introduction to juan de fuca ridge special section the u s geological survey and several other institutions have been studying the southernmost segment of the juan de fuca ridge for several years by almost every means available to marine geologists and geophysicists e g normark et al this issue hydrothermal vent areas along the ridge were located by camera tows by the presence of sulfides in dredge samples and by locating hydrothermal plumes by geophysical and geochemical tows clearly the only opportunity remaining for closer study of the vents and for in situ sampling of the sulfide minerals and associated hydrothermal fluids lay in the use of a deep manned submersible vessel accordingly the u s geological survey usgs sponsored a series of dives in the area in the fall of 1984 using dsrv alvin

the cleft revealed geologic magnetic and morphologic evidence for construction of upper oceanic crust along the southern juan de fuca ridge 1 the geology and structure of the cleft segment of the southern juan de fuca ridge jdfr have been examined using high resolution mapping systems observations by remotely operated vehicle rov rov mounted magnetometer and the geochemical analysis of recovered lavas bathymetric mapping using multibeam em300 coupled with in situ observations that focused on near axis and flank regions provides a detailed picture of 0 to 400 ka upper crust created at the southern terminus of the jdfr a total of 53 rock cores and 276 precisely located rock or glass samples were collected during three cruises that included sixteen rov dives our observations of the seafloor during these dives suggest that many of the unfaulted and extensive lava flows that comprise and or cap the prominent ridges that flank the axial valley emanate from ridge parallel faults and fissures that formed in the highly tectonized zone that forms the walls of the axial valley the geochemically evolved and heterogeneous nature of these near axis and flank eruptions is consistent with an origin within the cooler distal edges of a crustal magma chamber or mush zone in contrast the most recent axial eruptions are more primitive higher mgo chemically homogeneous lobate sheet and massive flows that generate a distinct magnetic high over the axial valley we suggest that the syntectonic capping volcanics observed off axis were erupted from near axis and flank fissures and created a thickened extrusive layer as suggested by the magnetic and seismic data this model suggests that many of the lavas that comprise the elevated ridges that bound the axial valley of the cleft segment were erupted during the collapse of a magmatic cycle not during the robust phase that established a new magmatic cycle

seismic evidence for variations in axial magma chamber properties along the southern juan de fuca ridge abstract multichannel seismic data collected along the cleft segment on the southern juan de fuca ridge show that this intermediate spreading center is underlain by a mid crustal reflector interpreted as the top of an axial magma chamber amc the amc reflection is present along most of the segment and deepens gently from 2 0 km near the southern end of the segment beneath the ridge cleft observatory site to 2 3 km at the northern end beneath the site of the mid 1980s submarine eruption we analyzed the one dimensional seismic structure of the amc at two locations with contrasting lava chemistry beneath two different hydrothermal vent fields at the northern site waveform modeling in the time intercept slowness p domain indicates that the amc is 100 m thick and it is characterized by a decrease in p wave velocity from 6 km s to 3 7 km s in contrast the p wave velocity within the shallower 100 m thick amc at the southern site is higher 5 0 km s the decrease in seismic velocity within the amc indicates that it is partially molten and that it is not a cracking front as previously suggested for other intermediate spreading segments the data show a coherent seismic phase interpreted as the p to s wave conversion at the amc pamcs stacking of this event shows that the pamcs is only present along the northern part of the segment our results thus suggest along axis variations in the crystallinity of the amc the amc along cleft varies from a high crystal content

geology of a vigorous hydrothermal system on the endeavour segment juan de fuca ridge a high precision high resolution geologic map explicitly documents relationships between tectonic features and large steep sided sulfide sulfate silica deposits in the vigorously venting endeavour hydrothermal field near the northern end of the juan de fuca ridge water depth in the vent field varies from 2220 to 2200 m location of the most massive sulfide structures appears to be controlled by intersections of ridge parallel normal faults and other fracture fissure sets that trend oblique to and perpendicular to the overall structural fabric of the axial valley the fractured basaltic substrate is primarily composed of well weathered pillow and lobate flows as presently mapped the field is about 200 by 400 m on a side and contains at least 15 large 1000 m3 sulfide edifices and many tens of smaller commonly inactive sulfide structures the larger sulfide structures are also the most vigorously venting features in the field they are commonly more than 30 m in diameter and up to 20 m in height actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller maximum venting temperatures of 375 c are associated with the smaller structures in the southeastern portion of the field highest temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20 30 c lower hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture controlled black smoker activity near the top of the structures a different type of high temperature venting takes place from the upper sides of the structures in the form of overflow from fully exposed quiescent pools of buoyant 350 c vent water trapped beneath overhanging sulfide sulfate silica ledges or flanges these flanges are attached to the upper outer walls of the large sulfide edifices two types of diffuse venting in the endeavour field include a lower temperature 8 15 c output through colonies of large tubeworms and 25 50 c vent fluid that seems to percolate through the tops of overhanging flanges the large size and steep walled nature of the these structures evidently results from sustained venting in a mature hydrothermal system coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development

water column hydrothermal plumes on the juan de fuca ridge hydrographic surveys on the juan de fuca ridge jdfr carried out from 1980 to 1987 show a complex pattern of sup 3 he and mn rich water column plumes produced by venting from several submarine hot spring areas in the vicinity of axial volcano at latitude 46 degree n distinct plumes were detected in 1980 1982 and 1983 with sup 3 he signatures up to delta sup 3 he 64 at approximately 1 500 m depth at distances of approximately 10 km from the seamount summit however the same plumes had no detectable thermal signature a paradox which is attributed to the high sup 3 he heat ratios and low salinities of the fluids venting within the caldera of axial volcano profiles directly over the seamount show hydrothermal sup 3 he in the water column up to 300 m above the caldera floor with the sup 3 he signal increasing with depth to very high and uniform ratios of delta sup 3 he 108 150 below the approximately 1 500 m caldera sill depth another apparent locus of hydrothermal input is helium basin a depression on the northeast flank of axial volcano which had delta sup 3 he 51 when first sampled in 1980 however subsequent hydrocasts into helium basin in 1982 and 1983 yielded lowermore helium enrichments suggesting either a decrease in hydrothermal input or flushing of the basin via a mixing event to the south of axial volcano high delta sup 3 he values of approximately 40 observed over the ridge axis at 45 degree 18 prime n and 45 degree 39 prime n indicate venting on this previously unexplored section of the ridge the water column plumes over the us geological survey vent site at approximately 44 degree 40 prime n on the southern jdfr have very high mn sup 3 he ratios of 4 600 mol cm sup 3 an apparently unique characteristic which can be used to distinguish these plumes from those originating at other jdfr vent fields less

geochemistry of some gases in hydrothermal fluids from the southern juan de fuca ridge five samples of hydrothermal fluids from two vent areas on the southern juan de fuca ridge were analyzed for dissolved gases concentrations in the end member hydrothermal fluid of h2 270 527 mol kg ch4 82 118 mol kg and co2 3920 4460 mol kg are well above values in ambient seawater and are similar to concentrations reported for other ridge crest hydrothermal systems the carbon isotopic ratios of the ch4 13c 17 8 to 20 8 and co2 13c 3 6 to 4 7 suggest that at least some of the ch4 and co2 in the fluids is basalt derived the range of 13c values for the basalt derived co2 is 6 8 to 9 7 calculated by assuming conservation of recharge co2 during hydrothermal circulation apparent temperatures of equilibration between the ch4 and the basalt derived co2 range from 640 c to 750 c small amounts of ethane c2h6 ch4 0 9 10 3 2 2 10 3 propane and butane detected in the samples may also have formed in the basalt one sample of almost pure 95 5 hydrothermal fluid contained a significant fraction up to 63 and 74 respectively of the recharge ar and n2 this suggests that the fluid has not undergone extensive vapor liquid phase separation

axial summit trough of the east pacific rice 9 10 n geological characteristics and evolution of the axial zone on fast spreading mid ocean ridge the nature and morphological characteristics of axial summit troughs on fast 90 130 mm yr 1 full spreading rate and superfast spreading 130 mm yr 1 mid ocean ridge crests reflect the time integrated effects of long term magmatic cycles short term volcanic episodicity and the tensional stress regime imposed on young ocean crust two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid ocean ridge crests 1 narrow axial troughs 300 2000 m wide and 30 100 m deep on the east pacific rise crest are classified as axial summit graben the dimensions of axial summit graben as well as the morphological and structural character of their walls and floors suggest a primary tectonic origin an axial summit graben may contain a nested axial summit collapse trough implying that processes responsible for these endemic features may be linked near bottom side looking sonar and observational data collected using the towed vehicle argo i and submersible alvin have been used to characterize the axial summit trough of the fast spreading east pacific rise between 9 and 10 n a four stage model is presented for the evolution of this axial summit collapse trough as well as for other well studied portions of the east pacific rise crest from 21 n to 20 s we propose that the transition from a narrow surface collapse dominated axial trough to a broader fault bounded graben is controlled by the relative importance of diking volcanism hydrothermal cooling and tectonism along a ridge segment over time periods 104 years

archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec part i geologic relations and formation of the vein system the sigma deposit consists of an archean gold bearing vein system in metavolcanic rocks intruded by an irregular body of porphyritic diorite both affected by regional deformation and in turn cut by younger undeformed feldspar porphyry dikes the veins cut all rock types and are composed chiefly of quartz and tourmaline with minor amounts of carbonates pyrite chlorite scheelite and free gold they were emplaced during or after a greenschist facies metamorphism postdating the intrusion of feldspar porphyry dikes the veins were not significantly affected by any subsequent metamorphism or tectonism two principal types of veins occur in the mine subvertical veins and subhorizontal veins they are contemporaneous and cogenetic subvertical veins occur within steeply dipping east west trending ductile shear zones along which reverse subvertical displacements have taken place the veins occupy openings created by overriding of undulations and irregularities of foliation and slip planes during progressive shearing movements within the shears subhorizontal veins occupy extensional fractures which developed between ductile shear zones preferentially in the more competent host rocks these veins were formed by one or several episodes of open space filling geologic relations indicate that the mineralized veins were emplaced progressively in a dynamic tectonic environment during and shortly after the formation of ductile shear zones a structural analysis has shown that the ductile shear zones and the two vein types can be related to a single strain ellipsoid of the following orientation the z axis trending north south and horizontal the y axis plunging 10 degrees west and the x axis plunging 80 degrees east the deformation responsible for the generation of ductile shear zones and emplacement of the veins is ascribed to a late north south trending compression postdating regional deformation

temporal and spatial variability of hydrothermal manganese and iron at cleft segment juan de fuca ridge a unique data set for hydrothermal mn and fe was collected at cleft segment on the juan de fuca ridge between 1983 and 1991 the data set includes observations of focused and diffuse venting fluids and neutrally buoyant plumes formed by chronic and episodic venting manganese heat and iron heat ratios for plumes from the north end of the cleft segment were combined with independently determined estimates of plume heat flux to yield annually averaged chronic venting fluxes for mn of 0 36 0 17 mol s 1 and for fe of 0 61 0 34 mol s 1 over 6 years of plume measurements at north cleft segment observed episodic hydrothermal discharge accounted for 15 of the total vented mn and 35 of vented fe the chronic fluxes for mn and fe at a second venting center located at the south end of the cleft segment were estimated to be approximately equal to the fluxes at north cleft segment chronic plumes at north cleft segment are mixtures of focused and diffuse discharge that contribute heat mn and fe in variable proportions similar examination of south cleft segment data strongly suggests the presence of an as yet unobserved venting source relatively depleted in mn and fe but contributing substantially to the overall heat temporal and spatial variations in the concentrations of mn and fe and in mn heat and fe heat ratios for focused seafloor vents were difficult to resolve within complex chronic plumes manganese heat and iron heat ratios of megaplumes suggest they may have derived from reservoirs of diffuse fluids while smaller event plumes may have formed by different processes and have properties similar to chronic plumes the accurate assessment of segment scale hydrothermal fluxes of mn and fe requires coordinated measurements of representative seafloor sources and the neutrally buoyant plume that integrates all seafloor discharge

manganese microchimneys buried in the central pacific pelagic sediments evidence of intraplate water circulation abstract manganese oxide deposits of an irregular tube like shape with unique composition and structure were found within pliocene siliceous sediments from an abyssal hill in the central pacific manganese nodule province effective separation of mn from fe and probably a rapid rate of mn deposition are suggested from the chemical composition a very high mn fe ratio up to 400 and low concentrations of transition metals the ree pattern is similar to other marine low temperature hydrothermal and sub oxic diagenetic mn deposits low total ree abundance and strong negative ce anomaly the deposit consists of major todorokite with minor buserites showing a hydrothermal signature blade like crystals of the minerals typically grow within open tubes several mm thick and lack detrital minerals all data indicate a low temperature hydrothermal or cold seep origin or less likely a sub oxic diagenetic origin this unusual manganese deposit from a pelagic environment is thought to have formed on the sea floor from mn bearing waters that passed through sediments or basement rocks we speculate that intraplate water circulation took place during the late pliocene to pleistocene due to the uplift and or block faulting after the deposition of pelagic pliocene sediments

an ocean bottom gravity study of the southern juan de fuca ridge we use seafloor and sea surface gravity data to model density structure along the southern juan de fuca ridge we determine the average density of the shallow oceanic crust at the ridge using seafloor gravity measurements and then use these data in conjunction with sea surface gravity observations to construct density structure models of 63 seafloor gravity measurements obtained 42 observations were distributed over the 3 km left lateral overlapping rift zone orz located at latitude 45 03 n and separating the juan de fuca ridge into the cleft and vance segments a 21 measurement seafloor gravity transect was made perpendicular to the ridge strike at latitude 44 52 n a region of linear ridge geometry on the cleft segment these seafloor gravity measurements which are sensitive to shallow crustal density variations were augmented by roughly 800 km of sea surface gravity measurements using a bathymetry bouguer anomaly analysis of the seafloor gravity data we determined the average density of the shallow roughly upper 2 km oceanic crust at these two locations to be 2630 kg m 3 50 kg m 3 within the uncertainties of the density determinations there is no difference between the average shallow oceanic crustal density at the linear cleft segment and at the cleft vance orz using the seafloor measurements we modeled the study area s underlying density to provide constraints on its fine scale structure localized porosity of up to 17 within the upper 500 m of oceanic crust layer 2a provide a possible explanation for the observed gravity anomalies at the cleft segment two dimensional density models of the crust underlying the linear cleft segment 44 52 n show that no deep source is required to explain the topographical asymmetry observed between the east and west sides of the ridge axis for the cleft vance orz three dimensional modeling suggests low density material between the rifts in agreement with thickened layer 2a from seismic observations in contrast layer 2a variations do not explain the low density inferred at the northern end of the cleft segment since seismic observations suggest a thin layer 2a the magmatic activity recently observed in this region may have a distinct deeper source

seismic structure and anisotropy of the juan de fuca ridge at 45 n a seismic refraction experiment was conducted with air guns and ocean bottom seismometers on the juan de fuca ridge at 45 n at the northern cleft segment and at the overlapping rift zone between the cleft and vance segments these data determine the average velocity structure of the upper crust and map the thickness variability of the shallow low velocity layer which we interpret as the extrusive volcanic layer the experiment is unique because a large number of travel times were measured along ray paths oriented at all azimuths within a small 20 km by 35 km area these travel times provide evidence for compressional velocity anisotropy in the upper several hundred meters of oceanic crust presumed to be caused by ridge parallel fracturing compressional velocities are 3 35 km s in the ridge strike direction and 2 25 km s across strike travel time residuals are simultaneously inverted for anisotropy as well as lateral thickness variations in the low velocity layer extrusive layer thickness ranges from approximately 200 m to 550 m with an average of 350 m the zone of the thinnest low velocity layer is within the northern cleft segment axial valley in a region of significant hydrothermal activity layer thickness variability is greatest near the cleft vance overlapping rift zone where changes of 300 m occur over as little as several kilometers laterally these low velocity layer thickness changes may correspond to fault block rotations in an episodic spreading system where the low side of each fault block accumulates more extrusive volcanics

crustal fissuring and its relationship to magmatic and hydrothermal processes on the east pacific rise crest 9 12 to 54 n the deep towed argo i optical acoustical vehicle and a geographic information system gis have been used to establish the abundance widths and spatial distribution of fissures as well as the relative age distribution of lavas along the narrow 500 m wide axial zone of the east pacific rise epr from 9o12 to 9o54 n on a second order scale 78 km long wider but less numerous fissures are found in the northern portion of the survey area this changes to narrower more abundant fissures in the south a profile of the cumulative width added by fissures to the axial zone exhibits minima in three areas along strike near 9o49 9o35 and 9o15 n where the most recent eruptions have occurred above sites of magmatic injection from the upper mantle filling and covering older fissures on a fourth order scale 5 15 km long the mean density of fissuring on a given segment is greater where relative axial lava age is greater fissure density also correlates with hydrothermal vent abundance and type increased cracking toward segment tips is observed at the second order scale whereas fourth order segments tend to be more cracked in the middle cracking on a fourth order scale may be driven by the propagation of dikes rather than by the far field plate stresses the above relations constrain the model of haymon et al 1991 in which individual fourth order segments are in different phases of a volcanic hydrothermal tectonic cycle

changing brine inputs into hydrothermal fluids southern cleft segment juan de fuca ridge author s wheat cg zierenberg ra paduan jb caress dw clague da chadwick ww abstract 2020 the authors in 2016 temperature recorders were recovered temperatures were measured and fluid samples were collected from vent 1 a high temperature 338 c hydrothermal discharge site on the southern cleft segment of the juan de fuca ridge coupled with previous sampling efforts this collection represents a 32 year record of discharge from a single chimney structure the longest record to date remarkably the fluid has remained brine dominated for more than three decades this brine formed during phase separation and segregation prior to initial observations in 1984 although the chloride concentration of the discharging fluid has decreased with time the fluid temperature has remained nearly constant for at least 3 3 years and probably for 15 or even 22 years compositions of the discharging fluids are consistent with inputs from a deep sourced brine which was last equilibrated at g400 c at a depth consistent with the base of the sheeted dikes and the brittle ductile transition this brine mixed diffusion or dispersion with a likely non phase separated hydrothermal fluid prior to discharge a survey of hydrothermal endmember fluids with chlorinities in excess of 700 mmol kg shows with the exception of fe a single trend between major ion concentrations and chlorinity even though data are from a range of crustal compositions spreading rates and water and magma depths calculated deep sourced brines from hydrothermal fluid data are similar to data based on fluid inclusions and estimates of brine assimilation in magmas a better understanding of brines is required given their potential duration of discharge and capacity for mobilizing metals

development of conjugate shear bands during bulk simple shearing abstract in rocks possessing a strong planar fabric shear bands of constant shear sense and oriented at an oblique angle to the foliation are considered by many authors to be characteristic of a non coaxial bulk deformation history whereas conjugate shear bands are considered to indicate coaxial shortening however in two areas where bulk deformation history appears to be non coaxial cap corse corsica and ile de groix brittany conjugate shear bands are observed in order to investigate this problem experiments were performed by bulk simple shearing using plasticine as a rock analogue when slip between layers of the model is permitted shear bands of normal fault geometry form with both the same and opposite shear sense as the bulk simple shearing at approximately the same angle with the layering 40 irrespective of layer orientation in the undeformed state for initial orientations of 50 30 and 15 shear bands are initially formed within individual layers and may propagate across layer interfaces when further movement along these is inhibited the existence of conjugate shear bands in corsica and ile de groix is therefore not incompatible with a model of bulk simple shearing for these two regions in field studies one should perhaps exercise care in using shear bands to determine the kind of motion or the sense of bulk shearing

high resolution studies of the summit of axial volcano sea beam bathymetry sea marc i side scan 30 khz towed camera and submersible data from axial volcano juan de fuca ridge jdfr have been integrated to produce detailed maps of the volcano s summit region in order to elucidate its volcanic and tectonic evolution the proximity of axial volcano the youngest edifice of the cobb eickelberg seamount chain to the jdfr superposes a hotspot derived radial stress pattern onto the linear stress field of a mid ocean ridge spreading center the morphology of the summit of axial volcano reflects the differing eruptive styles produced from this superposition long rift zones begin at the edges of the caldera and extend as much as 75 km to the north and south axial volcano has a distinct summit plateau similar to those found on many other submarine volcanos the process by which summit plateaus are constructed is illustrated by the lava flows originating from the south rift zone which have partially filled the southern half of the caldera and have smoothed the flanks the summit plateau contains penetrative ne sw fracturing that is approximately parallel to the jdfr the unusual rectangular caldera is 3 8 km in size and its trend is about 160 three possible mechanisms are discussed for the caldera s formation 1 canary island type trapdoor formation 2 overlapping spreading center interaction and 3 magma source migration although none of these models is fully compatible with the present data base the overlapping spreading center model is preferred postcaldera volcanism probably began with the formation of the central caldera eruptive complex which was characterized by relatively low volume high viscosity flows the most recent volcanism has apparently taken place along the northern and southern rift zones which have erupted more fluid lavas resulting in areally extensive flows covering up to 60 of the caldera floor extensive low temperature venting and areas of high temperature venting near the northern and southern rift zones and caldera walls suggest a shallow magmatic source beneath the summit of axial volcano

a 6 year time series of hydrothermal plumes over the cleft segment of the juan de fuca ridge the causal links between magmatic tectonic processes and hydrothermal venting along the mid ocean ridge axis will be difficult to discern without broadband observations of the temporal variability of hydrothermal activity i report here on temporal variability in the intensity and distribution of chronic and event plumes generated by continuous and episodic hydrothermal discharge at two vent fields on the cleft segment of the juan de fuca ridge hydrothermal temperature and light attenuation c anomalies were comprehensively mapped between 1 and 4 times per year during nine cruises from 1986 to 1991 intensity of and c in the chronic plume overlying the north cleft vent field showed more variability between years than within a given year both and c were exceptionally high in 1986 fell to moderate levels during 1988 to 1990 and declined to minimum values in 1991 the chronic plume over the south cleft vent field varied similarly superimposed on the chronic plumes were brief and intense releases of hydrothermal fluid that created event plumes distinguishable by their symmetry and exceptional rise height event plumes at the north cleft field ranged from the 1986 megaplume with a rise height exceeding 1000 m to smaller events in 1987 and 1989 with rise heights of 400 m chronic hydrothermal heat 660 277 1 mw and mass 204 86 1 g s 1 fluxes from the north cleft field were determined by assuming these fluxes were equivalent to the long term net advection of a mean cross section of the vertically integrated and c in the plume this 6 year history of hydrothermal plumes indicates that episodic discharge has been an important component of the recent hydrothermal flux and that the aggregate heat and mass fluxes declined significantly though not consistently between 1986 and 1991

comparative mineralogy and geochemistry of gold bearing sulfide deposits on the mid ocean ridges a comparative study of the mineralogy and geochemistry of sulfide deposits on mid ocean ridges in the northeast pacific and the mid atlantic reveals common characteristics associated with primary gold enrichment average gold contents of 0 8 to 5 ppm au occur in sulfides from southern explorer ridge and axial seamount northeast pacific and from the tag hydrothermal field and snakepit vent field mid atlantic ridge the enrichment of gold in these deposits is consistently related to a phase of late stage low temperature 1 ppm au occur exclusively in pyritic assemblages and commonly with abundant fe poor sphalerite and a suite of complex pb sb as sulfosalts amorphous silica and locally barite or carbonate are important constituents of the gold rich precipitates but do not contain gold themselves high temperature 350 c black smoker assemblages consisting dominantly of pyrite chalcopyrite pyrrhotite isocubanite and abundant anhydrite are uniformly gold poor 0 2 ppm au to the extent that individual sulfides can be mechanically separated chemical analyses by neutron activation indicate that gold is most abundant in sphalerite up to 5 7 ppm au but also occurs in pyrite and marcasite samples of sphalerite with abundant inclusions of fine grained sulfosalts locally contain up to 18 ppm au suggesting that sulfosalts may be repositories for gold no free gold has been observed at 4000 magnification of polished specimens indicating that the gold is present only as submicroscopic inclusions or as a chemical constituent within the sulfides samples from gold rich deposits in the northeast pacific and mid atlantic are compared with similar but relatively gold poor sulfides from the galapagos rift and 13 n on the east pacific rise epr and with barren sulfides from 11 n epr 21 n epr the endeavour ridge and the southern juan de fuca ridge trace element analyses of more than 170 samples show that gold enrichment in almost all of the deposits is associated with high concentrations of ag as sb pb and zn and locally with high cd hg tl and ga in contrast gold is typically depleted in samples with high co se and mo the close association of au with ag as sb and pb may reflect the common behavior of these metals as aqueous sulfur complexes e g au hs 2 at low temperatures similar mineralogical and geochemical associations are observed in sulfide deposits from modern back arc settings and in the ancient geologic record

volcanic and structural morphology of the south flank of axial volcano juan de fuca ridge results from a sea marc i side scan sonar survey a 1500 km2 sea marc i side scan sonar survey south of axial volcano investigated the geological structure and constitution of the south axial rift zone sarz and the northern 30 km of the vance spreading segment relative age assignments based on structural relationships indicate that recent volcanism on the southern flank of axial volcano has been restricted to the sarz the surficial volcanic morphology of the sarz changes downrift from linear volcanic ridges north to small 1 km diameter cratered cones south perhaps indicating a variation in eruptive vent geometry from fissures to point sources downrift recent lavas on the northern and central sarz erupted along preexisting faults in the underlying crust by acting as pathways for sarz magmas these faults may have controlled the orientation of sarz volcanic ridges and perhaps the orientation of the entire sarz edifice the vance spreading segment terminates at 45 40 n and is not linked with a transform fault the axis of spreading between 45 40 n and 46 00 n is offset west of the vance segment and is constrained to lie between 129 54 w and 130 06 w a discrete structurally defined spreading axis however is not evident over the southern flank of axial volcano the divergent plate boundary may now underlie the sarz massif although south helium basin an embayment in the southeast flank of axial volcano has undergone and may continue to accommodate crustal extension a 48 km2 lava field discovered east of the sarz is composed of lavas that are inferred to have erupted from the sarz these lavas can be traced eastward from the sarz into the axial valley of the northern vance segment and partially fill the valley with lavas up to 60 m thick the eruption of these lavas which occupy an estimated volume of 1 8 km3 may have contributed to the formation of axial volcano s summit caldera

manganiferous garnet underlying the bousquet gold orebody quebec metamorphosed manganese sediment as a guide to gold ore garnet quartz muscovite rock stratigraphically underlies a volcanogenic pyritic gold orebody within bousquet township abitibi greenstone belt quebec garnet constitutes only 3 of the rock within muscovite rich layers near its east and west ends whereas the rock contains 12 of garnet near its center trails of quartz carbonate and sulphide inclusions and parallel bands of an ultrafine black opaque mineral produce a layered appearance in the garnet garnet consists of 25 mno and is zoned with maximum mno content at its center furthermore mno content is greatest in garnet near the center and stratigraphic top of the rock and decreases systematically to the east west and stratigraphic base pb zn and ba exhibit similar enrichment trends from stratigraphic base to top of the rock however ba remains abundant in the overlying orebody the interpretation is that the garnet rock was a primary mn rich sediment that accumulated near a sea floor hydrothermal vent by analogy with modern hydrothermal mn d

composition and dissolution of black smoker particulates from active vents on the juan de fuca ridge during two atlantis ii alvin cruises to the juan de fuca ridge in 1984 active high temperature 140 284 c vents were sampled for black smoker particulates using the grassle pump individual mineral phases were identified using standard x ray diffraction and petrographic procedures in addition elemental compositions and particle morphologies were determined by x ray energy spectrometry and scanning electron microscope x ray energy spectrometry techniques the vent particulates from the southern juan de fuca ridge vent sites were highly enriched in s si fe zn and cu and were primarily composed of sphalerite wurtzite pyrite pyrrhotite barite chalcopyrite cubanite hydrous iron oxides and elemental sulfur two additional unidentified phases which were prevalent in the samples included an fe si phase and a ca si phase the grain sizes of the individual particle phases ranged from 100 m for the fe si particles grain size and current meter data were used in a deposition model of individual phase dispersal for many of the larger sulfide and sulfate particles the model predicts dispersal to occur over length scales of only several hundreds of meters the high temperature black smokers from the more northerly endeavour segment vents were highly enriched in fe s ca cu and zn and were primarily composed of anhydrite chalcopyrite sphalerite barite sulfur pyrite and other less abundant metal sulfide minerals the grain sizes of the individual particles ranged from 10 m to slightly larger than 500 m the composition and size distributions of the mineral phases are highly suggestive of high temperature mixing between vent fluids and seawater a series of field and laboratory studies were conducted to determine the rates of dissolution of several sulfate and sulfide minerals the dissolution rates ranged over more than 3 orders of magnitude from 3 2 10 8 cm s 1 for anhydrite to 1 2 10 12 cm s 1 for chalcopyrite the results indicate that for some minerals particularly anhydrite and marcasite total dissolution occurs within a few hours to a few weeks of their formation for other more stable minerals including pyrite sphalerite and chalcopyrite the time required for total dissolution is much longer and consequently individual crystals may be expected to persist in the sediments for considerable periods of time after deposition

the influence of glacier surface processes on the short term evolution of supraglacial tree vegetation the case study of the miage glacier italian alps supraglacial debris cover allows vegetation to colonize glacier surface and whenever it is enough stable and thick also shrubs and trees can germinate and grow supraglacial tree growth and distr

no spreading across the southern juan de fuca ridge axial cleft during 1994 1996 direct path acoustic measurements between seafloor transponders observed no significant extension 10 14 mm yr from august 1994 to september 1996 at the southern juan de fuca ridge 44 40 n and 130 20 w the acoustic path for the measurement is a 691 m baseline straddling the axial cleft which bounds the pacific and juan de fuca plates given an expected full spreading rate of 56 mm yr these data suggest that extension across this plate boundary occurs episodically within the narrow 1 km region of the axial valley floor and that active deformation is occurring between the axial cleft and the plate interior a cleft parallel 714 m baseline located 300 m to the west of the cleft on the pacific plate monitored system performance and as expected observed no motion 5 7 mm yr between the 1994 and 1996 surveys

strain pattern in the aar granite central alps orthogneiss developed by bulk inhomogeneous flattening the central part of the aar granite aar valley shows lens shaped domains of low strain separated by anastomosing domains of high strain a comparable pattern is found on all scales of observation deformation gradients are outlined by a gradual change from isotropic granite to orthogneiss and locally ultramylonites ultramylonites are concentrated within conjugate ductile shear zones where moderate the strain is concentrated in very narrow small scale faults which affect relatively undeformed granite within foliated domains finite strain measurements on deformed xenoliths indicate that i strain intensity increases gradually with grain size reduction and ii finite strain ellipsoids are of flattening type irrespective of the degree of mylonitization the pattern of small scale faulting the trends of the regional foliation and the orientation of ductile shear zones are consistent with a bulk strain field of flattening type throughout progressive deformation

submersible observations along the southern juan de fuca ridge 1984 alvin program in september 1984 the research submersible alvin provided direct observations of three major hydrothermal vent areas along the southernmost segment of the juan de fuca ridge jfr the submersible operations focused on specific volcanologie structural and hydrothermal problems that had been identified during the preceding 4 years of photographic dredging acoustic imaging and geophysical studies along a 12 km long section of the ridge a continuously maintained from 1981 to the present net of seafloor anchored acoustic transponders allowed the observations from alvin to be directly tied to all previous u s geological survey data sets and national oceanic and atmospheric administration water column surveys from 1984 to the present the three vent areas studied are the largest of at least six areas identified by previous deep towed camera surveys that lie within a deep cleft which marks the axis of symmetry of the jfr in this region the cleft appears to be the locus of eruption for this segment of the jfr the vent areas at least in part are localized near what appear to be previous volcanic eruptive centers marked by extensive lava lake collapse features adjacent to the cleft at these sites each hydrothermal area has several active discharge sites and sulfide deposits occur as clusters 15 100 m2 of small chimneys individual large chimneys or clusters of large branched chimneys we review the dive program and present a brief synthesis of the geology of the vent sites together with sample and track line compilations

the effect of hydrothermal processes on midwater phosphorus distributions in the northeast pacific abstract the distributions of dissolved particulate and sedimentary phosphorus were measured in the region of the juan de fuca ridge to determine the impacts of hydrothermal processes on the phosphorus cycle in the oceans significant negative dissolved phosphate anomalies ranging from 0 to 60 nmol l were observed in the water column at depths between 1900 and 2300 db the largest anomalies

craton scale distribution of archean greenstone gold deposits predictive capacity of the metamorphic model genetic models should not only attempt to explain features of archean gold mineralization on the scale of mines or districts but should also explain its marked spatial and temporal heterogeneity in greenstone belts the application of genetic models developed with such a philosophy can have predictive capacity in gold exploration the metamorphic model one of a number of current genetic models for archean gold deposits involves generation of auriferous h 2 o co 2 fluids by devolatilization of the stratigraphically lowermost greenstones during metamorphism and broadly synchronous synkinematic granitoid intrusion crustal scale fault shear systems are required to induce channeled fluid flow and to focus ore fluids toward suitable host rocks and structural sites declining temperature below the amphibolite greenschist transition combined with sulfidation of fe rich host rocks is a particularly efficient au depositing mechanism the model suggests that the major controlling factors in declining order of importance are 1 high temperature low pressure generally low strain metamorphic regimes 2 crustal scale fault systems 3 unaltered fe rich rocks tholeiites banded iron formation at a suitable stratigraphic metamorphic level and 4 abundant au enriched komatiites and s rich sediments in source regions the low pressure metamorphic conditions combined with the occurrence of elongate high strain zones in most if not all generally low strain greenstone belts may explain the ubiquitous occurrence of gold deposits in archean terranes all four favorable parameters appear to act in concert to produce the largest gold deposits and most abundant gold mineralization in 2 7 or 0 1 b y old rift phase greenstone belts interpreted to have formed under conditions of high crustal extension and extensive crustal thinning i e with predicted high geothermal gradients high fault density limited synvolcanic alteration and abundant komatiites and sulfidic sediment horizons more widespread commonly older platform phase greenstone belts which formed under conditions of lesser crustal extension and thinning are less mineralized as predicted by the metamorphic model the ability of the model to explain the regional distribution of known archean gold deposits lends credence to its predictive capacity for example 1 greenstone belts in any one craton can be broadly ranked in order of likely relative gold potential 2 the likelihood of discovery of large deposits in poorly exposed or poorly explored belts of known type can be estimated 3 areas with the highest potential can be defined in rift phase greenstone belts by the coincidence of critical parameters such as specific host rock sequences and metamorphic facies 4 prospective structural sites for gold mineralization can be postulated in platform phase greenstone belts affected by diapiric tectonics and 5 the possible spacing of large deposits can be estimated further the potential use of the model in exploration emphasizes serious gaps in our knowledge of parameters crucial to understanding greenstone belt metallogeny including alteration patterns metamorphic p t paths and regional scale structural regimes

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

compositions growth mechanisms and temporal relations of hydrothermal sulfide sulfate silica chimneys at the northern cleft segment juan de fuca ridge three active hydrothermal vents forming sulfide mounds and chimneys monolith fountain and pipe organ and more widely distributed inactive chimneys are spatially related to a system of discontinuous fissures and young sheet flow lavas at the northern cleft segment juan de fuca ridge the formation of zoned tubular curich chimneys type i on the monolith sulfide mound is related to focused flow of high temperature to 328 c fluid bulbous chimneys type ii or beehives at the monolith and fountain vents are products of diffuse high temperature to 315 c discharge a broader zone of vigorous mixing between the hydrothermal fluid and seawater results in quench crystallization of anhydrite rich shells columnar zn sulfide rich chimneys with narrow channelways type iii are constructed where focused and relatively low temperature 261 c fluid vents directly from the basalt substrate the bulk chemistry low cu high pb ag and sio2 contents mineralogy pyrite marcasite wurtzite amorphous silica anglesite colloform and filamentous textures and oxygen isotope characteristics of inactive type iv chimneys indicate a low temperature 250 c origin involving diffuse and sluggish flow patterns and conductive cooling seafloor observations and 210pb data indicate that 1 type iv chimneys are products of an earlier period of hydrothermal activity that ended no more than 60 years ago but prior to the sheet flow eruption 2 the high temperature monolith and fountain vents are manifestations of the same heating event shallow emplacement of magma that led to the sheet flow eruption and recent megaplumes and 3 the pipe organ vent is in a very youthful stage of development and chimney deposition postdates the sheet flow eruption

frozen bed fennoscandian and laurentide ice sheets during the last glacial maximum the areal extents of the laurentide and fennoscandian ice sheets during the last glacial maximum about 20 000 years ago are well known1 but thickness estimates range widely from high domed2 to thin3 with large implications for our reconstruction of the climate system regarding for example northern hemisphere atmospheric circulation and global sea levels this uncertainty stems from difficulties in determining the basal temperatures of the ice sheets and the shear strength of subglacial materials4 a knowledge of which would better constrain reconstructions of ice sheet thickness here we show that in the absence of direct data the occurrence of ribbed moraines in modern landscapes can be used to determine the former spatial distribution of frozen and thawed bed conditions we argue that ribbed moraines were formed by brittle fracture of subglacial sediments induced by the excessive stress at the boundary between frozen and thawed bed conditions resulting from the across boundary difference in basal ice velocity maps of glacial landforms from aerial photographs of canada and scandinavia reveal a concentration of ribbed moraines around the ice sheet retreat centres of quebec keewatin newfoundland and west central fennoscandia together with the evidence from relict landscapes that mark glacial areas with frozen bed conditions the distribution of ribbed moraines on both continents suggest that a large area of the laurentide and fennoscandian ice sheets was frozen based and therefore high domed and stable during the last glacial maximum

lateglacial nunataks in central scandinavia biostratigraphical evidence for ice thickness from lake flafattjonn tynset norway abstract this study reveals for the first time biostratigraphical evidence of nunataks emerging from the late weichselian ice sheet in central scandinavia almost 1 m of minerogenic basal layers from lake flafattjonn 1110 m a s l at tynset norway was deposited in the lateglacial possibly as early as 16 000 cal yr bp the sediments contain well preserved remains of plants diatoms and chironomids and indicate 1 a mosaic pioneer vegetation of dwarf shrub tundra snow bed vegetation and xerophytes on wind blown ridges 2 an ice free lake during summers with clear water at low ph due to little inwash 3 a continental climate with summers slightly warmer than today ca 10 c and most probably with very cold winters according to indications of permafrost and 4 a hiatus reduced sedimentation during at least the first half of the younger dryas when the lake was sealed by a more or less permanent ice cover the existence of lateglacial nunataks indicates a thin and multi domed lateglacial ice sheet in scandinavia early migration of arctic alpine plants to nunataks could explain their centric distribution in s norway furthermore vegetation establishment on the flafatet lateglacial nunatak indicates that s norway could have been vegetated by successions along the retreating ice from alpine nunataks and from coastal and southern areas the upland and lowland vegetation met at the final deglaciation

uniformity and diversity in the composition of mineralizing fluids from hydrothermal vents on the southern juan de fuca ridge abundances of li ni k rb ca sr ba mn fe zn and si have been determined in fluid samples from seven vents located in three areas on the southern juan de fuca ridge the hydrothermal component estimated from the mg contents of the samples ranges from 7 to 76 concentrations of fe and si among the other elements in acid stabilized solutions appear to be generally representative of the parental hydrothermal fluids but some zn determinations and most ba values appear to be too low thermodynamic calculations indicate that the acidified samples remain supersaturated with respect to silica barite and pyrite unacidified samples are supersaturated in addition with respect to zns fes and many silicate phases within the constraints of limited sampling there appear to be differences in fluid compositions both within and between the three vent areas some uniform differences in the elemental abundances predicted for hydrothermal end member fluids might be due to inmixing of fresh seawater at depth in the hydrothermal system the juan de fuca hydrothermal fluids contain more fe but otherwise have relative elemental abundances fairly similar to those in 13 sup 0 n east pacific rise fluids albeit at higher levels in contrast fluids from 21more sup 0 n east pacific rise and galapagos have lower k rb and much lower sr and na abundances these compositional features probably result from interaction of these fluids with a different mineral assemblage possibly more mature greenstone copyright american geophysical union 1987 less

chemistry of hydrothermal solutions from the southern juan de fuca ridge fluids from three vent fields on the southern juan de fuca ridge were sampled in september 1984 using the dsrv alvin the fluids are uniquely high in both chloride which ranges up to twice the seawater value and in metal content simple vapor liquid phase separation could not have produced both the high chlorinity and gas concentrations observed in these fluids the cause of the elevated chlorinity can not be uniquely identified but may be the result of either or a combination of two processes 1 subsurface formation of a degassed brine and subsequent mixing of a small amount of this brine with a hydrothermal seawater which has not undergone a phase separation and 2 dissolution of a chloride rich phase combined with a possible small loss of gas during sampling procedures although measured temperatures were all less than 300 sup 0 c quartz geothermometry suggests that the fluids have equilibrated at greater than 340 sup 0 c quartz geobarometry is also in agreement with geophysical estimates of depth to the local magma chamber copyright american geophysical union 1987

faunal composition and organic surface encrustations at hydrothermal vents on the southern juan de fuca ridge examination of a small collection of macroinvertebrates from three vents of the southern juan de fuca vent field reveals differences between the vents with respect to species composition species habits and microbial and metallic deposits on their surfaces two apparently new vestimentiferan species were found and for the first time the juan de fuca palm worm was observed to dwell on smokers high acidity values recorded in this system may interfere with the process of shell calcification in an archaeogastropod snail the surfaces of vestimentifer an tubes at two vents are heavily encrusted with microbial and metallic accumulations scanning electron microscope transmission electron microscope and energy dispersive x ray microanalysis observations show that iron based crusts on orange tubes are built from accumulations of an fe rich particle of distinctive size and shape morphological evidence is presented for the microbial origin of fe rich particles zn rich particles found on black tubes are not of microbial origin we suggest that iron deposition on surfaces in the vent environment is initially biocatalytic but subsequent deposits may build by simple inorganic reactions

deglaciation of fennoscandia abstract to provide a new reconstruction of the deglaciation of the fennoscandian ice sheet in the form of calendar year time slices which are particularly useful for ice sheet modelling we have compiled and synthesized published geomorphological data for eskers ice marginal formations lineations marginal meltwater channels striae ice dammed lakes and geochronological data from radiocarbon varve optically stimulated luminescence and cosmogenic nuclide dating this is summarized as a deglaciation map of the fennoscandian ice sheet with isochrons marking every 1000 years between 22 and 13 cal kyr bp and every hundred years between 11 6 and final ice decay after 9 7 cal kyr bp deglaciation patterns vary across the fennoscandian ice sheet domain reflecting differences in climatic and geomorphic settings as well as ice sheet basal thermal conditions and terrestrial versus marine margins for example the ice sheet margin in the high precipitation coastal setting of the western sector responded sensitively to climatic variations leaving a detailed record of prominent moraines and other ice marginal deposits in many fjords and coastal valleys retreat rates across the southern sector differed between slow retreat of the terrestrial margin in western and southern sweden and rapid retreat of the calving ice margin in the baltic basin our reconstruction is consistent with much of the published research however the synthesis of a large amount of existing and new data support refined reconstructions in some areas for example the lgm extent of the ice sheet in northwestern russia was located far east and it occurred at a later time than the rest of the ice sheet at around 17 15 cal kyr bp we also propose a slightly different chronology of moraine formation over southern sweden based on improved correlations of moraine segments using new lidar data and tying the timing of moraine formation to greenland ice core cold stages retreat rates vary by as much as an order of magnitude in different sectors of the ice sheet with the lowest rates on the high elevation and maritime norwegian margin retreat rates compared to the climatic information provided by the greenland ice core record show a general correspondence between retreat rate and climatic forcing although a close match between retreat rate and climate is unlikely because of other controls such as topography and marine versus terrestrial margins overall the time slice reconstructions of fennoscandian ice sheet deglaciation from 22 to 9 7 cal kyr bp provide an important dataset for understanding the contexts that underpin spatial and temporal patterns in retreat of the fennoscandian ice sheet and are an important resource for testing and refining ice sheet models

along axis hydrothermal flow at the axis of slow spreading mid ocean ridges insights from numerical models of the lucky strike vent field mar the processes and efficiency of hydrothermal heat extraction along the axis of mid ocean ridges are controlled by lithospheric thermal and permeability structures hydrothermal circulation models based on the structure of fast and intermediate spreading ridges predict that hydrothermal cell organization and vent site distribution are primarily controlled by the thermodynamics of high temperature mid ocean ridge hydrothermal fluids using recent constraints on shallow structure at the slow spreading lucky strike segment along the mid atlantic ridge we present a physical model of hydrothermal cooling that incorporates the specificities of a magma rich slow spreading environment using three dimensional numerical models we show that in contrast to the aforementioned models the subsurface flow at lucky strike is primarily controlled by across axis permeability variations models with across axis permeability gradients produce along axis oriented hydrothermal cells and an alternating pattern of heat extraction highs and lows that match the distribution of microseismic clusters recorded at the lucky strike axial volcano the flow is also influenced by temperature gradients at the base of the permeable hydrothermal domain although our models are based on the structure and seismicity of the lucky strike segment across axis permeability gradients are also likely to occur at faster spreading ridges and these results may also have important implications for the cooling of young crust at fast and intermediate spreading centers

pb sr and nd isotopes in basalts and sulfides from the juan de fuca ridge pb sr and nd isotopes of seven basalt glasses collected by the submersible alvin from the southern juan de fuca ridge sjfr are almost identical 206pb 204pb 18 45 207pb 204pb 15 47 208pb 204pb 37 81 87sr 86sr 0 70249 143nd 144nd 0 51315 whereas all basalts appear cogenetic four of the samples have uniform abundances of u th rb nd sm pb and sr indicating that they are also comagmatic two basalt glasses dredged previously at the sjfr have similiar isotopic compositions but higher concentrations of u th and pb the 206pb 204pb ratios are intermediate between generally less radiogenic ridge basalts from south of the juan de fuca ridge jfr and often more radiogenic basalts from the northern jfr and ne pacific seamounts sr and nd isotopic compositions closely resemble data of other ridge basalts from the northernmost east pacific rise and are intermediate between isotopically more diverse seamount basalts produced nearby eight sulfides collected from three hydrothermal vent sites at sjfr have very similar pb isotopic compositions to that of the associated basalts sr and nd isotopic ratios of a single sulfide sample lie on a mixing curve between seawater and basalt data indicating that the sulfide precipitated after mixing of hydrothermal solutions and ambient seawater

lake store finnsjoen a key for understanding lateglacial early holocene vegetation and ice sheet dynamics in the central scandes mountains the lateglacial lg deglaciation and vegetation development in the scandes mountains has been debated for a century here we present new evidence from microfossils radiocarbon dated plant macrofossils and sedimentary ancient dna from laminated sediments in lake store finnsjoen 1260 m a s l at dovre central norway combined with previous results from three other dovre lakes this allows for new interpretations of events during and immediately after the lg deglaciation the finnsjoen sediments present the first uninterrupted record of local vegetation development in the scandes mountains from the late younger dryas yd ca 12 000 cal years bp to the early holocene around 9700 cal years bp the local vegetation in late yd early holocene was extremely sparse with pioneer herbs e g artemisia norvegica beckwithia campanula cf uniflora koenigia oxyria papaver saxifraga spp and dwarf shrubs betula nana salix including salix polaris from 11 300 cal years bp local vegetation rapidly closed with dominant dryas saxifraga spp and silene acaulis from ca 10 700 cal years bp open birch forests with juniper empetrum nigrum and other dwarf shrubs developed pine forests established within the area from 10 300 cal years bp we identified the cold preboreal oscillation pbo not earlier described from pollen data in south norway around 11 400 cal years bp by a regional pollen signal distinct local vegetation changes were not detected until the post pbo warming around 11 300 cal years bp apparently the earlier warming at the yd holocene transition at 11 650 cal years bp was too weak and short lived for vegetation closure at high altitudes at dovre for the first time we demonstrate a regional glacier readvance and local ice cap formations during the yd in the scandes mountains in two of the deep lakes with small catchments yd glaciation blocked sedimentation without removing old sediments and caused a hiatus separating sediments of the ice free lg interstadial lgi from those of the ice free holocene period both regional glaciers and local ice caps caused hiati ice free pre yd conditions at dovre followed by a yd readvance point to a scenario that is intermediate between the maximum ice model postulating a thick glacier during the entire lg and the minimum ice model postulating thin and multi domed early lg ice

structure of the southern juan de fuca ridge from seismic reflection records twenty four channel seismic reflection records were obtained from the axial region of the southern juan de fuca ridge two profiles are normal to the strike of the spreading center and intersect the ridge at latitude 44 40 n and 45 05 n a third profile extends south along the ridge axis from latitude 45 20 n and crosses the blanco fracture zone processing of the axial portions of the cross strike lines resolved a weak reflection centered beneath the axis the reflector is at a depth similar to seismically detected magma chambers on the east pacific rise and a lau basin spreading center we suggest that the reflector represents the top of an axial magma chamber in the migrated sections the top of the probable magma chamber is relatively flat and 1 2 km wide and the subbottom depth of the chamber is greater where the depth to the ridge axis is greater

evolution of hydrothermal activity on the juan de fuca ridge observations mineral ages and ra isotope ratios we discuss observations of hydrothermal activity from sites along the juan de fuca ridge and report the ages and radium isotope ratios of samples from eight sites collected on a series of 11 alvin dives in september 1988 this was a pilot program to assess whether uranium and thorium series isotopes could provide age information on the duration of active venting the ages are based on the ingrowth of 228th from 228ra in both active and inactive chimneys and chimney talus composed of barite anhydrite chalcopyrite wurtzite and pyrite samples were obtained from from north to south the fairycastle site northern endeavour ridge the endeavour hydrothermal site and the southern juan de fuca sites named vent 3 vent 1 and plume areas samples from active chimneys 20 yielded 228th 228ra ages in the range 0 2 to 1 2 years we recorded no ages in the range 1 2 to 12 years for inactive chimneys or rubble our tentative conclusions are that the individual chimneys form rapidly and that the presently active chimney structures are 2 years old in the fairycastle site there is at least one episode of hydrothermal activity recorded in the rubble which occurred at least 20 years earlier large sulfide edifices in the endeavour hydrothermal site probably record a minimum of 20 years of discharge for each particular structure chimney structures in the southern juan de fuca ridge may be older they show no evidence of significant recent growth except for an outer barite rich layer the results of this study suggest that hydrothermal chimneys of diverse mineralogy can be dated by measuring the ingrowth of 228th from 228ra decay the technique is most accurate for chimneys that are less than 10 years old the technique in conjunction with detailed petrological analyses could be used to assess the growth rates of individual chimney walls variations in the 228ra 226ra activity ratios among the three areas reflect either differences in fluid residence time depth of generation of hydrothermal fluids or interaction with basaltic rocks previously altered at low temperatures

assessing glacier features supporting supraglacial trees a case study of the miage debris covered glacier italian alps the number of debris covered glaciers featuring supraglacial trees is increasing worldwide as a response of high mountain environments to climate warming generally their distribution on the glacier surface is not homogeneous thus suggesting that some glacier parameters influence germination and growth of trees in this study we focused our attention on the widest italian debris covered glacier the miage glacier mont blanc massif we analyzed the ablation area in the range from 1730 to 2400 m a s l where continuous debris coverage is present and trees are found using data obtained by remote sensing investigations and field surveys we defined a record of glacier parameters to be analyzed with respect to the presence and abundance of trees we found that supraglacial trees are present at the miage glacier 1 whenever exceeding a debris thickness threshold 19 cm 2 with a gentle slope 10 3 with a low glacier surface velocity 7 0 m yr and 4 where the vertical changes due to glacier

fine scale magnetic anomaly field over the southern juan de fuca ridge axial magnetization low and implications for crustal structure sea surface deep tow and submersible magnetic surveys show that the spreading axis of the cleft segment of the southern juan de fuca ridge sjdf is overlain by a distinctive magnetic anomaly low in contrast to the more typical magnetic anomaly high found over the adjacent vance segment and most mid ocean ridge spreading centers forward iterative thickness modeling based on the near bottom magnetic data shows that this axial magnetic low can be produced by a thin magnetic source layer beneath the cleft axial valley thick crust is required to produce the magnetic anomaly high over the vance axial valley this magnetic modeling demonstrates that variations in the base of the magnetic source layer which is assumed to be equivalent to the base of the extrusive lavas can produce a significant proportion of the observed short wavelength magnetic anomaly signal these magnetic layer thickness models also correlate quite well with nearby seismic results which show that seismic layer 2a thins beneath the axial valley of the cleft and is thicker beneath the vance axial valley recent seismic results over other fast and medium spreading rifts also show rapid changes in the thickness of the uppermost crust near the axis of spreading which could produce significant magnetic contrasts the results of this study and the correlation with independent seismic measurements suggest that while magnetization may vary due to geomagnetic behavior or because of petrology or alteration first order change in upper crustal thickness is one of the most important variables to consider in the production of fine scale magnetic anomalies in young ocean crust

along axis variations within the plate boundary zone of the southern segment of the endeavour ridge abstract the width of and along axis variations within the plate boundary zone of the southern segment of the endeavour ridge of the juan de fuca ridge system have been quantified using sea marc i sidelooking sonar imagery the sonar imagery was calibrated with video and photographic surveying to provide visual corroboration of the activity spacing and offset of faults and fissures this ridge segment contains an axial high bisected along its length by a summit depression whose width relief of its rims depth of its floor and spacing of faults and fissures vary systematically away from the midpoint of the ridge segment toward its tips the axial high and its summit depression are likened to an elongated shield volcano that is being disrupted by collapse of its crest into a widening linear caldera the observed distribution of faults and fissures could be produced by mechanisms which stretch young ocean crust which is characterized by rheological properties which vary with distance from the center of the elongated volcano reprints jhd

high resolution bathymetric surveys using scanning sonars lava flow morphology hydrothermal vents and geologic structure at recent eruption sites on the juan de fuca ridge the coaxial and cleft segments of the juan de fuca ridge have isolated chronic high temperature and focused hydrothermal vent sites both segments also have experienced recent volcanic eruptions which produced extensive ephemeral low temperature and diffuse hydrothermal venting to study the geologic setting of these sites high resolution bathymetric surveys at eight locations on the coaxial and cleft segments were collected between 1993 and 1999 two 675 khz scanning sonar systems were used mesotech on the submersible alvin and imagenex on the remotely operated vehicle jason the bathymetry from these surveys can be gridded at a scale of 2 4 m and contoured at 1 m and thus can resolve many fine scale features on the seafloor that are indistinguishable in multibeam bathymetry collected at the sea surface bathymetric data at this resolution are particularly useful for identifying geologic features related to diking faulting and lava flow emplacement for example the high resolution bathymetric maps show that submarine fissure eruptions that form pillow lavas last long enough to become localized and to produce point source constructs along their length and their extrusion rate is low enough that no significant drainback occurs in contrast lobate sheet flows are formed by short lived high effusion rate eruptions in which no localization of output occurs along the eruptive fissure and inflation is quickly followed by drainback resulting in extensive collapse features however if the process of submarine lava flow inflation occurs at a slower rate and over a longer period of time it can create lava rises up to 25 m high with distinctive structure and morphology the scanning sonar data also show that fissures and grabens have formed or reactivated where dikes approach the surface adjacent to recent eruptive sites the fine scale bathymetry establishes that all the hydrothermal vent sites studied at the coaxial and cleft segments are located along prominent volcanic or tectonic extensional structures which provide the physical pathway for fluids from the subsurface to the seafloor furthermore the fine scale morphology of recent lava flows can be used as a qualitative indication of eruption duration

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

mineralogical studies of sulfide samples and volatile concentrations of basalt glasses from the southern juan de fuca ridge sulfide samples obtained from the u s geological survey s dsrv alvin dives on the southern juan de fuca ridge closely resemble those from the same area described by koski et al 1984 major minerals include sphalerite wurtzite pyrite marcasite isocubanite anhydrite and chalcopyrite equilibrium if attained at all during deposition of most sulfides was a transient event over a few tens of micrometers at most and was perturbed by rapid temperature and compositional changes of the circulating fluid two new minerals were found one a hydrated zn fe hydroxy chlorosulfate and the other a mn mg fe hydroxide or hydroxy hydrate both were formed at relatively low temperatures lizardite starkeyite and anatase were found for the first time in such an environment sulfide geothermometry involving the system cu fe s indicates a vent temperature of 328 c for one sample fluid inclusion studies on crystals from the same vicinity of the same sample give pressure corrected homogenization temperatures of 268 and 285 c ice melting temperatures on inclusions from the same sample are about 2 8 c indicating that the equivalent salinity of the trapped fluid is about 50 greater than that of seawater volatile concentrations from vesicle free basalt glass from the vent field are about 0 013 wt co2 and 0 16 wt h2o co2 contents in these samples yield an entrapment depth of 2200 m of seawater which is the depth from which the samples were collected

englacial and proglacial glaciotectonic processes at the snout of a thermally complex glacier in svalbard many glaciers in svalbard which tend to be thermally complex with both warm and cold ice produced complex push moraines or thrust moraine complexes at the time of their maximum neoglacial advance and during their subsequent recession uv rsbreen which nearly reaches the coast of northwest spitsbergen has a push moraine 50 m high comprising gravel and sand of fluvial origin and diamicton derived from basal ice as well as thrust masses of dead ice the push moraine is almost entirely a product of the tectonic processes especially thrusting that occurred in the snout of the glacier and in the frozen sediments immediately in journal of quaternary science front thrusting is probably initiated where the glacier switches from a sliding mode to a state of being frozen to the bed thrusting in the foreland is an extension of the same processes that occur within the ice itself and a sole thrust probably extends from the frozen sediment that underlies the base of the glacier near its snout thrusting results in subglacial sediment being uplifted to a high level within the glacier or even to the surface subsidiary near horizontal thrusts parallel to the bedding plane link thrusts dipping up glacier and result in local thickening of individual subglacial sedimentary units gravels incorporated in this way retain their integrity but sands are folded whereas uplifted debris rich basal ice is intensely folded the distinctive tectonic style of low dip bedding in gravels truncated by high angle 30 40 thrusts and only localised folding in less competent material differs from most descriptions of such ridges but may be typical of many push moraines composed principally of fluvial material

erratum structural control of gold mineralization at the bousquet mine abitibi quebec the bousquet gold deposits are structurally controlled disseminated and vein type lodes located within a 500 m wide anastomosing deformation zone ore is located within narrow zones of high strain surrounded by lozenge shaped panels of less deformed rock strain characteristics are those of the bulk inhomogeneous flattening style ore lenses are spatially related to highly sheared fractured and altered mafic and felsic volcanic and volcaniclastic rocks of contrasting rheologic properties deformation features can be ascribed to multistage progressive ductile brittle deformation strain markers and kinematic indicators show that the principal displacement within the deformation zone was reverse faulting with a minor sinistral throw a structural analysis demonstrates that the deformation responsible for the development of a pervasive regional foliation brittle fractures and oblique reverse faults can be attributed to a north south compression metamorphic minerals such as andalusite kyanite garnet

petrology and geochemistry of basalts from the southern juan de fuca ridge controls on the spatial and temporal evolution of mid ocean ridge basalt three morphologically distinct regions within the neovolcanic zone of the cleft segment of the southern juan de fuca ridge were investigated and sampled in detail using the dsrv alvin additional along strike and off axis samples were recovered by dredge the southernmost region the southern cleft site is characterized by a 3 km wide axial valley floored by ponded sheet flows and bisected by a 30 to 50 m wide cleft farther north at the young sheet flow site the ridge axis is characterized by a distinct 500 m wide inner graben that is largely covered by distinctly younger looking sheet flows the northernmost of the three regions is defined by a linear series of discontinuous constructional pillow mounds that extend the trend of the cleft segment well into the zone of overlap with the neighboring vance segment the pillowed lavas at the young pillow mound site represent the most recent episode of volcanism along the cleft segment strong correlations exist between degrees of fractionation relative ages of lavas and latitude lavas are progressively younger looking and more mafic to the north the compositional range of mid ocean ridge basalts from the neovolcanic zone can generally be accounted for by 35 40 low pressure fractional crystallization of relatively primitive but not primary depleted n type melts scatter of the geochemical data about calculated liquid lines of descent is probably the result of mixing of magmas with slightly different parental compositions generated from small scale mantle heterogeneities furthermore the chemical variability may be the result of mixing of very depleted and more enriched sources or melts that are present beneath the southern juan de fuca ridge the more primitive nature of the young pillow mound basalts and their slightly different chemical characteristics indicates they cannot be simply related to the older southernmost lavas by along axis flow in a continuous axial magma chamber or conduit rather the data suggest lavas were derived from discrete magma chambers or lenses each in a different stage of evolution the youngest events may be associated with a new influx of magma into the northern part of the segment and subsequent northward diking and propagation to form the new pillow mounds the oldest stage 40 additional crystallization of the most mafic composition is associated with focused hydrothermal activity and tectonic extension whereas the youngest events are characterized by ridge inflation diking and dispersed hydrothermal activity geochemical and tectonomagmatic features observed along the cleft segment are similar to those recently documented along the east pacific rise from 9 30 n 10 n suggesting the scales processes and stages of magmagenesis are similar along medium to fast spreading ridge segments

in situ observations of dissolved iron and manganese in hydrothermal vent plumes juan de fuca ridge in situ measurements of dissolved manganese and total dissolved iron were conducted in hydrothermal plumes over the juan de fuca ridge using a submersible chemical analyzer scanner the scanner was deployed as part of a ctd transmissometer rosette instrument package on both tow yos and vertical casts during the vents leg i cruise in 1989 dissolved manganese and total dissolved iron concentrations along with temperature and light attenuation anomalies were determined over the ridge crest every 5 s discrete samples for laboratory analyses of dissolved iron ii total dissolved iron ii iii and manganese were also collected metal to heat ratios me q measured in situ were extremely variable in one steady state plume while an event plume had constant me q uniform values of mn q in the event plume demonstrate that mn behaves conservatively in the near field plume variability in the mn q ratios in a steady state plume indicated the presence of at least two hydrothermal sources with distinct me q values a simple mixing model shows that the contribution of mn from high me q sources with a composition characteristic of black smoker vents varies between 1 and 99 within the core of the steady state plume with an average value of 55 on average over 50 of the excess heat within the plume originates from low me q ratio sources with a composition characteristic of low temperature diffuse flow vent fluids less than 4 of the volume of hydrothermal fluids in the plume originates from black smokers the fe ii concentrations were used to provide an estimate of plume age on a transect across the ridge axis plume ages were about 2 5 days on axis and 12 days off axis these plume ages were modeled to provide estimates of plume transport and horizontal diffusion and show excellent agreement with ages determined using 222rn

woody vegetation fuel and fire track the melting of the scandinavian ice sheet before 9500 cal yr bp abstract new studies indicate the presence of early holocene ice free areas far north in scandinavia post glacial fire and vegetation were investigated based on sedimentary charcoal and pollen from two small lakes in northern sweden accumulation of organic sediment started around 10 900 and 9200 cal yr bp showing that both lake valleys were ice free extremely early given their northerly location fire events started after 9600 cal yr bp and became less common around the 8 2 ka event woody vegetation provided fuel that contributed to fires the first vegetation in our pollen record consisted of hippophae dryas grasses and sedges subsequently broadleaved trees betula salix increased in abundance and later pinus alnus ferns and lycopodium characterized the vegetation pollen from larix picea and malus were also found the change in vegetation composition was synchronous with the decrease in lake water ph in the region indicating ecosystem scale processes this occurred during a period of net global and regional warming the changes in fire frequency and vegetation appear independent of regional trends in precipitation the reconstructed fire history and vegetation support the scenario of early ice free areas far north in scandinavia during early holocene warming creating favorable conditions for woody plants and wildfires

u th and pb isotopes in hot springs on the juan de fuca ridge the concentrations and isotopic compositions of u th and pb in three hydrothermal fluids from the juan de fuca ridge were determined the samples consisted of 10 2 57 6 of the pure hydrothermal end members based on mg contents the pb contents of the samples ranged from 34 to 87 ng g u from 1 3 to 3 0 ng g and th from 0 2 to 7 7 pg g these samples showed large enrichments of pb and th relative to deep sea water and some depletion of u they did not show coherent relationships with mg however indicating nonideal mixings between the hot hydrothermal fluids and cold ambient seawater particles filtered from these hydrothermal fluids contained significant amounts of th and pb which may effectively increase the concentration of these elements in the fluids when acidified the sup 234 u sup 238 u values in all samples show a sup 234 u enrichment relative to the equilibrium value and have a seawater signature the pb isotopic composition of the juan de fuca hydrothermal fluids resembles that of 21 sup 0 n east pacific rise and has a uniform mid ocean ridge basalt signature the hydrothermal systems at oceanic spreading ridges have circulated through a large volume of basalts therefore pb in these fluids may representmore the best average value of the local oceanic crust from the effects of u deposition from seawater to the crust and pb extraction from rock to the ocean the u pb ratio in the hydrothermally altered oceanic crust may be increased significantly copyright american geophysical union 1987 less

particle size distributions within hydrothermal plumes over the juan de fuca ridge abstract the particle size distributions of suspended particles in neutrally buoyant hydrothermal plumes from two vent sites on the juan de fuca ridge were measured with a coulter counter hydrothermal particle populations were characterized by high volume concentrations of particles with diameters of m particle populations dominated by larger particles were rare and observed only very near active vent sources because of rapid settling and dilution particle size distributions from particle plumes of hydrothermal origin can be readily distinguished from those of benthic nepheloid layers by the fine particle tail which is expressed on a cumulative number distribution by a slope much steeper than the oceanic norm of 3 the predominance of particles with diameters m within neutrally buoyant hydrothermal plumes demonstrates that the bulk of hydrothermal precipitates are capable of being dispersed over wide areas

volcanic eruption of the mid ocean ridge along the east pacific rise crest at 9 45 52 n direct submersible observations of seafloor phenomena associated with an eruption event in april 1991 in april 1991 we witnessed from the submersible alvin a suite of previously undocumented seafloor phenomena accompanying an in progress eruption of the mid ocean ridge on the east pacific rise crest at 9 45 n 52 n the volume of the eruption could not be precisely determined although comparison of pre and post eruption seabeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were 10 m high effects of the eruption included 1 increased abundance and redistribution of hydrothermal vents disappearance of numerous vent communities and changes in characteristics of vent fauna and mineral deposits within the eruption area since december 1989 2 murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates 3 destruction of a vent community by lava flows mass wasting and possible hydrovolcanic explosion at a site known as tubeworm barbecue in the axial summit caldera asc at 9 50 6 n 4 near critical temperatures of hydrothermal vent fluids ranging up to 403 c 5 temporal variations over a 2 week interval in both temperatures and chemical isotopic compositions of hydrothermal fluids 6 unusual compositions of end member vent fluids with ph values ranging to a record low of 2 5 salinities ranging as low as 0 3 wt nacl one twelfth that of seawater and dissolved gases reaching high concentrations 65 mmol l for both co2 and h2s 7 venting at temperatures above 380 c of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices 8 disorganized venting of both high temperature fluids black and gray smoke and large volumes of cooler diffuse hydrothermal fluids directly from the basaltic seafloor rather than from hydrothermal mineral constructions 9 rapid and extensive growth of flocculent white bacterial mats species unknown on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid and 10 subseafloor downslope migration of magma normal to the ridge axis in a network of small scale 1 5 m diameter lava tubes and channels to distances at least 100 200 m outside the asc we suggest that in april 1991 intrusion of dikes in the eruption area to 200 m beneath the asc floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor rich hydrothermal fluids through the overlying rubbly cavernous lavas low salinities and gas rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid vapor phase separation boiling and phase segregation hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and or hydrovolcanic explosions these fissures and pits were lined with white mats of a unique fast growing bacteria that was the only life associated with the brand new vents the prolific bacteria which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids may thrive on high levels of gases in the vapor rich hydrothermal fluids initially escaping the hydrothermal system white bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic blizzard up to 50 m above the bottom the bacterial proliferation of april 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling and or by grazing as other organisms are re established in the biologically devastated area

upper crustal structure and axial topography at intermediate spreading ridges seismic constraints from the southern juan de fuca ridge 1 we use multichannel seismic reflection data to image the upper crustal structure of 0 620 ka crust along the southern juan de fuca ridge the study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow cleft and wide vance axial summit grabens asg along most of the axis of both segments we image the top of an axial magma chamber amc the amc along cleft deepens from south to north from 2 0 km beneath the ridge cleft observatory and hydrothermal vents near the southern end of the segment to 2 3 km at the northern end near the site of the 1980s eruptive event along the vance segment the amc also deepens from south to north from 2 4 to 2 7 km seismic layer 2a interpreted as the basaltic extrusive layer is 250 300 m thick at the ridge axis along the cleft segment and 300 350 m thick along the axis of the vance segment however off axis layer 2a is similar in both segments 500 600 m indicating 90 and 60 off axis thickening at the cleft and vance segments respectively half of the thickening occurs sharply at the walls of the asg with the remaining thickening occurring within 3 4 km of the asg along the full length of both segments layer 2a is thinner within the asg compared to the ridge flanks previous studies argued that the asg is a cyclic feature formed by alternating periods of magmatism and tectonic extension our observations agree with the evolving nature of the asg however we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply thus the asg rather than being formed by excess volcanism is a rifted flexural axial high the changes in axial morphology affect the distribution of lava flows along the ridge flanks as indicated by the pattern of layer 2a thickness the fluctuations in magma supply may occur at all spreading rates but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges

fennoscandian palaeoglaciology reconstructed using a glacial geological inversion model the evolution of ice sheet configuration and flow pattern in fennoscandia through the last glacial cycle was reconstructed using a glacial geological inversion model i e a theoretical model that formalises the procedure of using the landform record to reconstruct ice sheets the model uses mapped flow traces and deglacial melt water landforms as well as relative chronologies derived from cross cutting striae and till lineations as input data flow trace systems were classified into four types i time transgressive wet bed deglacial fans ii time transgressive frozen bed deglacial fans iii surge fans and iv synchronous non deglacial event fans using relative chronologies and aggregation of fans into glaciologically plausible patterns a series of ice sheet configurations at different time slices was erected a chronology was constructed through correlation with dated stratigraphical records and proxy data reflecting global ice volume geological evidence exists for several discrete ice sheet configurations centred over the scandinavian mountain range during the early weichselian the build up of the main weichselian fennoscandian ice sheet started at approximately 70 ka and our results indicate that it was characterised by an ice sheet with a centre of mass located over southern norway this configuration had a flow pattern which is poorly reproduced by current numerical models of the fennoscandian ice sheet at the last glacial maximum the main ice divide was located overthe gulf of bothnia a major bend in the ice divide was caused by outflow of ice to the northwest over the lowest part of the scandinavian mountain chain widespread areas of preserved pre late weichselian landscapes indicate that the ice sheet had a frozen bed core area which was only partly diminished in size by inward transgressive wet bed zones during the decay phase

morphometric variability within the axial zone of the southern juan de fuca ridge interpretation from sea marc ii sea marc i and deep sea photography the morphometric characteristics of the axial regions of oceanic spreading centers are determined by 1 the type of volcanic flows 2 the relation between primary volcanic relief on a scale of a few meters to tens of meters and degree of sediment cover and 3 the extent of surficial expression and timing of tectonic disruption of the young oceanic crust even within a single continuous linear spreading ridge segment with relatively uniform axial valley dimensions over a distance of 50 or more kilometers such as along the southern juan de fuca ridge the changes in morphometric characteristics along axis within the youngest crust indicate distinct variation in tectonic and volcanic activity over short distances within short time periods an integrated analysis of sea marc i sea marc ii and photographic data for the southernmost continuous segment of the juan de fuca ridge shows that generalizations about tectonic and volcanic processes at spreading ridges must consider both the temporal scale of processes as well as the physical scales of observations if predictive models are to be successful comparison of the morphometric expression within the major hydrothermal vent area and the rest of the southernmost ridge segment suggests that the mapped distribution of hydrothermal vents may reflect the extent of survey effort rather than uniqueness of geologic setting

deglaciation of the eurasian ice sheet complex the eurasian ice sheet complex eisc was the third largest ice mass during the last glacial maximum with a span of over 4500 km and responsible for around 20 m of eustatic sea level lowering whilst recent terrestrial and marine empirical insights have improved understanding of the chronology pattern and rates of retreat of this vast ice sheet a concerted attempt to model the deglaciation of the eisc honouring these new constraints is conspicuously lacking here we apply a first order thermomechanical ice sheet model validated against a diverse suite of empirical data to investigate the retreat of the eisc after 23 ka bp directly extending the work of patton et al 2016 who modelled the build up to its maximum extent retreat of the ice sheet complex was highly asynchronous reflecting contrasting regional sensitivities to climate forcing oceanic influence and internal dynamics most rapid retreat was experienced across the barents sea sector after 17 8 ka bp when this marine based ice sheet disintegrated at a rate of 670 gigatonnes per year gt a 1 through enhanced calving and interior dynamic thinning driven by oceanic atmospheric warming and exacerbated by eustatic sea level rise from 14 9 to 12 9 ka bp the eisc lost on average 750 gt a 1 peaking at rates 3000 gt a 1 roughly equally partitioned between surface melt and dynamic losses and potentially contributing up to 2 5 m to global sea level rise during meltwater pulse 1a independent glacio isostatic modelling constrained by an extensive inventory of relative sea level change corroborates our ice sheet loading history of the barents sea sector subglacial conditions were predominately temperate during deglaciation with over 6000 subglacial lakes predicted along with an extensive subglacial drainage network moreover the maximum eisc and its isostatic footprint had a profound impact on the proglacial hydrological network forming the fleuve manche mega catchment which had an area of 2 5 106 km2 and drained the present day vistula elbe rhine and thames rivers through the seine estuary during the bolling allerod oscillation after c 14 6 ka bp two major proglacial lakes formed in the baltic and white seas buffering meltwater pulses from eastern fennoscandia through to the younger dryas when these massive proglacial freshwater lakes flooded into the north atlantic ocean deglaciation temporarily abated during the younger dryas stadial at 12 9 ka bp when remnant ice across svalbard franz josef land novaya zemlya fennoscandia and scotland experienced a short lived but dynamic re advance the final stage of deglaciation converged on present day ice cover around the scandes mountains and the barents sea by 8 7 ka bp although the phase lagged isostatic recovery still continues today

basaltic lava domes lava lakes and volcanic segmentation on the southern east pacific rise meter scale dsl 120 sonar mapping and coregistered argo ii photographic observations reveal changes in eruptive style that closely follow the third order structural segmentation of the ridge axis on the southern east pacific rise 17 11 18 37 s near segment ends we observe abundant basaltic lava domes which average 20 m in height and 200 m in basal diameter and have pillow lava as the dominant lava morphology the ubiquity of pillow lava suggests low effusion rate eruptions the abundance of lava domes suggests that the fissure eruptions were of sufficient duration to focus and produce a line of volcanic edifices near segment centers we observe fewer but larger lava domes voluminous drained and collapsed lava lakes and smooth lobate and sheet lava flows with very little pillow lava the abundance of sheet flows suggests that high effusion rate eruptions are common fewer lava domes and large lava lakes suggest that fissure eruptions do not focus to point sources this pattern was observed on eight third order ridge segments suggesting that a fundamental volcanic segmentation of the ridge occurs on this scale the third order segment boundaries also correlate with local maxima in the seismic axial magma chamber reflector depth throughout the study area and decreased across axis width of the region of seismic layer 2a thickening along the one segment where sufficient cross axis seismic lines exist the geochemically defined magmatic segment boundaries in the study area match the locations of our volcanic segment boundaries although rock sampling density is not adequate to constrain the variation across all the third order volcanic segments that we identify these observations suggest that variation in the processes of crustal accretion along axis occurs at a length scale of tens of kilometers on superfast spreading 140 km myr full rate mid ocean ridges

chemical and mineralogical influences on concentrations of trace metals in hydrothermal fluids abstract concentrations of trace metals in hydrothermal solutions from the southern juan de fuca ridge sjfr and the tag hydrothermal field on the mid atlantic ridge are influenced by the combined effects of source rock composition brine seawater mixing and complex interplay among various metals and minerals deposited and remineralized in vent mounds and chimneys copper mo and co show predicted sharp decreases in concentration for vent fluids as temperatures decrease below 350 c however concentrations of cu in 363 c fluids at the tag site are well above values determined from seawater basalt interactions due to subseafloor refining processes and remineralization of chalcopyrite concentrations of zn cd pb as and ga do not show any temperature dependence between 214 to 363 c however zn values do correlate well with cl on an area by area basis due to subseafloor mixing of brine with altered seawater zinc levels also correlate well with concentrations of cd pb and as for all vent fluid samples as well as with basalt and zn rich sulfides implying some continuity in behavior from source rock to vent fluid to sulfide minerals gallium values correlate with zn and cl levels for the sjfr but are high relative to zn at the tag site due to subseafloor refining processes concentrations of tl follow cl k and rb consistent with the behavior predicted for tl overall the cl and zn rich fluids from the sjfr are characteristic of a brine dominated system formed during recent volcanism in contrast high temperature fluids from the tag area are representative of a system that has evolved over the past 100 000 yr with high concentrations of cu co and ga that result from higher temperatures and complex refining processes within a large vent mound

alpine debris covered glaciers as a habitat for plant life debris covered glaciers represent a significant increasing fraction of glaciers and can host plant life on their surface the goal of this work was to evaluate the suitability of supraglacial debris as a habitat for plant life and to discuss its ecological and biogeographic role the research was carried out on the miage glacier mont blanc massif western alps italy vegetation cover was sampled using a regular sampling grid recording plant species and number of individuals in 71 plots detailed glaciological parameters surface temperature debris thickness glacier surface velocity were recorded or derived from published data relationships between vegetation and environmental variables were assessed through generalized linear models principal components analysis and canonical correspondence analysis the glacier surface hosted a high biodiversity with 40 vascular plant species including trees and shrubs plant cover was arranged along an altitude glacier velocity gradient whilst debris thickn

dynamic character of the hydrothermal vent habitat and the nature of sulphide chimney fauna abstract a major substratum for vent organisms on juan de fuca and explorer ridges of the northeast pacific is the polymetallic sulphide chimney the deposition processes and subsequent growth of such chimneys provide a dynamic and extreme habitat that changes rapidly during initial venting stages colonization must await stabilization of the chimney structure as the porous anhydrite shell accumulates sulphide minerals protection from direct hot water contact increases for organisms colonizing the outer walls while sufficient vent water still diffuses for the support of chemosynthesis vestimentiferan worms are found on small sulphide mounds but the active growth and collapse of anhydrite spires is a major source of disruption and mortality high temperature venting and spire growth appear to attract alvinellid polychaetes which may be implicated in the process of strengthening and sealing the anhydrite spires fauna on the growing chimney is subject to changes in fluid flow patterns that can make the structure uninhabitable large chimneys are inhabited by more species indicating that a diversification of habitat results from variations in mineralization and fluid availability chimney fauna has become specialized in exploiting an unpredictable and unstable habitat in an extreme example of the control of biotic development by the physical environment

submarine hydrothermal activity and gold rich mineralization at brothers volcano kermadec arc new zealand brothers volcano of the kermadec intraoceanic arc is host to a hydrothermal system unique among seafloor hydrothermal systems known anywhere in the world it has two distinct vent fields known as the nw caldera and cone sites whose geology permeability vent fluid compositions mineralogy and ore forming conditions are in stark contrast to each other the nw caldera site strikes for 600 m in a sw ne direction with chimneys occurring over a 145 m depth interval between 1 690 and 1 545 m at least 100 dead and active sulfide chimney spires occur in this field and are typically 2 3 m in height with some reaching 6 7 m their ages at time of sampling fall broadly into three groups 4 23 and 35 years old the chimneys typically occur near the base of individual fault controlled benches on the caldera wall striking in lines orthogonal to the slopes rarer are massive sulfide crusts 2 3 m thick two main types of chimney predominate cu rich up to 28 5 wt cu and more commonly zn rich up to 43 8 wt zn geochemical results show that mo bi co se sn and au up to 91 ppm are correlated with the cu mineralization whereas cd hg sb ag and as are associated with the dominant zn rich mineralization the cone site comprises the upper cone site atop the summit of the recent main dacite cone and the lower cone site that straddles the summit of an older smaller more degraded dacite cone on the ne flank of the main cone huge volumes of diffuse venting are seen at the lower cone site in contrast to venting at both the upper cone and nw caldera sites individual vents are marked by low relief 0 5 m mounds comprising predominately native sulfur with bacterial mats vent fluids of the nw caldera field are focused hot 300 c acidic ph 2 8 metal rich and gas poor calculated end member fluids from nw caldera vents indicate that phase separation has occurred with cl values ranging from 93 to 137 of seawater values by contrast vent fluids at the cone site are diffuse noticeably cooler 122 c more acidic ph 1 9 metal poor and gas rich higher than seawater values of so4 and mg in the cone vent fluids show that these ions are being added to the hydrothermal fluid and are not being depleted via normal water rock interactions iron oxide crusts 3 years in age cover the main cone summit and appear to have formed from fe rich brines evidence for magmatic contributions to the hydrothermal system at brothers includes high concentrations of dissolved co2 e g 206 mm kg at the cone site high co2 3he negative d and 18oh2o for vent fluids negative 34s for sulfides to 4 6 sulfur to 10 2 and 15n2 to 3 5 vent fluid ph values to 1 9 and mineral assemblages common to high sulfidation systems changing physicochemical conditions at the brothers hydrothermal system and especially the cone site occur over periods of months to hundreds of years as shown by interlayered cu au and zn rich zones in chimneys variable fluid and isotopic compositions similar shifts in 3he 4he values for both cone and nw caldera sites and overprinting of magmatic mineral assemblages by water rock dominated assemblages metals especially cu and possibly au may be entering the hydrothermal system via the dissolution of metal rich glasses they are then transported rapidly up into the system via magmatic volatiles utilizing vertical 2 5 km long narrow 300 m diameter pipes consistent with evidence of vent fluids forming at relatively shallow depths the nw caldera and cone sites are considered to represent stages along a continuum between water rock and magmatic hydrothermal dominated end members

growth of plants on the late weichselian ice sheet during greenland interstadial 1 unglaciated forelands and summits protruding from ice sheets are commonly portrayed as areas where plants first establish at the end of glacial cycles but is this prevailing view of ice free refug

metamorphic incongruent solution diffusion and pressure solution stripes abstract the formation of pressure solution stripes is discussed on the basis of previosly published observations and of local chemical equilibria between solution and solid phases chemically driven mineral reactions can be localized at early sites of nucleation and be enhanced by stresses to create patterns of cleavage stripes these reactions cause transport of material over distances greater than single grains they provide suitable sites and suitable chemistry for later incongruent pressure solution potentially a much faster process than congruent pressure solution discussed in previous literature neither reaction nor straightforward incongruent pressure solution explains the solution of quartz commonly observed at cleavage stripes quartz pressure solution is explicable on certain assumptions about diffusivities and the effects of stresses this involves interaction between quartz and the established incongruent solution of other minerals an overall process termed interdependent pressure solution electrical potential differences will be greatest during early mineral reactions a feature which may provide a means to corroborate the chemical role in initiating pressure solution stripes

trace metals in hydrothermal solutions from cleft segment on the southern juan de fuca ridge concentrations of trace metals in fe and cl rich hydrothermal solutions from the southern juan de fuca ridge sjfr have been determined and corrected for residual precipitates formed in the sampler precipitate corrections for ag cd cu mo pb sb and zn were variable contributing an average of 20 to total solution concentrations whereas corrections for co averaged less than 5 and essentially no corrections were required for fe and mn values for cu co and mo in these solutions showed a strong dependence on temperature with sharp decreases in concentrations as temperatures decreased to less than 320 c in addition and unlike most other metals studied all vent fluids from the sjfr were almost completely depleted in mo relative to seawater values of about 110 nmol kg 1 in contrast to the cu co mo group concentrations of ag cd sb and pb correlated well with those for zn and are presumed to follow a distribution that is influenced less by temperature over the 246 to 332 c range encountered in this study and more by the combined chemical processes that control zn levels along the transport pathway from the deep reaction zone to the vent orifice

a chronological framework for the holocene vegetational history of central minnesota the steel lake pollen record abstract paleorecords from minnesota and adjacent areas have often been used to evaluate large scale climatic processes in the mid continent of north america however most of these records are compromised by chronological flaws making problematic any comparisons with climatic interpretations based on other records e g gisp2 in greenland we report here a high resolution pollen record with a secure chronology constrained by 26 14 c dates on terrestrial macrofossils from steel lake central minnesota about 11 2 ka cal bp 11 000 calibrated yr before present the late glacial picea forest near steel lake was succeeded abruptly by pinus banksiana and or resinosa the pinus forest began to open 9 4 ka cal bp with the expansion of prairie taxa and a pine parkland or savanna prevailed until about 8 ka cal bp when quercus replaced pinus to become the dominant tree in the prairie areas for 4500 yr the close chronological control permits the correlation of key vegetational changes with those at other reliably dated sites in the eastern dakotas and in minnesota suggesting that the abrupt decline of the spruce forest was time transgressive from southwest to northeast during 2000 yr and that the development of prairie was time transgressive in the same direction over 2600 yr correlation of key pollen horizons at steel lake with those in the high resolution pollen profiles of elk lake ca 50 km northwest of steel lake suggests that the well known elk lake varve chronology for the early holocene is about 1000 yr too young

evolution of the south central part of the archean abitibi belt quebec part i stratigraphy and paleogeographic model the region comprises parts of the southern zone of the abitibi greenstone belt and of the bellecombe gneiss belt and the belleterre angliers greenstone belt the stratigraphic sequence of the abitibi belt consists of 1 a lower ultramafic division 2 a middle tholeiitic division and 3 an upper diverse division these divisions lap onto an older volcanic sequence to the north and west in the bellecombe belt sediments overlie the lower ultramafic division these sediments coarsen toward the abitibi belt and toward the stratigraphic top the volcanic rocks of the belleterre angliers belt overlie the sediments of the bellecombe belt five paleogeographic phases have been recognized during phase i a deep marine ultramafic lava plain covered the south part of the abitibi belt and the bellecombe belt faulting took place at the duparquet destor break during phase ii tholeiitic basalts formed a deep marine lava plain in the west of the abitibi belt whereas flows erupted mainly from central volcanic c

characteristics of hydrothermal plumes from two vent fields on the juan de fuca ridge northeast pacific ocean deep ctd transmissometer tows and water bottle sampling were used during 1985 to map the regional distribution of the neutrally buoyant plumes emanating from each of two major vent fields on the southern symmetrical segment sss and endeavour segment es of the juan de fuca ridge at both vent fields emissions from point and diffuse hydrothermal sources coalesced into a single 200 m thick plume elongated in the direction of current flow and with characteristic temperature anomalies of 0 02 0 05 c and light attenuation anomalies of 0 01 0 08 m 1 10 80 g l above background temperature anomalies in the core of each plume were uniform as far downcurrent as the plumes were mapped 10 15 km downcurrent light attenuation trends were non uniform and differed between plumes apparently because different vent fluid chemistries at each field cause significant differences in the settling characteristics of the hydrothermal precipitates vent fluids from the sss are metal dominated and mostly precipitate very fine grained hydrous fe oxides that remain suspended in the plume vent fluids from the es are sulfur dominated and precipitate a high proportion of coarser grained fe sulfides that rapidly settle from the plume the integrated flux of each vent field was estimated from measurements of the advective transport of each plume heat flux was 1700 1100 mw from the es and 580 351 mw from the sss particle flux varied from 546 312 g s to 204 116 g s at the es depending on distance from the vent field and was 92 48 g s from the sss

relationships between lava types seafloor morphology and the occurrence of hydrothermal venting in the ashes vent field of axial volcano deep towed and submersible photographic surveys within the caldera of axial volcano have been integrated with high resolution bathymetry to produce a geological map of the most active vent field axial seamount hydrothermal expeditions ashes in the caldera the ashes vent field encompasses an approximately 200 m 1200 m area of active venting which is located adjacent to the southwest caldera wall locations for over 2000 photographs in and near the vent field were determined using a seafloor transponder network then each photograph was described utilizing a classification system which provides detailed information concerning lava type hydrothermal activity sediment cover geological structure and biology resulting data were entered into a digital data base and computer generated maps were created that portray spatial relationships between selected geological variables in general the entire ashes field is characterized by pervasive low temperature venting the most vigorous venting is concentrated in an approximately 80 m 80 m area where there are several high temperature vents including some which are producing high temperature vapor phase fluids derived from a boiling hydrothermal system lava types within the ashes vent field are grouped into three distinct morphologies 1 smooth flat surfaced ropy and whorled sheet flows 2 lobate flows and 3 jumbled sheet flows the most intense hydrothermal venting is concentrated in the smooth sheet flows and the lobate flows the location of the ashes field is mainly attributable to faulting which defines the southwest caldera wall but the concentration of intense venting appears to be related also to the spatial distribution of lava types in the vent field and their contrasting permeabilities other structural trends of faults and fissures within the field also influence the location of individual vents