combined effects of thermal and chemical degradation and se loss from the carbonaceous argillite source to a hydrothermal fluid including magmatic se i e leached and or magmatic hydrothermal with deposition at or near the paleoseafloor pristine unaltered black shales show little variation in 82se relative to bulk earth se accumulation and fractionation to more negative isotopic values is interpreted to have been produced by post sediment deposition but pre ore stage reduction of seawater se within the black shales

modes of extension at oceanic spreading centers evidence from the solea graben troodos ophiolite cyprus abstract the solea graben in the troodos ophiolite cyprus represents a preserved cretaceous age axial valley formed during a brief period of amagmatic seafloor spreading structural analysis of all crustal levels of the graben reveals two periods of deformation an extensive early phase of extensional deformation associated with seafloor spreading and a later phase of minor deformation associated with obduction and doming of the complex during the tertiary recent amagmatic extensional deformation produced networks of normal faults that dip toward the graben axis and sole into a regional detachment fault located at the sheeted dike plutonic contact and oriented subparallel to ophiolite pseudostratigraphy individual fault segments are essentially planar but link to approximate listric geometries locally bulk structural rotation of fault blocks bounding the graben occurred about a nnw trending axis parallel to the present graben trend average dips in the sheeted complex within the central portions of the graben suggest 40 50 of structural rotation although this is a minimum figure because present dike dips are not a simple function of rotation magnitude tensor analysis of fault populations associated with extensional deformation shows that 3 was oriented approximately perpendicular to the graben axis late near axis extensional deformation segmented the detachment fault along nnw trending normal faults obduction related uplift and doming of the troodos ophiolite is expressed in the solea graben by minor thrust reverse and strike slip faults principally developed during reactivation of pre existing low angle fault zones formed during seafloor deformation tensor analysis of this later phase of faulting shows that 1 was oriented approximately n s and that 2 and 3 alternated position during the deformation the style of deformation documented in the solea graben supports recent speculation for basin and range style detachment faulting at slow spreading ridge crests with low magma budgets such as parts of the mid atlantic ridge near large offset transform faults natural cross sectional exposure of the solea graben shows how extensional structures might link at depth beneath such slow spreading ridge segments and demonstrates the importance of extensive brittle deformation of the crust prior to establishment of large scale ore forming hydrothermal circulation cells

effects of magmatic volatile influx in mafic vms hydrothermal systems evidence from the troodos ophiolite cyprus abstract the troodos ophiolite cyprus is the principal on land analogue for mafic hosted volcanogenic massive sulfide vms deposits this study for the first time presents sulfur isotope 34s data on a regional scale from vms deposits and other mineralised zones across the troodos ophiolite in combination 34s se s ratios and trace element chemistry e g se cu and au of different hydrothermal sulfides are used to assess variations in magmatic volatile influx and the source of metals and sulfur in ancient hydrothermal systems sulfur isotope analyses n 180 across 19 mineral deposits indicate a variable source of sulfur in the troodos vms hydrothermal system this in turn allows a variable source of metals to be inferred pyrite 34s range from 5 5 to 13 2 with an average of 4 6 n 160 for all deposits investigated these 34s variations cannot only be explained by variable proportions of thermochemical seawater sulfate reduction 34s 18 to 19 and leaching of primary magmatic sulfur from igneous rocks 34s 0 1 consequently two processes are proposed explaining the trace metal and 34s variation across the troodos ore forming systems including i a variable source of metals in the sheeted dyke complex and ii the addition of a magmatic volatile rich phase to the vms hydrothermal systems two distinct lava units exist in the troodos stratigraphy namely the upper and lower pillow lavas upl and lpl the more primitive upl are enriched in au se and cu relative to as sb and zn that are concentrated in the lpl some vms deposits pre date the formation of the upl e g agrokipia a suggesting se cu and au depleted source rocks hence the stratigraphic position of vms deposits and the ratio of upl lpl affinity elements e g as zn sb vs cu se au imply a systematic relationship between trace element distribution and stratigraphic depth this relates to the relative proportion of upl and lpl affinity lavas within the metal source region 34s values 106 ratios 500 in pyrite suggest the contribution of a magmatic volatile component e g apliki and skouriotissa we demonstrate that the source of metals and sulfur in the troodos vms hydrothermal system is affected by regional scale processes related to i variable source lithologies and ii the contribution of a magmatic volatile phase to some troodos vms hydrothermal systems

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

constraints on the behavior of trace elements in the actively forming tag deposit mid atlantic ridge based on la icp ms analyses of pyrite the distribution of trace ore elements in different paragenetic stages of pyrite has been documented for the first time in the sub seafloor of the actively forming tag massive sulfide deposit trace element distributions have been determined by in situ laser ablation inductively coupled plasma mass spectrometry la icp ms of pyrite formed at different stages of mineralization and at different temperatures constrained by previously published fluid inclusion analyses the data reveal a strong dependence on paragenetic stage with distinct low and high temperature enrichments porous pyrite and marcasite formed at low temperatures 350 c at the base of the hydrothermal mound and in the stockwork zone is enriched in co se bi cu ni and sn a number of different sub types of pyrite also have characteristic trace element signatures e g the earliest pyrite formed at the highest temperatures is always enriched in co and se compared to later stages ablation profiles for co se and ni are smooth and indicate that these elements are present mainly in lattice substitutions rather than as inclusions of other sulfides profiles for as sb tl and cu can be either irregular or smooth indicating both lattice substitutions and inclusions lead and ag have mostly smooth profiles but because pb cannot substitute directly into the pyrite lattice it is interpreted to be present as homogeneously distributed micro or nano scale particles the behavior of the different trace elements mainly reflects their aqueous speciation in the hydrothermal fluids at different temperatures and for some elements like co and se strong partitioning into the pyrite lattice at elevated temperatures adsorption onto pyrite surfaces controls the distribution of a number of redox sensitive elements i e mo v ni u particularly in the upper part of the mound which is infiltrated by cold seawater where micro or nano scale inclusions of chalcopyrite sphalerite galena or sulfosalts are present there is still a strong temperature dependence on the inclusion population e g more abundant chalcopyrite in the highest temperature pyrite suggesting that the inclusions were co precipitated with pyrite rather than overgrown however at the deposit scale the trace element distributions are also strongly controlled by remobilization and chemical zone refining as previously documented in bulk geochemical profiles the results show that pyrite chemistry is a remarkably good model of the chemistry of the entire hydrothermal system for many trace elements the concentrations in pyrite are highly predictive in terms of the conditions of mineral formation over a wide range of temperatures from the stockwork zone to the cooler outer margins of the deposit calculated minimum concentrations of the trace elements in the fluids needed to account for the observed concentrations in pyrite show good agreement with measured vent fluid concentrations particularly pb as mo ag and tl however significantly higher concentrations are indicated for co and se than have been measured in sampled fluids confirming the strong partitioning of these elements into high temperature pyrite

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

geochemistry and intrusive directions in sheeted dikes in the troodos ophiolite implications for mid ocean ridge spreading centers 1 sheeted dikes at mid ocean ridge volcanoes represent the link between deep magma production and storage processes and shallow processes such as volcanism and hydrothermal activity as such they are crucial for the interpretation of many observations at mid ocean ridges or other volcanoes with pronounced rift zones including topography hydrothermal systems petrology and geochemistry we carried out a structural magnetic and chemical investigation of a 4 10 km sheeted dike section in the troodos ophiolite cyprus on the basis of major and trace element geochemistry we distinguish dikes that may be correlated with the basal high ti series hts lavas from those of the overlying low ti series lts lavas all dikes studied are nearly parallel to each other with vertical or steeply dipping planes whose strike likely indicates the orientation of the spreading center anisotropy of magnetic susceptibility measurements suggests that the hts and lts dikes intrude in fundamentally different ways hts dikes reflect the intrusive behavior of dikes in the vicinity of a magma supply system and define ridge parallel intrusive sheets that radiate out from the magma chamber lts dikes show a bimodal orthogonal set of intrusive directions one shallow and one near vertical near lateral propagating dikes provide a means for delivery of magma into distant portions of a rift system and near vertical dike propagation directions are probably associated with feeder dikes to down rift surface flows our study suggests that the types of dike intrusive behavior in the troodos ophiolite may also be typical for normal mid ocean ridges or other major shield volcanoes with well developed rift zones

geology of the mount st helens area record of discontinuous volcanic and plutonic activity in the cascade arc of southern washington the quaternary edifice of mount st helens volcano was built upon a deeply eroded terrane of gently folded and altered volcanic and plutonic rocks that represent the core of the tertiary cascade magmatic arc these rocks constitute an east dipping homoclinal sequence several kilometers thick of subaerially erupted mafic to silicic flows and volcaniclastic strata k ar ages from this section range from about 28 to 23 ma late oligocene and earliest miocene which corresponds to an apparent lull in cascade volcanism to the north of mount rainier volcanism was essentially continuous during this period of time and neither a well defined base nor top of the section is exposed within the mapped area basalt and basaltic andesite dominate the lower part of the mid tertiary section whereas andesitic and dacitic rocks comprise most of the upper part this section was intruded by numerous mafic to silicic dikes sills and irregular plutonic bodies most no more than a few million years younger than their host rocks and subjected to pervasive burial metamorphism and widespread hydrothermal alteration large areas of hornfelsed rock surrounding even relatively small intrusions indicate that the proportion of plutonic rock becomes significantly greater at shallow depth beneath the existing erosion surface a large granitic pluton intruded the mid tertiary section north of spirit lake at about 21 ma the earl porphyry copper deposit occurs within the pluton but appears too young 17 ma to be genetically related to it in contrast to the rather continuous and voluminous oligocene to early miocene activity volcanism since then in the mount st helens area has been localized and volumetrically minor products of three younger eruptive periods have been recognized a sequence of 15 m y old pyroxene andesite flows resting unconformably on mid tertiary strata south of mount st helens widespread shallow dikes and sills of pyroxene andesite between 10 and 8 m y old and compositionally diverse rocks erupted during the past 3 m y the quaternary lavas are more potassic than the tertiary lavas and typically contain phenocrysts of hornblende and biotite which are absent from the older rocks a number of tertiary structures define a broad nne trending zone that may reflect a deep seated lithospheric flaw that has controlled the locus of cascade magmatism in southern washington for the past 25 m y mount st helens lies within this zone at the intersection of the nnw striking st helens seismic zone shz and an ene trending alignment of pleistocene silicic plug domes no surface breakage has been detected along the shz which is apparently very young the linear zone of silicic vents is probably controlled by a fault that has been interpreted from seismic records to occur directly beneath the volcano this zone parallels the directions of regional maximum horizontal compressive stress and north america juan de fuca plate convergence mount st helens is an example of a low volume tectonically controlled magmatic system in an early stage of development

regional tectonic and earthquake hazard implications of a crustal fault zone in southwestern washington the st helens seismic zone shz is a crustal earthquake zone in southwestern washington that is defined by small to moderate magnitude 2 5 to 5 5 earthquakes and focal mechanisms this zone seismically active along a distance of 90 km is interpreted as a fault zone capable of generating a moderate to large magnitude shallow earthquake at the northern end of this seismic zone about 15 km north of the cowlitz river a large area of seismic quiescence for earthquakes above magnitude 3 0 has predominated since 1977 this quiescence leaves open the question whether the shz continues northward into southern puget sound earthquake focal mechanisms along the shz indicating nearly vertical strike slip faulting with the preferred fault planes striking north south from these focal mechanisms we infer that the direction of maximum compression is northeast approximately parallel with the direction of plate convergence between the north american and juan de fuca plates we interpret the focal mechanism data as evidence for locked subduction this tectonic model raises the possibility of a large magnitude subduction earthquake neither the possibility of a moderate to large shallow crustal earthquake nor a major subduction event is part of the current earthquake hazard assessment for washington

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

constraints on water depth of massive sulfide formation evidence from modern seafloor hydrothermal systems in arc related settings the results of three decades of seafloor research provide the most reliable information on the importance of water depth in massive sulfide formation available data from over 130 occurrences show that water depths of seafloor vent sites vary with plate tectonic setting and the regional magmatic and volcanic environment the shallowest hydrothermal systems in subduction related settings are hosted by arc volcanoes these shallow vent sites have a number of features in common with subaerial epithermal systems massive sulfide occurrences in arc related rifts the most likely setting for many ancient analogs are generally restricted to water depths from 700 to 2 000 m with rifts developing within old arc crust at the deeper end of this range back arc spreading centers proximal to arcs host massive sulfide deposits at depths of 1 500 to 2 000 m the deepest hydrothermal systems occur along mature back arc spreading centers distal to volcanic arcs where water depths range from 2 000 to 3 700 m these deeper vent sites probably represent the best modern analogues of ophiolite hosted massive sulfide deposits boiling of the hydrothermal fluids is common at volcanic arcs and in arc related rifts in these environments elevated magmatic gas contents of the hydrothermal fluids can contribute to the widespread occurrence of phase separation and associated gas loss by contrast the high ambient pressures in deep marine hydrothermal systems along mature back arc spreading centers prevent fluids from boiling during their ascent to the seafloor boiling controls the maximum temperature at which hydrothermal fluids discharge at the seafloor and therefore influences the metal content of seafloor sulfide deposits copper rich massive sulfides typically occur at water depths exceeding 1 000 m whereas zn and pb rich occurrences may form at any water depth boiling can be an important control on ag and au grades but is not the only factor controlling precious metal enrichment in massive sulfides shallow marine hot spring deposits can be highly enriched in trace metals such as as hg and sb submarine volcanic arc and back arc settings are geologically complex and significant variations in water depth can occur over short distances paleoenvironmental reconstruction of these environments in ancient volcanic terranes is hampered by the lack of unequivocal volcanological or sedimentological criteria that indicate water depth the relationships established here using modern seafloor observations provide important constraints on the paleoenvironmental setting of ancient volcanic hosted massive sulfide deposits

distribution of platinum group elements in magmatic and altered ores in the jinchuan intrusion china an example of selenium remobilization by postmagmatic fluids the division of platinum group elements pge between those hosted in platinum group minerals pgm versus those in solid solution in base metal sulfides bms has been determined for ores from the pge bearing ni cu rich jinchuan intrusion in northwest china all the bms are devoid of pt and ir and magmatic bms are also barren of rh these pge may have been scavenged by arsenic to form pgm during magmatic crystallization of the bms pd os and ru are recorded in bms and pd is predominantly in solid solution in pentlandite unlike the fresh magmatic ores in altered or serpentinized ores pd pgm are present froodite is hosted in magnetite formed during alteration of bms accompanied by sulfur loss and liberation of pd michenerite pd pt bite sperrylite ptas2 and au bearing pgm are located in altered silicates irarsite irass occurs mainly enclosed in bms padmaite pdbise identified at the junctions of magnetite and bms was the last pgm to form and locally partially replaces earlier non se bearing pgm we propose that padmaite formed under oxidizing conditions during late local remobilization of se from the bms se bearing pgm are rare and our review shows they are frequently associated with carbonate suggesting that pd and se can be mobilized great distances in low ph oxidizing fluids and may be precipitated on contact with carbonate s se ratios are used by researchers of magmatic ni cu pge ores to determine sulfur loss assuming se is immobile and representative of magmatic sulfur content this study shows that se as well as s is potentially mobile and this should be considered in the use of s se ratios

an aeromagnetic study of mount st helens aeromagnetic data from surveys flown by the u s geological survey over mount st helens washington before and after the climactic may 18 1980 eruption were used to determine the bulk magnetic properties of the volcano and to delineate a buried source we assumed that most of the edifice of preeruption and posteruption mount st helens has a magnetization direction near the present earth s field of 69 inclination and 20 declination and calculated its intensity as 4 1 a m after subtraction of magnetic anomalies due to topogaphy magnetized with this direction and intensity the preeruption and posteruption surveys revealed nearly identical residual magnetic highs and lows indicative that their sources were not altered or removed by the may 1980 eruption the residual highs were explained by a 200 m deep source lying mostly within the edifice of mount st helens the source could be terrain that predates mount st helens such as a buried ridge or a cone or a valley filled with lava we calculated the magnetization of the material removed by the may 1980 eruption and found its intensity to be 4 2 a m in a direction near the present earth s field similar to that assumed for the volcano as a whole this similar result confirmed the validity of the magnetization vector assumed for the entire edifice

dyke domains in the mitsero graben troodos ophiolite cyprus an off axis model for graben formation at a spreading centre abstract graben at oceanic spreading centres may form at on axis or off axis positions their formation is dependent on the tectonic processes active in the oceanic crust evidence for the position of formation of graben in oceanic analogues ophiolites can be obtained from the orientation of dykes within sheeted dyke complexes dyke orientation data and dyke relations from the mitsero graben and environs troodos ophiolite cyprus have been used to determine the tectonic setting of graben formation five major dyke domains were delineated on the basis of orientation of dykes in the sheeted dyke complex in the mitsero graben between domains the mean strike rotates counter clockwise by 25 in the easternmost domain dyke strikes are roughly 070 and progressively westward the mean strikes of dykes within domains are roughly 045 026 000 and 324 within domains the dyke trends show no systematic changes in strike toward the domain edges dyke cross cutting relations the degree of their rotation and relations to other features such as hydrothermal alteration leads to the suggestion that the mitsero graben formed late in the history of the area possibly in an off axis position

thermal structure of the low velocity zone derived from laboratory and seismic investigations laboratory measurements of seismic velocities in upper mantle peridotite can be used to constrain the thermal state in the low velocity zone the measurements carried out at high temperature and high confining pressure can be extrapolated to upper mantle conditions we compare laboratory velocities with seismic observations and estimate temperatures and partial melt fractions in the low velocity zone the comparison gives 1310 c and 7 vol partial melt at 45 km depth in 0 5 my asthenosphere beneath the iceland plateau and 1450 c and 3 vol partial melt at 85 km depth in 0 20 my asthenosphere beneath the pacific ocean the laboratory results show that up to a 6 velocity drop in the low velocity zone relative to the lithospheric velocity does not necessarily require the existence of partial melt this implies that partial melt may not exist in the low velocity zone older than 5 my and 50 my in the iceland plateau and the pacific ocean respectively

history of carbonate ion concentration over the last 100 million years abstract instead of having been more or less constant as once assumed it is now apparent that the major ion chemistry of the oceans has varied substantially over time for instance independent lines of evidence suggest that calcium concentration ca2 has approximately halved and magnesium concentration mg2 approximately doubled over the last 100 million years on the other hand the calcite compensation depth and hence the caco3 saturation has varied little over the last 100 my as documented in deep sea sediments we combine these pieces of evidence to develop a proxy for seawater carbonate ion concentration co32 over this period of time from the calcite saturation state which is proportional to the product of ca2 times co32 but also affected by mg2 we can calculate seawater co32 our results show that co32 has nearly quadrupled since the cretaceous furthermore by combining our co32 proxy with other carbonate system proxies we provide calculations of the entire seawater carbonate system and atmospheric co2 based on this reconstructed atmospheric co2 is relatively low in the miocene but high in the eocene finally we make a strong case that seawater ph has increased over the last 100 my

spheroidal textures in igneous rocks textural consequences of h2o saturation in basaltic melts abstract when melts reach h 2 o saturation before they are emplaced at the earth s surface they exsolve a hydrous fluid phase at elevated pressure and high temperature a hydrous fluid is not pure h 2 o but can contain some quantities of silicate and metal oxide solute notably if fluid saturation occurs at high pressure chemically and physically fluid saturated basalts behave like any other system with liquid immiscibility textures that can result after solidification are globules of former fluid dispersed in a former silicate melt or silicate melt orbs embedded in a former fluid matrix the troodos ophiolite on the island of cyprus generated in a back arc basin above a wet intra oceanic subduction zone carries in its magmatic stratigraphy lithologies distinctly orbicular and globular in shape many melt compositions parental to the ophiolite were h 2 o saturated near their liquidus temperatures before they reached their depths of emplacement within the ophiolite sequence spheroids are not uncommon in magmatic rocks but still debated as to how they form we propose that globular and orbicular textures in igneous worldwide may result when melts reach h 2 o saturation and exsolve an immiscible solute bearing h 2 o dominated fluid phase at elevated temperature and pressure before or while they are emplaced

the magnetite crisis in the evolution of arc related magmas and the initial concentration of au ag and cu the association of large au cu ag ore deposits with convergent margins is commonly attributed to the higher content of chalcophile elements in the parental magmas generated at subduction zones compared with mid ocean ridges we present new geochemical data for arc like relatively oxidized mantle derived basalt to rhyolite magmas from the pual ridge and vicinity eastern manus basin which show that the initial abundances of base and precious metals in the parental basalts are similar to those of mid ocean ridge basalt morb the contents of au cu and ag are built up in the evolving pual ridge liquids to levels considerably in excess of those in morb because unlike morb they are not saturated in a sulfide phase which is a consequence of their being more oxidized than morb the behaviour of s during the evolution of the pual ridge magmas is obscured by late stage so2 loss during eruption but we show that it may be inferred by using se as a proxy because this element follows s closely during magmatic evolution except it is not lost during low pressure near sea floor degassing the onset of magnetite fractionation at 60 wt sio2 and an mg number of 40 is accompanied by an abrupt decrease in the contents of au cu and ag previously attributed to separation of cu au rich fluid which is also shown by se implying that magnetite fractionation triggers sulfide saturation petrological modelling reveals that the amount of magnetite fractionation involved is sufficient to convert most of the s originally dissolved in the magma as sulfate so42 to sulfide s2 triggering saturation in a cu rich sulfide phase tentatively identified as bornite cu5fes4 this sulfide phase sequesters au and ag elements with the same valence as cu in sulfides but not other potentially chalcophile elements such as ni re and pt which suggests that the phase is crystalline rather than an immiscible sulfide melt the relatively high contents of cu and au characteristic of evolved convergent margin magmas requires no enrichment from subducted material instead the association of major cu au deposits with convergent margin magmatism results specifically from the process of magmatic evolution under oxidizing conditions this same property also leads to early magnetite fractionation triggering the abrupt saturation in the cu rich sulfide hence the easily recognizable trend of magmatic evolution under oxidizing conditions i e the sharp drop in chalcophile element concentrations may be an exploration guide to economic au ag cu provinces or a crucial pre enrichment step in the formation of such deposits the decrease in p2o5 and sr at the same stage in the fractionation sequence 60 wt sio2 indicates that saturation in apatite is concomitant with magnetite sulfide saturation in the eastern manus basin

magmatic system geometry at mount st helens modeled from the stress field associated with posteruptive earthquakes earthquakes following the may 18 1980 and june 12 1980 explosive eruptions of mount st helens were concentrated at depths below 6 km the hypocenters of these posteruption earthquakes define two seismic lobes separated by an aseismic volume of 2 km width that is located beneath the position of the crater the timing and location of the posteruption seismicity suggests that the earthquakes occurred in the country rock surrounding a magma body located within the aseismic volume the withdrawal of magma caused a pressure decrease within the reservoir and the earthquakes occurred as a brittle response to the resulting stress change focal mechanism solutions for posteruption earthquakes distributed within the seismic lobes are calculated using polarity and amplitude data results show that a perturbation from the regional stress field occurs between depths of 7 and 11 km this anomalous stress field is successfully modeled by a decrease in pressure within a cylindrical magma body located between the seismic lobes and subject to conditions representing the regional shear stress regime in the best fitting model the magma body is centered at 46 204 n 122 187 w 600 m to the north of the present dome and is under a regional shear stress of 20 bars the radius of the chamber is calculated to be between 0 65 and 0 75 km when using pressure drops within the cylinder of 220 and 150 bars respectively the magma body extends from 7 to 11 km and has a volume of between 5 and 7 km3 posteruption earthquakes located below 11 km fall on a northeast striking fault that is preferentially aligned with the regional tectonic stress regime this fault may be a conduit to transport magma to the shallow reservoir from greater depths within the crust

the susu knolls hydrothermal field eastern manus basin papua new guinea an active submarine high sulfidation copper gold system susu knolls comprises three steep sided conical volcanic peaks standing on a n nw trending ridge in the eastern manus basin a complex zone of convergence between the major indo australian and pacific plates the knolls consist of three porphyritic andesite to dacite domes each 1 0 to 1 5 km in diameter with crests ranging from 1 150 to 1 520 m below sea level an intense hydrothermal plume with peak transmission anomalies in excess of 40 originates from susu knolls and associated hydrothermal venting with fluid temperatures exceeding 300 c and sulfide mineralization has been detected on the crests of all three edifices the most important of these is the 2 47 million metric ton mt solwara 1 copper gold deposit on suzette knoll slated for mining by nautilus minerals the porphyritic dacite is commonly strongly altered by reaction with acidic fluids fragments of sulfide mineralization collected as part of the seafloor talus surrounding the crest of north su and south su are characterized by an assemblage of pyrite fukuchilite enargite covellite chalcopyrite by contrast there are actively venting chimneys on the crest of suzette that are typically zoned with chalcopyrite pyrite tennantite inner zones and barite dominated outer zones the knolls are covered by black sulfidic sediments that contain up to 2 4 wt cu and 3 2 ppm au primary feldspars have been obliterated by the hydrothermal activity that ranges from incipient to intense and which is characterized by natroalunite alunite north su only cristobalite tridymite and rare quartz and kaolinite most volcanic rocks exhibit a patchy surficial coating of native sulfur which may also fill vesicles no altered rocks were dredged from suzette because of the thick sulfidic sediment cover mass balance calculations for altered and unaltered rock from north and south su show that major and trace lithophile elements are depleted in the altered rocks except for ti al si and zr all near immobile in concert with the destruction of the primary minerals and removal of primary components by acidic fluids sulfur and chalcophile trace elements i e as au ba cd cu mo pb and se typically associated with magmatic cu au mineralization are enriched by orders of magnitude in both leached and mineralized rock sulfide and native sulfur 34s values from all three domes range from 7 4 to 0 4 indicating a magmatic component for the sulfur a small group of ore elements i e ag bi in sb and zn are strongly enriched in mineralized breccias and depleted in the altered dacites suggesting redistribution during alteration the presence of advanced argillic alteration the paragenesis of the fe cu as sulfide assemblage and presence of native sulfur together with the sulfur isotope evidence for magmatic input into the hydrothermal fluid at susu knolls are consistent with a submarine high sulfidation magmatic cu au hydrothermal system furthermore the sulfide assemblage pyrite enargite covellite chalcopyrite observed at north and south su and orders of magnitude enrichment of chalcophile elements in both altered and mineralized volcanic rocks relative to unaltered volcanic rock are also characteristic of subaerial high sulfidation epithermal mineralization the susu knolls hydrothermal field is a good example of a modern high sulfidation cu au submarine hydrothermal system

structural and geophysical expression of the solea graben troodos ophiolite cyprus field studies show that extensional structures in the north central part of the troodos ophiolite including steep normal faults grabens dikes and low angle detachment faults are related to e w present coordinates spreading the solea graben directly north of the mafic ultramafic rock outcrops around mount olympus consists of a 10 to 15 km wide western portion that is underlain by a low angle detachment fault between the sheeted dike complex and the plutonic section structural and paleomagnetic evidence indicates that the sheeted dikes above this detachment have been rotated 40 or more about subhorizontal axes the eastern portion of the graben contains sheeted dike blocks with varying axes of rotation and significantly different style of intrusive activity most structural features in the graben formed simultaneously with or shortly after cessation of spreading at the graben axis based on cross cutting ophiolitic dikes and hydrothermal mineralization timing of the uplift of the central part of the troodos ophiolite is uncertain but is probably related to the tectonic thinning of the upper crust by low angle normal faulting the solea graben spreading center provides a model for the structure of modern mid ocean ridges that are spreading amagmatically

causes for large scale metal zonation around mineralized plutons fluid inclusion la icp ms evidence from the mole granite australia detailed reconstruction of the fluid evolution in several ore deposits associated with the sn w and base metal mineralized mole granite shows that multiple factors contributed to the zonal metal distribution around this intrusion information about the mechanism of ore formation was obtained from field relationships detailed petrographic studies and laser ablation inductively coupled plasma mass spectrometry la icp ms microanalysis of single fluid inclusions premineralization fluid inclusions in sn deposits are characterized by high sn w ratios whereas early fluid inclusions from w deposits contain higher w concentrations and comparatively low sn w ratios this indicates that the metal ratio in these deposits already was predefined by the composition of the magmatic input fluid however in both types of deposits the input fluid actually contained much higher concentrations of base metals pb zn cu etc than sn and w demonstrating that metal precipitation was highly selective the metal zonation around the mole granite therefore reflects both a compositional variation of the source fluid as well as sequential metal precipitation from it an additional process contributing to metal fractionation occurred by phase separation of the hydrothermal fluid into saline brine and low density vapor whereby cu b li as s ag la preferentially partitioned into the vapor phase analyses on six assemblages of unambiguously coexisting vapor and brine inclusions allowed direct determination of vapor brine distribution coefficients for 24 elements

direct observation of the evolution of a seafloor black smoker from vapor to brine a single hydrothermal vent f vent occurring on very young crust at 9 16 8 n east pacific rise was sampled in 1991 and 1994 in 1991 at the measured temperature of 388 c and seafloor pressure of 258 bar the fluids from this vent were on the two phase curve for seawater these fluids were very low in chlorinity and other dissolved species and high in gases compared to seawater and most sampled seafloor hydrothermal vent fluids in 1994 when this vent was next sampled it had cooled to 351 c and was venting fluids 1 5 times seawater chlorinity this is the first reported example of a single seafloor hydrothermal vent evolving from vapor to brine the 1991 and 1994 fluids sampled from this vent are compositionally conjugate pairs to one another these results support the hypothesis that vapor phase fluids vent in the early period following a volcanic eruption and that the liquid phase brines are stored within the oceanic crust and vent at a later time in this case 3 years these results demonstrate that the venting of brines can occur in the same location in fact from the same sulfide edifice where the vapor phase fluids vented previously

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 mobilisation of au and other metals in supra subduction oceanic crust insights from the troodos ophiolite the troodos ophiolite is an ideal location to investigate the relationships between the mobilisation of metals by hydrothermal fluids from the lower oceanic crust and the formation of volcanogenic massive sulphide vms deposits the ophiolite hosts the classic cyprus type cu rich vms deposits as well as abundant zones of epidosite alteration in the lower sheeted dyke section that are significantly depleted in base metals including cu and zn and are considered to be the source of the metals enriched in the overlying deposits previous research indicates that the troodos vms deposits are irregularly enriched in au and related elements as sb and se but the behaviour of these elements during the hydrothermal alteration of the troodos ophiolite hitherto has been poorly investigated low detection limit whole rock analyses of fresh glass samples reveal that the troodos primitive crust has a similar metal content and distribution to modern day arc related environments such as the manus basin compared to mid oceanic ridge basalt morb the troodos primitive crust is enriched in as sb and pb most likely due to addition from a subducting slab during crustal formation during early stages of magmatic differentiation 9 3 5 wt mgo au as sb se cu zn and pb behave as incompatible elements due to the sulphide undersaturated nature of the melt the onset of magnetite crystallisation however at 3 5 wt mgo leads to sulphide segregation and depletion of strongly chalcophile elements au cu and se during continued differentiation 3 5 wt mgo whereas poorly chalcophile elements as sb zn and pb remain incompatible these differences in metal behaviour can account for the cu rich zn pb poor of the cyprus type vms deposits as the source area rocks show high cu fertility compared to zn and pb mobilisation of metals during hydrothermal alteration of the troodos ophiolite is more extensive than observed in hydrothermally altered morb mass balance calculations show that the epidosite zones are significantly depleted in au 88 16 as 89 23 sb 60 12 se 91 20 cu 84 18 zn 63 9 and pb 60 8 background altered diabase from outside epidosite zones shows similar metal depletions which suggests that the source areas of vms are not restricted to epidosite zones but are extended to the lower sheeted dyke section the masses of metals mobilised from a source area of 10 9 km3 composed of a 5 km3 epidosite zone and 5 9 km3 of background altered diabase in the solea graben are 47 t au 21 kt as 1200 t sb 3100 t se 2 4 mt cu 1 8 mt zn and 27 kt pb comparison of metal quantities mobilised from lower sheeted dike section in the solea graben with those hosted in vms deposits shows trapping efficiencies ranging from 4 to 37 indicating that most of the metals is lost by other processes

hydrothermal venting at pressure temperature conditions above the critical point of seawater 5 s on the mid atlantic ridge hydrothermal circulation within oceanic crust depends on pressure p and temperature t the critical point cp of seawater at 298 bar and 407 c represents the threshold between subcritical and supercritical conditions here we present data from the first hydrothermal system in which the sampled fluids fall on and above the cp the vent system discovered at 5 s on the mid atlantic ridge is characterized by multiple fluid emanations at variable temperatures in water depths of 3000 m vigorous vapor phase bubbling stable emanation of superhot fluid at 407 c and decreased salinity indicate phase separation at conditions above the cp at one site at another site the measured maximum t of 464 c during a 20 s interval is by far the hottest fluid ever measured at the seafloor and falls into the vapor phase supercritical region of seawater besides these two separate fields with ongoing phase separation and extremely hot fluids a third vent field emanates non phase separated fluids at 349 c and is used as a reference site fluid chemistry shows that supercritical fluids evolve differently than subcritical fluids making this vent system a unique natural laboratory to investigate processes at high p t conditions the stability of the high temperature and fluid geochemistry measured in 2005 and 2006 after the assumed seismic trigger event in 2002 supports this as an exceptional site along the mid atlantic ridge

multi scale reasonable attenuation tomography analysis murat an imaging algorithm designed for volcanic regions abstract the attenuation of body wave amplitudes with propagation distance can be used to provide detailed tomographic images of seismic interfaces fluid reservoirs and melt batches in the crust the high sensitivity of body wave energies to high scattering structures becomes an obstacle when we try to apply attenuation tomography to small scale volcanic media where we must take into account the complexities induced by strong heterogeneous scattering topography and uncertain source modeling in the recorded wave fields the murat code uses a source and site independent coda normalization method to obtain frequency dependent measurements of p to coda and s to coda energy ratios the code inverts these data for both the geometrical spreading factor and the spatially dependent quality factors q providing additional attenuation information in the regions where velocity tomography is available the high sensitivity of coda waves to highly heterogeneous structures highlights zones of anomalous scattering which may corrupt amplitude dependent attenuation measurements and where basal assumptions of linear optics may go unfulfilled a multi step tomographic inversion increases the stability of the results obtained in regions of high heterogeneity e g the volcanic edifice by the inclusion of data corresponding to either sources or stations located in regions of lower heterogeneity on the other hand a mere increase in the number of rays entirely contained in the heterogeneous structures affects both the stability and the effective resolution of the results we apply the code to two small waveform datasets recorded at an active mount st helens and at a quiescent mount vesuvius volcano the results show that the seismicity located inside or under the volcanic edifice produces an increase of the low frequency energy ratios with travel time in both areas in our interpretation the anomalous concentration of energy which affects any waveform recorded on the cone produced inside the volcanic edifice or in the feeding system of the volcano is due to seismic source or medium dependent resonance the results also provide spatial and frequency limits on the feasibility of attenuation tomography in these two regions with larger datasets

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

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

new geochemical evidence for the onset of volcanism in the central atlantic magmatic province and environmental change at the triassic jurassic boundary the late triassic early jurassic was a time of major global change however the fundamental processes driving these changes are less than clear we have determined the re and os abundances and os isotope compositions of marine mudrock samples that span the triassic jurassic boundary in the uk and have established that major isotopic and geochemical shifts in the composition of seawater took place in the latest triassic we argue that these shifts were caused by the sudden initiation of widespread igneous activity within the central atlantic magmatic province associated with rifting of the supercontinent pangea the os isotope composition of seawater responded rapidly to these events demonstrating that the seawater os isotope system has great potential in identifying the nature and precise timing of major environmental change