an experimental investigation of the behavior of amphibole in the system na 2 o mgo al 2 o 3 sio 2 h 2 o at high pressures the behavior of an amphibole in the system na2o mgo al2o3 sio2 h2o and its relationships with sodium micas have been examined at high pressures a sodium mica of composition intermediate between wonesite and preiswerkite is produced from low pressures to at least 28 kbar at 750 c from differing bulk compositions in water excess experiments minor amphibole has been found in water excess runs but a c2 m amphibole approaching glaucophane composition exists only in water undersaturated systems this amphibole coexists with quartz in runs on glaucophane composition from 18 kbar to 24 kbar at 700 c in this pressure range the amphibole is displaced from ideal glaucophane composition by substitution of naaaliv for si and mgm4mgm2 for nam4alm2 above 24 kbar the a site of this amphibole empties leading to the coexistence of amphibole jadeite quartz between 24 kbar and 28 kbar above 28 kbar talc forms while the modal amount of amphibole decreases tight buffering of the amphibole to ideal glaucophane composition does not occur and small amounts of naalsi 1 substitution are found in the talc this is reflected modally in the persistence of quartz with talc jadeite and amphibole amphibole and quartz continue to decrease in modal proportions with rising pressures but remain in the run products of synthesis experiments to at least 35 kbar at 750 c

a varve based calibration of the bridge river tephra fall a 1 cm thick layer of bridge river tephra is preserved in a core of varved lake sediments recovered from hector lake alberta the varve sequence extends from the core top to well below the tephra layer and has bseen calibrated to calendar years a varve count yields a calendar age of 2332 bp for the ash layer with a potential error of about 50 years this age is about 30 years younger than the current best radiocarbon based estimate of the time of the eruption however in light of the potential errors in both the varve count and the radiocarbon age the two ages are not inconsistent the tephra contains no biotite in the past biotite free bridge river tephra has been associated with a possible late ca 2000 bp tephra fall but the tephra from hector lake is clearly associated with the main 2300 2400 bp eruptive event

the tectonic implications of high pressure metamorphism in the western alps five mineralogical stages are recognized in the metamorphism of alpine ophiolites from one area in the zermatt saas zone constraints on p t values indicate an abrupt change at the metamorphic peak 550 600 c 17 20 kbar from a cold positive slope of 25 c kbar or less to cooling initially without discernible change of pressure correspondence of the prograde path to the p t trend of t in c 150 23 p in kbar shown by a variety of alpine units suggests that they all have p t histories for which an england richardson 1977 relaxation model also england thompson 1984 is inappropriate major tectonism at the peak pressure is indicated by association between minerals kyanite chloritoid lawsonite in eclogite vein growths fluid action and thrust surfaces the proposed metamorphic environment involves thrust repetition subcreting ophiolite to the over riding slab in a thermally mature subduction zone at a depth of approximately 70 km preservation is favoured when ocean closure leads to subduction of sialic crust so providing a combination of buoyant uplift with the continued refrigeration by underthrusting of cold surface materials

evidence for catastrophic volcanic debris flows in pemberton valley british columbia the mount meager volcanic complex in southern british columbia is snow and ice covered and has steep glaciated and unstable slopes of hydrothermally altered volcanic deposits three large volume 108 m3 volcanic debris flow deposits derived from the mount meager volcanic complex have been identified the volcanic debris flows travelled at least 30 km downstream from the volcanic complex and inundated now populated areas of pemberton valley clay content and mineralogy of the deposits indicate that the volcanic debris flows were clay rich 5 7 clay in the matrix and derived from hydrothermally altered volcanic material the youngest volcanic debris flow deposit is interpreted to be associated with the last known volcanic eruption 2360 calendar cal years bp the other two debris flows may not have been directly associated with eruptions volcanic debris flow hazard inundation maps have been produced using the geographic information system gis based modelling program laharz the maps provide esti

evidence for protracted prograde metamorphism followed by rapid exhumation of the zermatt saas fee ophiolite major and trace element zoning in garnet in combination with rb sr sm nd and lu hf geochronology provide evidence for a protracted garnet growth history for the zermatt saas fee zsf ophiolite western alps four new lu hf ages from pfulwe c 52 46 ma and one from chamois c 52 ma are very similar to a previously published lu hf age from lago di cignana overall the similarity of geochronological and garnet zoning patterns suggests that these three localities had a similar prograde tectonic history commensurate with their similar structural position near the top of the zsf samples from the lower part of the zsf at saas fee and st jacques however produced much younger lu hf ages c 41 38 ma neither differences in whole rock geochemistry which might produce distinct garnet growth histories nor rare earth element zoning in garnet can account for the age differences in the two suites this suggests a much later prograde history for the lower part of the zsf supporting the idea that it was subducted diachronously such a model is consistent with changes in subduction vectors based on plate tectonic reconstructions where early oblique subduction which produced long prograde garnet growth changed to more orthogonal subduction which corresponds to shorter prograde garnet growth six new rb sr phengite ages range from c 42 to 39 ma and in combination with previously published rb sr ages constrain the timing of the transition from eclogite to upper greenschist facies p t conditions the proximity of the zsf in the saas fee region to the underlying continental monte rosa unit and the similarity of peak metamorphic ages suggest these two units were linked for part of their tectonic history this in turn indicates that the monte rosa may have been partly responsible for rapid exhumation of the zsf unit

metabasalts axial hot springs and the structure of hydrothermal systems at mid ocean ridges knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid ocean ridge studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the mid atlantic ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy mineral abundances and mineral compositions especially for chlorite these relationships can be interpreted in terms of the distribution of alteration with respect to time temperature and water rock ratio in young crust assemblages of chl ab ep act chl ab ep act qtz chl ab qtz and chl qtz are produced at successively higher effective sea water to rock ratios within the temperature range of the greenschist facies 250 to 450 c toward higher ratios chlorites tend to become more mg rich pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending rather than the ascending limb of a convection system this type of alteration which reflects moderately high water rock ratios may characteristically occur above a still partially molten magma chamber as the magma chamber solidifies and is then penetrated more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water rock ratios producing solutions such as those recovered from springs on the galapagos rift and the east pacific rise at 21 n localized upwelling limbs of the convection system can produce veins filled with quartz sulfides and fe rich chlorite

the tephra stratigraphy of two lakes in south central british columbia canada and its implications for mid late holocene volcanic activity at glacier peak and mount st helens washington usa several mid late holocene glacier peak tephras along with mazama and mount st helens wn and p tephras were found in cores from cooley and rockslide lakes in southeastern british columbia 300 km northeast of glacier peak the sediments in cooley lake host the late holocene glacier peak a tephra 2010 calibrated cal years bp four separate glacier peak dusty creek gpdc tephras 5780 5830 cal years bp and a glacier peak set d tephra 6060 cal years bp this is the first report of glacier peak a and d tephras in british columbia the a tephra has been correlated on the basis of glass composition and age to a late holocene glacier peak tephra in the sediments of big twin lake 75 km northeast of glacier peak the glasses in the four gpdc tephra layers from cooley lake are compositionally indistinguishable from those in mount barr cirque and frozen lakes in southwestern british columbia the layers likely represent four eruptions taking place over 50 years although set d tephra has not been correlat

a continuous holocene glacial record inferred from proglacial lake sediments in banff national park alberta canada abstract sediment cores from three proglacial lakes in northern banff national park alberta preserve a record of holocene glacial activity upvalley which is more continuous and better dated than available surficial records dating of the cores is based on tephrochronology and 16 ams14c ages of terrestrial macrofossils all cores contain a threefold sequence of lacustrine sediments overlying a late pleistocene diamicton basal lacustrine sediments 10 10014c yr old contain little organic matter sediment composition indicates a large glacigenic contribution a sharp increase in organic content marks the beginning of the altithermal interval at all three lakes this transition occurred abruptly at about 10 10014c yr b p at crowfoot lake and possibly more gradually at the other lakes altithermal sediments contain relatively little glacigenic material and during most of the altithermal glaciers may have been absent above crowfoot and bow lakes glaciers draining into hector lake appear to have persisted through the altithermal a subsequent decrease in organic content in each lake reflecting increased clastic sedimentation marks the end of the altithermal and the onset of neoglacial ice advances the transition took place between about 5800 and 400014c yr b p and may be time transgressive beginning earlier in hector lake than in crowfoot lake changing neoglacial clastic sedimentation rates through the neoglacial interval indicate two main periods of increased glacier extent between ca 3000 and 1800 varve yr ago ca 2900 190014c yr b p and during the last several hundred years during the intervening period glaciers were less extensive but much more extensive than during the recessions of the altithermal interval

diffusion limited ree uptake by eclogite garnets and its consequences for lu hf and sm nd geochronology garnets from the zermatt saas fee eclogites contain narrow central peaks for lu yb tm er and at least one additional small peak towards the rim the ree sm eu gd tb dy are depleted in the cores but show one prominent peak close to the rim these patterns cannot be modeled using rayleigh fractionation accompanied by mineral breakdown reactions instead the patterns are well explained using a transient matrix diffusion model where ree uptake is limited by diffusion in the matrix surrounding the porphyroblast observed profiles are well matched if a roughly linear radius growth rate is used the secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism the model predicts anomalously low 176lu 177hf and 147sm 144nd ratios in garnets where growth rates are fast compared to diffusion of the ree and these results have important implications for lu hf and sm nd geochronology using garnet

the high pressure stability of hydrous phases in orogenic belts an experimental approach on eclogite forming processes abstract metamorphic evolution of orogenic belts is characterized by reactions involving hydrous phases stability relations of hydrous phases depend on h 2 o activity availability during metamorphism however hydrous phases do occur in synthetic and natural systems also at water undersaturated conditions h 2 o undersaturated experiments in the cao al 2 o 3 sio 2 h 2 o model system were performed in order to predict the role of hydrous phases in h 2 o undersaturated compositions at constant pressure and temperature these experiments yielded different assemblages of hydrous anhydrous phases depending on the experimental bulk h 2 o content in natural eclogites of mafic to intermediate bulk composition common hydrous phases include amphibole lawsonite zoisite mg chloritoid paragonite and talc complex continuous reactions delimit the stability fields of the different assemblages involving these hydrous phases such continuous reactions can be used as powerful petrogenetic tools to derive p t conditions of natural eclogites derivation of h 2 o undersaturated phase relationships for natural and synthetic systems show that hydrous phases also occur in h 2 o undersaturated bulk compositions at eclogite facies conditions comparison of experimentally determined phase relationships in mafic rocks with pressure temperature evolution of orogenic terrains demonstrates that at subsolidus conditions a trend toward h 2 o saturation is necessarily maintained during prograde p t paths in orogenic belts as far as dehydration reactions occur any prograde p t path necessarily leads to a maximized occurrence of hydrous phases and thus a maximized h 2 o preservation is achieved during metamorphism

dating of ultramafic rocks from the western alps ophiolites discloses late cretaceous subduction ages in the zermatt saas zone the zermatt saas zone was part of the middle to late jurassic tethyan lithosphere that underwent oceanic metamorphism during mesozoic time and subduction during eocene time hp to uhp metamorphism in upper valtournanche serpentinite metarodingite and eclogite record a dominant s2 foliation that developed under 2 5 0 3 gpa and 600 20 c during alpine subduction serpentinites contain clinopyroxene and rare zircon porphyroclasts clinopyroxene porphyroclasts show fringes within s2 with similar compositions to that of grains defining s2 zircon cores show zoning typical of magmatic growth and thin fringes parallel to the s2 foliation these features indicate crystallization of clinopyroxene and zircon fringes during hp syn d2 metamorphism related to the alpine subduction the u pb zircon dates for cores and fringes reveal crystallization at 165 3 2 ma and 65 5 5 6 ma respectively the middle jurassic dates are in agreement with the known ages for the oceanic accretion of the tethyan lithosphere the late cretaceaous paleocene dates suggest that the zermatt saas zone experienced high pressure to ultra high pressure hp uhp metamorphism at c 16 ma earlier than previously reported this result is in agreement with the evidence that in the western alps the continental sesia lanzo zone reached the subduction climax at least from 70 ma and was exhumed during ongoing oceanic subduction our results are further evidence that the zermatt saas ophiolites diachronically recorded heterogeneous hp uhp metamorphism

an interpretive analysis of magnetotelluric data a simple analysis to determine the dimensionality of magnetotelluric resistivity variations to separate the near surface resistivity variation from the variations at depth and to find the relative resistivity variations with depth is described parameters derived from the magnetotelluric impedance tensor are mapped in pseudosection and planar view for two dimensional 2 d model data and for survey data from montana these data illustrate procedures that can qualitatively map complex resistivity structures in the lateral distance frequency domain or equivalently the lateral distance scaled vertical distance domain phase dependent parameters have better vertical resolution than parameters related to scalar apparent resistivity but the latter allow estimates of the resistivities encountered phase parameters for the montana data provide a semiquantitatively accurate cross section of a geologically known anticlinal structure this suggests a means of subsurface structural mapping with magnetotelluric data the use of these parameters with sufficiently dense and accurate data provides a first order indication of structure for subsequent quantitative modeling

late quaternary volcanic ash in the peatlands of central alberta three distinct volcanic ash layers were identified in peat deposits through visual microscopic and chemical means these layers were related to known volcanic events through the 14c dating of 34 peat samples the upper layer was probably bridge river ash deposited about 2350 years bp the middle layer was possibly st helens y tephra deposited around 3400 years bp the lower ash was related to mazama tephra deposited about 6600 years bp ash enriched layers were formed in the accumulating peat as aerial deposition of tephra occurred often for several hundred years before and after the presumed main eruption as shown by 14c dates and peat accumulation rates this implies periodic eruptions by the volcanoes not all of which produced distinct ash layers in areas distant from the source redeposition from wind eroded beds of mazama ash during the dry postglacial climatic maximum is a possibility

holocene and latest pleistocene fluctuations of stutfield glacier canadian rockies the holocene and late glacial history of fluctuations of stutfield glacier are reconstructed using moraine stratigraphy tephrochronology and dendroglaciology stratigraphic sections in the lateral moraines contain tills from at least three glacier advances separated by volcanic tephras and paleosols the oldest pre mazama till is correlated with the crowfoot advance dated elsewhere to be younger dryas equivalent a neoglacial till is found between the mazama tephra and a paleosol developed on the bridge river tephra a log dating 2400 bp from the upper part of this till indicates that this glacier advance correlated with the peyto advance culminated shortly before deposition of the bridge river tephra radiocarbon and tree ring dates from overridden trees exposed in moraine sections indicate that the initial cavell little ice age lia advance overrode this paleosol and trees after a d 1271 three subsequent phases of the cavell advance were dated by dendrochronology the maximum glacier extent occurred in the mid 18th century predating 1743 on the southern lateral although ice still occupied and tilted a tree on the north lateral in 1758 subsequent glacier advances occurred ca 1800 1816 and in the late 19th century the relative extent of the lia advances at stutfield differs from that of other major eastward flowing outlets of the columbia icefield which have maxima in the mid late 19th century this is the first study from the canadian rockies to demonstrate that the large morphologically simple lateral moraines defining the lia glacier limits are actually composite features built up pro gressively but discontinuously over the holocene and contain evidence of multiple holocene and crowfoot age gla cier advances resume la stratigraphie des moraines la tephrochronologie et la dendroglaciologie des moraines ont servi a recons truire les fluctuations a l holocene et au cours de l histoire glaciaire tardive du glacier stutfield les sections stratigra phiques dans les moraines laterales contiennent des tills d au moins trois avancees glaciaires separees par des tephras volcaniques et des paleosols le plus ancien le till pre mazama est correle a l avancee crowfoot dont on a determine l ge ailleurs comme etant un equivalent du dryas recent un till de l epoque neo glaciaire se retrouve entre le tephra mazama et un paleosol developpe sur le tephra de bridge river un billot date 2400 bp provenant de la partie supe rieure de ce till indique que cette avancee glaciaire correlee a l avancee de peyto a culmine peu de temps avant la deposition du tephra de bridge river des datations au 14 c et des series chronlogiques provenant d arbres recouverts puis exposes dans des sections de moraine indiquent que l avancee cavell initiale petit ge glaciaire a chevauche ce paleosol et les arbres apres l an 1271 la dendrochronologie a permis la datation de trois phases subsequentes de l avancee cavell l etendue glaciaire maximale eut lieu au milieu du 18e siecle avant 1743 sur le cote sud bien que la glace fut encore presente sur le cote nord et y ait bascule un arbre en 1758 des avancees glaciaires subsequentes ont eu lieu vers 1800 1816 et vers la fin du 19e siecle l etendue relative des avancees du petit ge glaciaire a stutfield differe de celles d autres glaciers majeurs du champ de glace columbia s ecoulant vers l est et qui ont eu leur maxi mum dans la deuxieme moitie du 19e siecle cette etude est la premiere provenant des rocheuses canadiennes a de monter que les grandes moraines laterales morphologiquement simples definissant les limites du glacier au petit ge glaciaire sont en fait des elements composites developpes progressivement mais de facon discontinue tout au cours de l holocene et qu elles contiennent des evidences d avancees glaciaires multiples d ge holocene et crowfoot

mid holocene glacier peak and mount st helens we tephra layers detected in lake sediments from southern british columbia using high resolution techniques abstract a glacier peak tephra has been found in the mid holocene sediment records of two subalpine lakes frozen lake in the southern coast mountains and mount barr cirque lake in the north cascade mountains of british columbia canada the age depth relationship for each lake suggests an age of 5000 5080 14 c yr b p 5500 5900 cal yr b p for the eruption which closely approximates the estimated age 5100 5500 14 c yr b p of the dusty creek tephra assemblage found near glacier peak the tephra layer which has not been reported previously from distal sites and was not readily visible in the sediments was located using contiguous sampling magnetic susceptibility measurements wet sieving and light microscopy the composition of the glass in pumice fragments was determined by electron microprobe analysis and used to confirm the probable source of this mid holocene tephra layer using the same methods the a d 1481 1482 mount st helens we tephra layer was identified in sediments from dog lake in southeastern british columbia suggesting the plume drifted further north than previously thought this high resolution method for identifying tephra layers in lake sediments which has worldwide application in tephrachronologic paleoenvironmental studies has furthered our knowledge of the timing and airfall distribution of holocene tephras from two important cascade volcanoes

varve studies at hector lake alberta canada and the relationship between glacial activity and sedimentation glacial lake sedimentary records from hector lake in banff national park alberta are examined with the aim of clarifying the relationship between glacial activity and outwash sedimentation rhythmic couplets in lake bottom sediments provide a method for accurate documentation and dating of recent sedimentation chronology as analysis of 137cs content indicates that the couplets are of annual periodicity i e varves a comparison of sedimentation rate records with recent regional glacial history indicates that high sedimentation rates of one to a few decades duration occur either during and immediately following periods of moraine deposition ie maximum ice stands or during periods of rapid ice recession comparison of sedimentary records with somewhat longer term glacial and vegetation records indicates that sedimentation rate variations of several centuries duration closely parallel changes in upvalley ice extent over this time scale high sedimentation rates occur during periods of relatively increased ice extent low rates during periods of reduced ice extent within this general pattern however sedimentation rates may remain very high for nearly a century following maximum ice stands due to the exposure of unstable glaciogenic deposits to fluvial reworking during ice recession

sequence and style of major post nappe structures simplon pennine alps abstract in the upper pennine nappe complex of the simplon pennine alps switzerland and italy at least three phases of major post nappe folding in places associated with thrusting can be distinguished these are superimposed on an earlier formed partly chaotic complex of tectonic units including the bernhard and monte rosa continental flakes and the zermatt saas and antrona ophiolite complexes the earliest post nappe folds were essentially isoclinal throughout the whole region and were accompanied by a strong schistosity which is the main foliation in most areas later two successive phases of back folding led to the present overall structure both phases typically show rapid variations in style from open folds lacking axial planar schistosity to very tight structures with complete foliation transposition this has been demonstrated by systematically mapping the major axial traces over the whole region successively removing the major structures in reverse order shows that the ophiolite complexes were originally part of a continuous unit marking an important suture between the bernhard and monte rosa nappes

the eclogitized pillows of the betic ophiolitic association relics of the tethys ocean floor incorporated in the alpine chain after subduction in the volcanic sequence of the betic ophiolitic association boa volcanic structures and textures are preserved in spite of being metamorphosed to the edogite facies the original quenched glassy margins of the pillows and lava flows are still recognizable by the darker colour fine grained textures and scarcity of phenocrysts the boa eclogitized pillows have chemical compositions very similar to the basalts enriched in lil elements erupted nowadays at the mid oceanic ridges magmatism which generated the boa most likely began under continental rift conditions at the triassic jurassic boundary and continued under ocean floor extensional conditions during the lower and middle jurassic in age and petrological characteristics this magmatism is equivalent to that of the western tethys ophiolites during the late cretaceous due to the collision of african and iberian plates the boa ophiolites were subducted and underwent a metamorphism in the eclogite fades whose climax in the lugros outcrop can be estimated at 650 700 c and about 20 kb

zoned high pressure assemblages in pillow lavas of the zermatt saas ophiolite zone switzerland pillow lavas of the zermatt saas ophiolite zone of the western swiss alps in places consist of zoned high pressure parageneses these assemblages which developed subsequent to the metamorphic peak are of lawsonite eclogite in the cores of pillows and dolomite bearing blueschist or glaucophane eclogite in the rims the matrix between the pillows comprises dolomite glaucophane rich material an analysis of reactions in the system na2o cao feo mgo al2o3 sio2 h2o co2 ncfmas h2o co2 indicates that one possible model is that these assemblages formed cofacially due to systematic intra pillow variations in the chemical potentials of h2o and co2 pillow rims equilibrated at higher values of co2 and lower values of h2o than pillow cores textural evidence suggests that infiltration has controlled the distribution of co2 generated by the breakdown of dolomite bearing assemblages during the return of the rocks to the surface variation in h2o content was caused by ocean floor hydrothermal alteration in these rocks therefore blueschist assemblages have been stabilised by higher values of co2 and eclogites by higher values of h2o if fluids were present as suggested by various lines of evidence then blueschist pillow rims equilibrated at higher co2 co2 h2o than eclogitic pillow cores

volcanology of the 2350 b p eruption of mount meager volcanic complex british columbia canada implications for hazards from eruptions in topographically complex terrain the pebble creek formation previously known as the bridge river assemblage comprises the eruptive products of a 2350 calendar year b p eruption of the mount meager volcanic complex and two rock avalanche deposits volcanic rocks of the pebble creek formation are the youngest known volcanic rocks of this complex they are dacitic in composition and con tain phenocrysts of plagioclase orthopyroxene amphi bole biotite and minor oxides in a glassy groundmass the eruption was episodic and the formation com prises fallout pumice bridge river tephra pyroclastic flows lahars and a lava flow it also includes a unique form of welded block and ash breccia derived from col lapsing fronts of the lava flow this merapi type brec cia dammed the lillooet river collapse of the dam triggered a flood that flowed down the lillooet valley the flood had an estimated total volume of 10 9 m 3 and inundated the lillooet valley to a depth of at least 30 m above the paleo valley floor 5 5 km downstream of the blockage rock avalanches comprising mainly blocks of plinth assemblage volcanic rocks an older formation making up part of the mount meager vol canic complex underlie and overlie the primary vol canic units of the formation both rock avalanches are unrelated to the 2350 b p eruption although the post eruption avalanche may have its origins in the over steepened slopes created by the explosive phase of the eruption much of the stratigraphic complexity evident in the pebble creek formation results from deposition in a narrow steep sided mountain valley containing a major river

kyanite eclogite to amphibolite fades evolution of hydrous mafic and pelitic rocks adula nappe central alps in the southern adula nappe central alps two stages of regional metamorphism have affected mafic and pelitic rocks earlier eclogite facies with a regional zonation from glaucophane eclogites to kyanite hornblende eclogites was followed by a tertiary overprint which varied from greenschist to high grade amphibolite facies despite a common metamorphic history contrasting equilibration conditions are often recorded by high pressure mafic eclogite and adjacent predominantly lower pressure pelite assemblages this pressure contrast may be explained by different overprinting rates of the two bulk compositions during unloading the rates are controlled by a mechanism in which dehydrating metapelites provide the h2o required for simultaneous overprinting of enclosed mafic eclogites by hydration

high pressure metamorphism of the zermatt saas ophiolite zone switzerland within the zermatt saas ophiolite zone of the western alps a record of polystadial high pressure metamorphism is well preserved early blueschist assemblages retained as inclusions in garnets were succeeded by eclogitic assemblages which in some rocks contained lawsonite kyanite talc and chloritoid these eclogitic assemblages formed by prograde reaction from the early blueschists subsequently reaction of these eclogites with a mixed h 2 o co 2 vapour phase led to the replacement of kyanite by paragonite and the partial replacement of omphacite garnet paragenses by assemblages containing glaucophane paragonite and ankerite high pressure eo alpine metamorphism took place under conditions of 550 600 c and 17 5 20 kbar values of a h 2 o between 0 55 and 1 are high enough to accommodate equilibration with a water rich vapour phase under the highest grade conditions the presence of such a fluid phase is locally indicated by the presence of quartz rich veins containing omphacite and kyanite this vapour phase was absorbed during retrogression later rehydration is limited to areas close to albite veins tectonic contacts and bodies of metasediment the metamorphic conditions determined for the zermatt saas zone are compatible with those suggested for over and under lying units these conditions and the p t path inferred by comparing the reaction histories with a petrogenetic grid for the system na 2 o cao mgo a1 2 o 3 sio 2 h 2 o suggest that metamorphism occurred during subduction to depths of between 60 and 70 km and subsequent exhumation during the alpine orogeny

a re examination of the three most prominent holocene tephra deposits in western canada bridge river mount st helens yn and mazama abstract volcanic ash deposits tephra in western canada are instrumental in providing independent chronologic control for many archaeological and paleoenvironmental sites in alberta tephra are a key chronologic tool in a region where radiocarbon dates are often unreliable because of the prevalence of carbonate rich bedrock and other old carbon sources such as coal however many studies using tephra for age control particularly archaeological projects identify tephra simply through field characteristics or light microscopy in both alberta and british columbia many radiocarbon dates that were used to date key tephra deposits were bulk conventional ages on peat and lake sediments which are not always reliable these factors have led to uncertainty in the age and number of bridge river and mount st helens msh set y tephra present in the region and incomplete distribution maps new major element geochemical analyses from archaeological and sedimentary sites across south central alberta complemented by new analyses of tephra from british columbia and saskatchewan refine the distribution of the bridge river msh yn and mazama tephra new geochemical data radiocarbon dates and a detailed overview of proximal msh set y stratigraphy and geochemistry show that only one msh layer yn is present in this region rather than two msh set y tephra as previously suggested additionally re assessment of age data combined with new geochemical analyses confirm that there is also only one bridge river tephra a bayesian modelled age estimate is determined for msh yn based on new ams dates on the tephra and vetted existing conventional ages providing a revised age estimate for msh yn of 3805 3535 cal bp mean of 3660 cal bp

diatom pollen and chemical evidence of postglacial climatic change at big lake south central british columbia canada abstract postglacial climatic conditions were inferred from cores taken from big lake in southern british columbia low concentrations of nonarboreal pollen and pigments near the base of the core suggest that initial conditions were cool increases in both aquatic and terrestrial production suggest warmer and moister conditions until 8500 cal yr b p hyposaline diatom assemblages increases in nonarboreal pollen and increased concentrations of pigments suggest the onset of arid conditions from 8500 to 7500 cal yr b p slightly less arid conditions are inferred from 7500 until 6660 cal yr b p based on the diatoms small increases and greater variability in biogenic silica and pigments and higher percentages of arboreal pollen at 6600 cal yr b p changes in diatoms pigments biogenic silica and organic matter suggest that big lake became fresh deep and eutrophic until 3600 cal yr b p when water levels and nutrients decreased slightly our paleoclimatic inferences are similar to pollen based studies until 6600 cal yr b p however unlike these studies our multiple lines of evidence from big lake imply large changes in effective moisture since 6000 cal yr b p

serpentinites of the zermatt saas ophiolite complex and their texture evolution the zermatt saas serpentinite complex is an integral member of the penninic ophiolites of the central alps and represents the mantle part of the oceanic lithosphere of the tethys metamorphic textures of the serpentinite preserve the complex mineralogical evolution from primary abyssal peridotite through ocean floor hydration subduction related high pressure overprint meso alpine greenschist facies metamorphism and late stage hydrothermal alteration the early ocean floor hydration of the spinel harzburgites is still visible in relic pseudomorphic bastite and locally preserved mesh textures the primary serpentine minerals were completely replaced by antigorite the stable assemblage in subduction related mylonitic serpentinites is antigorite olivine magnetite diopside the mid tertiary greenschist facies overprint is characterized by minor antigorite recrystallization textural and mineral composition data of this study prove that the hydrated mineral assemblages remained stable during high pressure metamorphism of up to 2 5 gpa and 650 c the zermatt saas serpentinites thus provide a well documented example for the lack of dehydration of a mantle fragment during subduction to 75 km depth

the little ice age in the canadian rockies abstract this paper reviews the evidence and history of glacier fluctuations during the little ice age lia in the canadian rockies episodes of synchronous glacier advance occurred in the 12th 13th early 18th and throughout the 19th centuries regional ice cover was probably greatest in the mid 19th century although in places the early 18th century advance was more extensive glaciers have lost over 25 of their area in the 20th century selective preservation of the glacier record furnishes an incomplete chronology of events through the 14th 17th centuries in contrast varve sequences provide continuous annually resolved records of sediments for at least the last millennium in some highly glacierized catchments such records have been used to infer glacier fluctuations evaluation of recent proxy climate reconstructions derived from tree rings provides independent evidence of climate fluctuations over the last millennium most regional glacier advances follow periods of reduced summer temperatures reconstructed from tree rings particularly ca 1190 1250 1280 1340 1690s and the 1800s reconstructed periods of higher precipitation at banff alberta since 1500 are 1515 1550 1585 1610 1660 1680 and the 1880s glacier advances in the early 1700s the late 1800s and in places the 1950 1970s reflect both increased precipitation and reduced summer temperatures negative glacier mass balances from 1976 to 1995 were caused by decreased winter balances the glacier fluctuation record does not contain a simple climate signal it is a complex response to several interacting factors that operate at different timescales evaluation of climate proxies over the last millennium indicates continuous variability at several superimposed timescales dominated by decade century patterns only the 19th century shows a long interval of sustained cold summers this suggests that simplistic concepts of climate over this period should be abandoned and replaced with more realistic records based on continuous proxy data series the use of the term lia should be restricted to describing a period of extended glacier cover rather than being used to define a period with specific climate conditions

optical dating studies of postglacial aeolian deposits from the south central interior of british columbia canada abstract tests of the validity of optical dating of the fine grained 4 11 m component of aeolian deposits from six sites in south central british columbia using 1 4 ev infrared excitation of potassium feldspars were made making use of known age tephra beds at one site on the western side of the fraser valley samples bracketing the mazama 7 5 7 6 ka cal yrs and bridge river tephras 2 3 ka cal yrs yielded optical ages in accordance with the known ages when a correction for thermal transfer was used the same method was applied to aeolian deposits on the fraser plateau known to have been deposited between 14 and 2 3 ka cal yrs the ages obtained from five samples from four separate exposures spanning 10 km varied from 30 to 94 ka clearly indicating that this sediment was not exposed to sufficient sunlight prior to burial as a result of a detailed investigation it is suggested that the reason is that some grains had been transported over short distances

improved age estimates for the white river and bridge river tephras western canada new 14c ages date the eruptions that produced the white river and bridge river tephras two important holocene marker beds in western canada the 14c ages were obtained on trees in growth position buried in coarse tephra and a pyroclastic flow near the source vents the mean calendric age of the white river eruption based on four 14c ages is 1147 cal years bp calibrated years approximately equivalent to calendric years or ad 803 the 2 age range obtained from the two most precise 14c ages is 1014 1256 cal years bp or ad 694 936 the mean age of the bridge river eruption determined both from i the single most precise outer ring 14c age and ii the weighted mean of six outer ring 14c ages is 2360 cal years bp or 411 bc 2 age range 2349 2704 cal years bp or 755 400 bc