late pleistocene and holocene vegetation history of the bale mountains ethiopia abstract a sediment core recovered from garba guracha a glacial lake at 3950 m altitude in the bale mountains of ethiopia at the boundary of the ericaceous and afroalpine vegetation belts provides a 16 700 year pollen record of vegetation response to climatic change the earliest vegetation recorded was sparse and composed mainly of grasses amaranthaceae chenopodiaceae and artemisia indicating an arid climate at 13 400 cal bp amaranthaceae chenopodiaceae pollen declined sharply and cyperaceae increased suggesting a change to moister conditions the younger dryas interval is represented by a small increase in artemisia and reduced cyperaceae indicating aridity just after the start of the holocene 11 200 cal bp the upper altitudinal limit of the ericaceous belt rose and woody ericaceous vegetation extended across the sanetti plateau in response to increased moisture and temperature the marked change from clastic to organic lake sedimentation at this time reflects the increase in woody vegetation cover in the lake catchment accompanied by soil stabilisation and increased leaf litter and soil humus content from about 6000 cal bp and especially after 4500 cal bp mid altitude dry afromontane juniper podocarpus forests developed on the northern slopes of the mountains in response to reduced rainfall in a shortened wet season erica shrub and forest decreased in area and altitude and the afroalpine ecosystem expanded on the plateau podocarpus declined from about 2000 cal bp as juniperus increased to its present dominance at 2500 3300 m altitude human impact on the high altitude afroalpine and ericaceous vegetation has been relatively minor confirming that the endemic biodiversity of the ethiopian mountains is a legacy of natural holocene vegetation change following repeated expansion and contraction of the upland ecosystems during the quaternary

quaternary glaciation of the bale mountains ethiopia central ethiopia comprises a high plateau at 2000 3000 m formed from tertiary lava flows and bisected by the eastern african rift ten volcanic mountains rise to altitudes of just over 4000 m but on only three has quaternary glaciation been substantiated by published field observations on the bale mountains 4400 m a previous report based on limited evidence proposed an ice cap extending to 600 km2 based on aerial photographs and ground surveys this paper reports evidence of a more complex situation a wide spread of large erratic boulders on the plateau records a central ice cap of 30 km2 though ice probably extended for a further 40 km2 further north two groups of deeply incised and clearly glaciated valleys contain moraines and roches moutonnees 60 km2 on interfluves between them and on the open north slopes are moraines from an earlier stage of the same glaciation or from a distinct older event altogether about 180 km2 may have been glaciated cores dated by 14c from inside and outside the glaciated area suggest that at least the northern valley glaciers may date from the last glacial maximum estimated equilibrium line altitudes for these glaciers and the ice cap are 3750 4230 m

causal links between nile floods and eastern mediterranean sapropel formation during the past 125 kyr confirmed by osl and radiocarbon dating of blue and white nile sediments abstract it has long been hypothesised that beds of highly organic mud or sapropels seen in marine sediment cores retrieved from the floor of the eastern mediterranean accumulated during times of high nile fluvial discharge our recent fieldwork in the valleys of the blue nile the white nile and the main nile has for the first time revealed a sequence of extreme flood episodes synchronous with sapropel units s5 124 kyr s4 102 kyr s3 81 kyr s2 55 kyr and s1 13 5 6 5 kyr there are more weakly defined links with nile floods and sapropel units s9 240 kyr s8 217 kyr s7 195 kyr s6 172 kyr but the dating error terms are too large to allow us to be too definite during times of extreme floods over the past 125 kyr wide distributary channels of the blue nile flowed across the gezira alluvial fan in central sudan and transported a bed load of sand and gravel into the lower white nile valley the sands were reworked by wind to form source bordering dunes all of which contain heavy minerals of ethiopian provenance these source bordering dunes were active at 115 105 kyr 60 kyr and 12 7 kyr all times of extreme blue nile floods the flood and dune sediments were dated using a combination of optically stimulated luminescence osl and radiocarbon analyses the quaternary record of nile floods discussed here shows a precessional signal and reflects episodes of stronger summer monsoon and more northerly seasonal movement of the itcz linked to times of higher insolation in northern tropical latitudes progressive aggradation of holocene nile channels in northern sudan has had a profound influence upon human settlement in the last 8 kyr

quaternary glaciation in africa key chronologies and climatic implications multiple episodes of quaternary glaciation are evidenced on 10 distinct mountain localities throughout africa with the best dated sites from kilimanjaro and mt kenya in equatorial east africa a general paucity of radiogenic dates constrains the glacial chronology and regional sequences have largely been based on correlations by relative weathering of features excellent glacial moraine preservation and other features of erosion put limits on the spatial extent of palaeoglaciers and have facilitated advances in palaeo equilibrium line altitude ela analyses all radiocarbon dates n 30 provide minimum ages of deglaciation and are consistent with a widespread synchronous advance during the last glacial maximum lgm 21 2 ka k ar dates on kilimanjaro lava flows initially recognised up to three glaciations prior to the lgm over 100 cosmogenic radionuclide crn 36 cl exposure ages on glacial erratics and bedrock from kilimanjaro and mt kenya date 16 landforms confirming maximum glacial extent prior to marine isotope stage mis 10 no glacial geological features have been dated to mis 6 while on kilimanjaro the crn ages confirm a clear lgm mis 2 maximum mt kenya lacks definitive lgm moraines lateglacial advances or stillstands on both mountains are suggested by low relief moraines dating to 14 6 1 2 ka two boulders on mt kenya and 16 3 1 9 ka and 15 8 2 5 ka on kilimanjaro multiple moraines upvalley have early holocene ages ranging from 11 2 0 8 to 8 6 0 2 ka isolated and equivocal dates on both mountains also provide speculative evidence that the maximum extent since mis 5e may have occurred close to 30 ka similar to sites in south america new palaeoclimatic understanding from lake and ocean proxies suggests hypotheses for the source of low latitude climate changes causing the advances that can be tested with improved dates a high degree of intra regional variance in ela is observed with differences by aspect reflecting precipitation gradients future work should include more crn dates improved mapping and inverse climate mass balance modelling copyright 2008 john wiley sons ltd

carbon geochemistry of serpentinites in the lost city hydrothermal system 30 n mar abstract the carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered at the lost city hydrothermal field lchf and drilled at iodp hole 1309d at the central dome of the atlantis massif mid atlantic ridge 30 n was examined to characterize carbon sources and speciation in oceanic basement rocks affected by long lived hydrothermal alteration our study presents new data on the geochemistry of organic carbon in the oceanic lithosphere and provides constraints on the fate of dissolved organic carbon in seawater during serpentinization the basement rocks of the atlantis massif are characterized by total carbon tc contents of 59 ppm to 1 6 wt and 13ctc values ranging from 28 7 to 2 3 in contrast total organic carbon toc concentrations and isotopic compositions are relatively constant 13ctoc 28 9 to 21 5 and variations in 13ctc reflect mixing of organic carbon with carbonates of marine origin saturated hydrocarbons extracted from serpentinites beneath the lchf consist of n alkanes ranging from c15 to c30 longer chain hydrocarbons up to c40 are observed in olivine rich samples from the central dome iodp hole 1309d occurrences of isoprenoids pristane phytane and squalane polycyclic compounds hopanes and steranes and higher relative abundances of n c16 to n c20 alkanes in the serpentinites of the southern wall suggest a marine organic input the vent fluids are characterized by high concentrations of methane and hydrogen with a putative abiotic origin of hydrocarbons however evidence for an inorganic source of n alkanes in the basement rocks remains equivocal we propose that high seawater fluxes in the southern part of the atlantis massif likely favor the transport and incorporation of marine dissolved organic carbon and overprints possible abiotic geochemical signatures the presence of pristane phytane and squalane biomarkers in olivine rich samples associated with local faults at the central dome implies fracture controlled seawater circulation deep into the gabbroic core of the massif thus our study indicates that hydrocarbons account for an important proportion of the total carbon stored in the atlantis massif basement and suggests that serpentinites may represent an important as yet unidentified reservoir for dissolved organic carbon doc from seawater

late quaternary floods and droughts in the nile valley sudan new evidence from optically stimulated luminescence and ams radiocarbon dating our results show that the late pleistocene nile in northern sudan was shifting position and actively aggrading at 145 20 kyr 83 24 kyr 32 8 kyr and 20 7 0 2 kyr and indicate for the first time a phase of high energy flow in the white nile at 27 8 3 2 kyr with still high but somewhat reduced flow in that river at 13 3 kyr 10 kyr and 4 8e4 0 kyr beach ridges associated with a 386 m strandline of the white nile have osl ages of 27 5 2 7 kyr and 14 5 1 6 kyr the holocene terraces and former channels of the main nile have ages of 11 kyr 6 5e5 0 kyr and 4 8e4 0 kyr after which there was a general decline in flood discharge the now arid main nile valley in northern sudan was significantly wetter during the early to middle holocene with a lake up to 450 km 2 in area fed by an overflow channel from the early holocene nile between 9 5 kyr and 7 5 kyr previously stable late pleistocene dunes were reactivated at intervals during the holocene with five samples from the white nile valley indicating brief phases of holocene dune activity at 9 9 2 0 kyr 9 0 2 8 kyr 6 6 0 9 kyr 4 8 0 9 kyr and 2 9 0 5 kyr the earliest of which occurred within periods of generally wetter climate and higher nile flow the youngest freshwater shells on the khor abu habl alluvial fan west of the white nile correspond to a time of regionally wetter climate between 1 7 and 1 0 kyr our results suggest that millennial scale climatic instability may have been characteristic of holocene climates in this region 2010 elsevier ltd all rights reserved

using equilibrium thermodynamics in the study of metasomatic alteration illustrated by an application to serpentinites abstract metasomatic rocks in which mineralogical change is driven by infiltration of an externally derived fluid are challenging for the application of equilibrium thermodynamics because fluid driven changes in rock bulk composition can be faster than diffusive within grain re equilibration nevertheless careful definition of systems in terms of appropriate length scales an informed choice regarding the controlling variables and detailed petrological analysis can provide useful results thermodynamic calculations using methods that solve for equilibrium between a set of independent end member reactions produce valid results regardless of the identity of the controlling variables but determination of the relationship between cause and effect in metasomatic rocks is best performed with some knowledge of the parameters that drive the evolution of the system the correspondence between observations of serpentinised harzburgites from new caledonia and the results of the application of thermodynamic techniques suggest that equilibrium thermodynamics can be applied to these rocks on appropriate small length scales varying from micron to cm the primary drivers for mineralogical change are proposed to be 1 infiltration of a h 2 o fluid which is buffered to lower a o 2 by the rock with resultant production of h 2 and the resulting change in the redox budget of the rocks and 2 gradients in a sio 2 caused by mm to cm scale variation in si mg fe inherited from the harzburgite protolith

serpentinization and heat generation constraints from lost city and rainbow hydrothermal systems abstract the discovery of ultramafic hosted hydrothermal systems at rainbow 36 n mar and lost city a vent site approximately 15 km west of the mar at 30 n provides unique perspectives on chemical and heat generating processes associated with serpentinization at a range of chemical and physical conditions heat balance calculations together with constraints imposed by geochemical modeling indicate that significant changes in temperature are not likely to occur at either vent system as a result of the exothermic nature of olivine hydrolysis at rainbow the relatively high temperatures in subseafloor reaction zones in excess of 400 c which must be linked to magmatic processes inhibit olivine hydrolysis effectively precluding mineralization induced heating effects geochemical modeling of the lost city vent fluids indicates temperatures in excess of those measured 40 75 c the relatively high subseafloor temperatures 200 50 c requires conductive cooling of the fluids on ascent to the seafloor a scenario in keeping with the mineralization of chimney structures actually observed although the intermediate temperatures predicted for subseafloor reaction zones at lost city could be expected to enhance olivine to serpentine conversion dissolved cl k cl and na cl ratios of the lost city vent fluids are virtually unchanged from seawater values and indicate little hydration of olivine which is a necessary condition for exothermic heat generation by serpentinization apparently the fluid rock mass ratio is too high or fluid residence times too low for this to occur to any significant extent thus in spite of the off axis location of the lost city vents and apparent lack of a localized heat source mineralization reactions likely play an insignificant role in accounting for hydrothermal circulation it is more likely that tectonic processes associated with the slow spreading mar permit access of seawater to relatively deep and still hot lithospheric units and or near axis magmatic heat sources before venting additional chemical and physical temperature flow rate data for lost city and similar hydrothermal systems are needed to test key elements of the proposed model

late quaternary glaciation in southern africa moraine ridges and glacial deposits at mount enterprise in the drakensberg of the eastern cape province south africa late quaternary moraines and diamictons containing striated clasts are described from near elliot in the drakensberg of south africa an equilibrium line altitude ela of 2109 m is suggested for the palaeoglacier associated with the innermost moraine this glacier was fed by a very extensive snowblow area and the ela may reflect the temperature precipitation wind ela rather than the temperature precipitation ela and be considerably below the regional ela mean annual air temperatures when glaciation occurred were probably at least 10 0 c below those of the present copyright 2001 john wiley sons ltd

geochemical constraints on the modes of carbonate precipitation in peridotites from the logatchev hydrothermal vent field and gakkel ridge the ultramafic hosted logatchev hydrothermal field lhf at 15 n on the mid atlantic ridge and the arctic gakkel ridge gr feature carbonate precipitates aragonite calcite and dolomite in voids and fractures within different types of host rocks we present chemical and sr isotopic compositions of these different carbonates to examine the conditions that led to their formation our data reveal that different processes have led to the precipitation of carbonates in the various settings seawater like 87sr 86sr ratios for aragonite in serpentinites 0 70909 to 0 70917 from the lhf are similar to those of aragonite from the gr 0 70912 to 0 70917 and indicate aragonite precipitation from seawater at ambient conditions at both sites aragonite veins in sulfide breccias from lhf also have seawater like sr isotope compositions 0 70909 to 0 70915 however their rare earth element ree patterns show a clear positive europium eu anomaly indicative of a small 1 hydrothermal contribution in contrast to aragonite dolomite from the lhf has precipitated at much higher temperatures 100 c and yet its 87sr 86sr ratios 0 70896 to 0 70907 are only slightly lower than those of aragonite even higher temperatures are calculated for the precipitation of deformed calcite veins in serpentine talc fault schists form north of the lhf these calcites show unradiogenic 87sr 86sr ratios 0 70460 to 0 70499 indicative of precipitation from evolved hydrothermal fluids a simple mixing model based on sr mass balance and enthalpy conservation indicates strongly variable conditions of fluid mixing and heat transfers involved in carbonate formation dolomite precipitated from a mixture of 97 seawater and 3 hydrothermal fluid that should have had a temperature of approximately 14 c assuming that no heat was transferred the much higher apparent precipitation temperatures based on oxygen isotopes 100 c may be indicative of conductive heating probably of seawater prior to mixing the hydrothermal calcite in the fault schist has precipitated from a mixture of 67 hydrothermal fluid and 33 seawater which should have had an isenthalpic mixing temperature of 250 c the significantly lower temperatures calculated from oxygen isotopes are likely due to conductive cooling of hydrothermal fluid discharging along faults rare earth element patterns corroborate the results of the mixing model since the hydrothermal calcite which formed from waters with the greatest hydrothermal contribution has ree patterns that closely resemble those of vent fluids from the lhf our results demonstrate for the first time that 1 precipitation from pure seawater 2 conductive heating of seawater and 3 conductive cooling of hydrothermal fluids in the sub seafloor all can lead to carbonate precipitation within a single ultramafic hosted hydrothermal system

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization crystal size distributions csds measured in metamorphic rocks yield quantitative information about crystal nucleation and growth rates growth times and the degree of overstepping t of reactions during metamorphism csds are described through use of a population density function n dn dl where n is the cumulative number of crystals per unit volume and l is a linear crystal size plots of ln n vs l for olivine pyroxene and magnetite in high temperature 1000 c basalt hornfelses from the isle of skye define linear arrays indicating continuous nucleation and growth of crystals during metamorphism using the slope and intercept of these linear plots in conjunction with growth rate estimates we infer minimum mineral growth times of less than 100 years at t 10 c and nucleation rates between 10 4 and 10 1 cm3 s garnet and magnetite in regionally metamorphosed pelitic schists from south central maine have csds which are bell shaped we interpret this form to be the result of two processes 1 initial continuous nucleation and growth of crystals and 2 later loss of small crystals due to annealing the large crystals in regional metamorphic rocks retain the original size frequency distribution and may be used to obtain quantitative information on the original conditions of crystal nucleation and growth the extent of annealing increases with increasing metamorphic grade and could be used to estimate the duration of annealing conditions if the value of a rate constant were known finally the different forms of crystal size distributions directly reflect differences in the thermal histories of regional vs contact metamorphosed rocks because contact metamorphism involves high temperatures for short durations resulting csds are linear and unaffected by annealing similar to those produced by crystallization from a melt because regional metamorphism involves prolonged cooling from high temperatures primary linear csds are later modified by annealing to bell shapes

the geological setting of the ultramafic hosted logatchev hydrothermal field 14 45 n mid atlantic ridge and its influence on massive sulfide formation the logatchev hydrothermal field 14 degrees 45 n on the mar is one of a few submarine hydrothermal systems associated with ultramafic rocks it is situated on the eastern inner flank of the rift valley wall 7 km away from the spreading axis and its formation has previously been linked to detachment faulting and core complex formation geological mapping during various rov dives geological sampling and shallow drilling reveal a structural control of hydrothermal activity as well as its location in a debris flow consisting of heterogeneous ultramafic and mafic intrusive rocks the mixed mafic ultramafic host rock lithology is in agreement with published vent fluid and gas chemical data showing characteristics for interaction with mafic as well as with ultramafic rocks massive sulfide formation is more focused than previously thought and likely limited to a thin veneer at the seafloor the logatchev hydrothermal field shows a number of peculiarities that are unusual for most other hydrothermal systems one of these are so called smoking craters seafloor depressions that are several meters wide characterized by an elevated crater rim made up partly of sulfide talus but also of abundant wall rock material at these smoking craters hydrothermal venting occurs directly from holes within the craters and from small cm to dm high cu rich chimneys occurring at the crater rim based on geological mapping and sampling we suggest that these smoking craters are the product of processes related to the regional and local geological setting in an ultramafic hosted off axis location with abundant landslides as well as off axis gabbroic intrusions providing the heat for the hydrothermal convection cell c 2009 elsevier b v all rights reserved

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

timing of glaciation during the last glacial cycle evaluating the concept of a global last glacial maximum lgm abstract it has long been known that mountain glaciers and continental ice sheets around the globe reached their respective maximum extent at different times during the last glacial cycle often well before the global last glacial maximum lgm c 23 19 ka which is formally defined by peaks in global sea level and marine oxygen isotope records however there is increasing evidence from around the world that it was not only mountain glaciers which were asynchronous with the global lgm but also some regions of the large continental glaciers the barents kara ice sheet in northern eurasia together with a majority of ice masses throughout asia and australasia reached their maximum early in the last glacial cycle a few thousand years before the global lgm period the east antarctic ice sheet also reached its maximum extent several millennia before the global lgm in numerous mountainous regions at high mid and low latitudes across the world glaciers reached their maximum extent before marine isotope stage mis 2 in mis 5 4 and 3 this is in contrast to most sectors of the laurentide ice sheet the cordilleran ice sheet the se sector of the fennoscandinavian ice sheet and the alpine ice sheet in central europe which appear to have reached their maximum close to the global lgm in mis 2 the diachronous maximum extents of both mountain glaciers and continental ice sheets during the last glacial cycle means that the term and acronym last glacial maximum lgm has limited chronostratigraphical meaning when correlating glacial deposits and landforms

30 000 years of hydrothermal activity at the lost city vent field strontium carbon and oxygen isotope data and radiocarbon ages document at least 30 000 years of hydrothermal activity driven by serpentinization reactions at lost city serpentinization beneath this off axis field is estimated to occur at a minimum rate of 1 2 10 4 cubic kilometers per year the access of seawater to relatively cool fresh peridotite coupled with faulting volumetric expansion and mass wasting processes are crucial to sustain such systems the amount of heat produced by serpentinization of peridotite massifs typical of slow and ultraslow spreading environments has the potential to drive lost city type systems for hundreds of thousands possibly millions of years

carbon oxygen isotopic covariation in hydrothermal calcite during degassing of co 2 the effect of the outgassing of co2 from a hydrothermal fluid on the c and o isotopic compositions of calcite which is precipitated from this fluid is quantitatively modelled in terms of batch and rayleigh distillation equations both co2 degassing and calcite precipitation are considered to be the removal mechanisms for dissolved carbon species from the fluid combined degassing precipitation models are then developed by taking h2co3 and hco 3 respectively as the dominant dissolved carbon species a positive correlation array between 13c and 13o values of calcite can be yielded by the precipitation of calcite from a h2co 3 dominant fluid accompanied by a progressive decrease in temperature during co2 degassing whereas calcite precipitated from a hco 3 dominant fluid under the same conditions tends to display much smaller variation in 13c values than in 18o values the combined processes of co2 degassing and calcite precipitation result in lowering the 13c value of calcites with respect to those precipitated in a closed system simply due to temperature effect carbon and oxygen isotopic data for calcite from the kushikino gold mining area in japan illustrate the application of quantitative modelling and degassing of co2 is suggested as a more likely cause for the precipitation of the calcite and quartz in this mining area

high resolution sedimentary record of the last deglaciation from a high altitude lake in ethiopia sedimentological and geochemical studies conducted on a 15 82 m long core collected from lake garba guracha ethiopia associated with a precise ams 14c time scale document a unique record of the sedimentary processes linked to the progressive retreat of a high altitude glacier in the bale mountains since 17 000 yr cal bp lake sedimentation is interpreted as the result of discharges of meltwater and glaciogenic sediment which progressively filled the accommodation space generated by glacier retreat within the basin monogenic sediment originated from glacial erosion of the trachytic tuff forming the cirque floor ice melting ended progressively between 12 600 and 11 800 cal bp as suggested by the decrease in sedimentation energy followed by a sharp change in sedimentary facies from 11 800 cal bp the lake reached its maximum development and clastic input was replaced by organic rich sedimentation this relates to a major increase of lake productivity which lasted up to 4500 cal bp from this period a lowering in productivity reflects the widespread dryness which occurred throughout the east african tropics

behavior of li and its isotopes during serpentinization of oceanic peridotites analyses of li and li isotopes in serpentinized peridotites have been performed using thermo ionisation mass spectrometry tims and secondary ion mass spectrometry sims techniques on samples collected from the southwest indian ridge swir in the bulk samples li concentrations range from 0 6 to 8 2 ppm while whole rock 6li values range from 2 9 to 14 in situ analyses display a greater range in both li concentration 0 1 19 5 ppm and li isotopic composition 27 to 19 with the serpentinized portions having higher li concentrations than the associated relict phases these variations may reflect changes in li partitioning and isotopic fractionation between serpentine and fluid with temperature and water rock ratio they may also be explained by changes in the composition of the serpentinizing fluid over the course of serpentinization as the serpentine forms by interaction with a circulating fluid it preferentially removes 6li causing the li in the fluid to become isotopically heavier the isotopic composition of the initial hydrothermal fluid is dominated by basalt derived li which easily overwhelms the very low li content originally present in seawater as this fluid circulates through ultramafic rocks it induces the formation of serpentine that incorporates this mantle derived li hence li in serpentine is mainly derived from oceanic crust rather than from seawater and serpentinization involves li recycling within this crust consequently li isotopes are good tracers of the hydrothermal contribution in serpentinizing fluid these results imply that serpentinized peridotites are probably only a minor sink of oceanic li

evidence from fluid seeps atop serpentine seamounts in the mariana forearc clues for emplacement of the seamounts and their relationship to forearc tectonics abstract the mariana forearc is a non accretionary forearc with numerous seamounts dredge hauls from these seamounts are composed primarily of serpentinite indicating a non volcanic origin in addition dredged carbonate samples reveal 1 the presence of aragonite in water depths greater than the aragonite compensation depth 2 unusual porosity and cement types 3 light carbon and heavy oxygen isotopic compositions and 4 an unusual trace element geochemistry of aragonite and calcite these mineralogic petrologic and geochemical characteristics formed the basis of a prediction that methane bearing fluids are venting from these seamounts subsequent to this prediction submersible dives on a mariana forearc seamount found and sampled chimneys seeping cool water that contains dissolved methane the serpentine substrate beneath the chimneys appears to form from cold gravitational flows of serpentine material fluid inclusion contents of chimney samples also show an earlier history of methane as well as longer chain hydrocarbons aromatics and acetate ions drilling into the flank and the summit of this seamount by the ocean drilling program during leg 125 confirmed the presence of methane other light alkanes and organic acids in the present day interstitial pore fluid of the cores the presence of aromatic compounds and organic acids if derived from the maturation of organic compounds indicates a thermogenic origin for these volatiles strontium isotopic ratios of the aragonitic chimney samples and of the present day interstitial waters range from present day seawater values to significantly lower values the strontium isotopic ratios of the pore waters within the seamount trend toward a relatively less radiogenic composition with increasing depth below the seafloor this strontium isotopic signature is probably not the consequence of near surface processes but more likely a contribution from an igneous source deeper in the lithosphere the lack of a large accretionary prism in the mariana forearc removes the complication of fluid contribution from dewatering of the prism by compression during convergence as in the oregon washington or barbados margins the organic compounds in the fluid inclusions and interstitial pore fluids as well as the isotopic and the geochemical composition of the carbonates and the dissolved constituents suggest that fluids are derived from dewatering compression and dehydration of subducted sediments and the subducting plate furthermore the composition of the fluid is influenced by thermal maturation of organics in subducted sediments as well as by a contribution from a deep igneous source

glacial and periglacial geomorphology and its paleoclimatological significance in three north ethiopian mountains including a detailed geomorphological map abstract geomorphological investigations and detailed mapping of past and present peri glacial landforms are required in order to understand the impact of climatic anomalies the ethiopian highlands show a great variety in past and contemporary climate and therefore in the occurrence of glacial and periglacial landforms however only a few mountain areas have been studied and detailed geomorphological understanding is lacking in order to allow a fine reconstruction of the impact of the past glacial cycle on the geomorphology vegetation complexes and temperature anomalies a detailed geomorphological map of three mountain areas mt ferrah amba 12 51 n 39 29 e mt lib amba 12 04 n 39 22 and mt abuna yosef 12 08 n 39 11 e was produced in all three study areas inactive solifluction lobes presumably from the last glacial maximum lgm were found in the highest study area of abuna yosef three sites were discovered bearing morainic material from small late pleistocene glaciers these marginal glaciers occurred below the modeled snowline and existed because of local topo climatic conditions evidence of such pleistocene avalanche fed glaciers in ethiopia and africa has not been produced earlier current frost action is limited to frost cracks and small scale patterned ground phenomena the depression of the altitudinal belts of periglacial and glacial processes during the last cold period was assessed through periglacial and glacial landform mapping and comparisons with data from other mountain areas taking latitude into account the depression of glacial and periglacial belts of approximately 600 m implies a temperature drop around 6 c in the last cold period this cooling is in line with temperature depressions elsewhere in east africa during the lgm this study serves as a case study for all the intermediate mountains 3500 4200 m of the north ethiopian highlands

compositional controls on hydrogen generation during serpentinization of ultramafic rocks abstract where ultramafic rocks are exposed to water at temperatures 2 formation are not well established we used thermodynamic computations to examine the equilibrium mineral assemblages mineral compositions and the chemistry of fluids during serpentinization of 21 different ultramafic rock compositions and 10 distinct compositions of olivine between 25 c and 400 c at 50 mpa our models predict some systematic differences between serpentinization of olivine dominated lithologies i e peridotite and of orthopyroxene dominated lithologies i e pyroxenite most notably it is predicted that serpentinization of peridotite causes the formation of serpentine having elevated fe 3 fe 3 fe 2 values fe bearing brucite at temperatures ca 320 c and magnetite at temperatures ca 200 c while serpentinization of pyroxenite does not produce magnetite but instead forms fe rich serpentine with relative low fe 3 fe 3 fe 2 values and fe poor talc the predicted activities of dissolved hydrogen a h 2 aq dissolved silica a sio 2 aq as well as the ph vary accordingly at temperatures ca 350 c fluids interacting with peridotite are more reducing have lower a sio 2 aq and higher ph than fluids interacting with pyroxenite a direct correlation between the iron content of olivine its stability relative to water temperature and a h 2 aq is apparent from our calculations in contrast to forsterite rich olivine fayalite rich olivine can be stable to temperatures as low as 180 c in the presence of water as a consequence the predicted ah 2 aq for serpentinization of fayalite is maximal at temperatures 180 c

fossil oceanic core complexes recognized in the blueschist metaophiolites of western alps and corsica tethyan ophiolites show an apparent poorly organized association of ultramafic andmafic rocks by contrast tothe complete mantle crustal sections of semail type ophiolite sheets tethyan ophiolites are characterized by asmaller amount of mafic rocks gabbros and basalts by the absence of any sheeted dyke complex and by thefrequent occurrence of oceanic sediments stratigraphically overlying mantle derived peridotites and associatedgabbroic intrusions therefore they are considered as typical remnants of oceanic lithosphere formed in slowspreadingenvironment or in ocean continent transition at distal passive margins in the very first models offormation of the tethyan ophiolites in the years 1980 the geodynamical processes leading to mantle unroofingwere poorly understood due to the paucity of data and concepts available at that time from the present dayoceans in particular at that time little work had focused on the distribution origin and significance ofmafic rocks with respect to the dominant surrounding ultramafics here we reconsider the geology of sometypical metaophiolites from the western alps and corsica and we show how results from the past decadeobtained in the current oceans ask for reassessing the significance of the tethyan ophiolites in general revisitedexamples include a set of representative metaophiolites from the blueschists units of the western alps queyrasregion and from alpine corsica golo valley field relationships between the ophiolitic basement andthe metasedimentary metavolcanic oceanic cover are described outlining a typical character of the tethyanophiolite lithological associations jurassic marbles and polymictic ophiolite metabreccias are unconformablyoverlying the mantle gabbo basement in a way strictly similar to what is observed in the non metamorphicappennine ophiolites or chenaillet massif this confirms that very early tectonic juxtaposition of ultramafic andmafic rocks occurred in the oceanic domain before subduction this juxtaposition resulted from tectonic activitythat is now assigned to the development of detachment faults and to the formation of oceanic core complexes occs at the axis of slow spreading ridges this fundamental plate tectonics process is responsible for the exhumationand for the axial denudation of mantle rocks and gabbros at diverging plate boundaries in addition fieldrelationships between the discontinuous basaltic formations and the ultramafic mafic basement indicate that thistectonic stage is followed or not by a volcanic stage we discuss this issue in the light of available field constraints

petrologic and stable isotopic studies of a fossil hydrothermal system in ultramafic environment chenaillet ophicalcites western alps france processes of carbonate cementation abstract the late jurassic chenaillet ophiolitic complex western alps represents parts of an oceanic core complex of the liguria piemonte domain a model for the origin and evolution of the chenaillet ophicalcites based on textural and isotopic characterization is presented the chenaillet ophicalcites correspond to brecciated serpentinized peridotites that record seafloor shallow serpentinization at a minimum temperatures of 150 c followed by authigenic carbonation carbonation starts with a network of micrometric to millimetric pre or syn clast formation calcite veins accompanied by a pervasive carbonation of residual olivine and serpentine inside the serpentinite mesh core a matrix of small calcite 13 c vpdb values that range between 5 and 0 4 the lower values were obtained for calcite within the serpentinite clasts the 18 o vsmow values have a range between 11 and 16 in carbonated clasts the 18 o values in the matrix are fairly homogeneous with an average at 12 and the late calcite veins have values between 12 5 and 15 5 these values suggest a relatively high temperature of formation for all the carbonates carbonates within clast are mainly characterized by a formation temperature in the range of 110 c to 180 c assuming a 18 o value of seawater of 0 the matrix forms at a temperature of ca 165 c late veins are characterized by a formation temperature ranging between 120and 155 c we propose a model where serpentinization is followed by discrete carbonation then brecciation and cementation as a consequence of continuous hydrothermal fluid circulation in the serpentinite basement this is comparable to observations made in the stockwork of present day long lived oceanic hydrothermal systems

late quaternary rapid climate change in northern chile analyses of terrigenous sediments from the chilean continental slope off the southern border of the atacama desert 27 5 s focusing on illite crystallinity and the fe al ratio of the sediments reveal a high frequency variability of the position of the southern westerlies which is very similar to the coeval short term climatic events known from greenland ice cores and from north atlantic sediments besides showing dominantly precession driven variability in precipitation over the andes these analyses also reveal rapid changes in weathering intensity along the chilean coastal range during the last 80 000 years these rapid changes occur at much shorter timescales than the 19 100 kyr orbital forcing of the milankovitch cycles

cemented mounds and hydrothermal sediments on the detachment surface at kane megamullion a new manifestation of hydrothermal venting 2 long lived detachment faults are now known to be important in tectonic evolution of slow spreading mid ocean ridges and there is increasing evidence that fluid flow plays a critical role in development of detachment systems here we document a new manifestation of low temperature hydrothermal venting associated with the detachment fault that formed kane megamullion 3 3 2 1 m y ago in the western rift valley wall of the mid atlantic ridge hydrothermal effects on the detachment surface include 1 cemented mounds of igneous rock and chalk debris containing hydrothermal mn oxides and fe oxyhydroxides and 2 layered deposits of similar fe mn minerals interbedded chalks mounds are roughly conical 1 10 m high and contain primarily basalts with lesser gabbro serpentinite and polymict breccia the layered fe mn rich sediments are flat bedded to contorted and locally are buckled into low relief linear or polygonal ridges we propose that the mounds formed where hydrothermal fluids discharged through the detachment hanging wall near the active fault trace hydrothermal precipitates cemented hanging wall debris and welded it to the footwall and this debris persisted as mounds as the footwall was exhumed and surrounding unconsolidated material sloughed off the sloping detachment surface some of the layered fe mn rich deposits may have precipitated from fluids discharging from the hanging wall vents but they also precipitated from low temperature fluids venting from the exposed footwall through overlying chalks observed natural disturbance and abnormally thin hydrogenous fe mn crusts on some contorted hydrothermal fe mn rich chalks on 2 7 ma crust suggest diffuse venting that is geologically recent results of this study imply that there are significant fluid pathways through all parts of detachment systems and that low temperature venting through fractured detachment footwalls may continue for several million years off axis

ophicalcites from the northern pyrenean belt a field petrographic and stable isotope study brecciated and fractured peridotites with a carbonate matrix referred to as ophicalcites are common features of mantle rocks exhumed in passive margins and mid oceanic ridges ophicalcites have been found in close association with massive peridotites which form the numerous ultramafic bodies scattered along the north pyrenean zone npz on the northern flank of the pyrenean belt we present the first field textural and stable isotopic characterization of these rocks our observations show that pyrenean ophicalcites belong to three main types 1 a wide variety of breccias composed of sorted or unsorted millimeter to meter sized clasts of fresh or oxidized ultramafic material in a fine grained calcitic matrix 2 calcitic veins penetrating into fractured serpentine and fresh peridotite and 3 pervasive substitution of serpentine minerals by calcite stable isotopic analyses o c have been conducted on the carbonate matrix veins and clasts of samples from 12 pyrenean ultramafic bodies we show that the pyrenean ophicalcites are the product of three distinct genetic processes 1 pervasive ophicalcite resulting from relatively deep and hot hydrothermal activity 2 ophicalcites in veins resulting from tectonic fracturing and cooler hydrothermal activity and 3 polymictic breccias resulting from sedimentary processes occurring after the exposure of subcontinental mantle as portions of the floor of basins which opened during the mid cretaceous we highlight a major difference between the eastern and western pyrenean ophicalcites belonging respectively to the sedimentary and to the hydrothermal types our data set points to a possible origin of the sedimentary ophicalcites in continental endorheic basins but a post depositional evolution by circulation of metamorphic fluids or an origin from relatively warm marine waters cannot be ruled out finally we discuss the significance of such discrepancy in the characteristics of the npz ophicalcites in the frame of the variable exhumation history of the peridotites all along the pyrenean realm

oxygen self diffusion in calcite dependence on temperature and water fugacity oxygen self diffusion in natural calcite single crystals was studied hydrothermally at 400 800 c and 10 350 mpa confining pressure diffusion coefficients d were determined from18o concentration profiles measured with an ion microprobe at 100 mpa confining pressure the arrhenius parameters yield an activation energy q 173 6 kj mole and pre exponential factor d0 7 10 9 m2 s over the temperature range 400 800 c and there is no measurable anisotropy constant temperature experiments at 700 c indicate that there is a strong linear correlation slope 0 9 of d with water fugacity over the range 4 240 mpa consistent with the interpretation that in the presence of water the dominant oxygen bearing transport species in calcite is molecular water a sample containing 1180 ppm mn shows a marked increase in observed d values at temperatures below 600 c however when pre annealed at 700 c the d values obtained at 550 c are within a factor of four of the value obtained for the low 100 ppm mn sample the results of this study confirm that in contrast to carbon oxygen diffusion rates in calcite are greatly enhanced when water is present the difference in the effect of water on doxygen and dcarbon in calcite may provide valuable information for evaluating the role of fluids in the thermal histories of calcite bearing rocks

the late quaternary glaciation of africa a regional synthesis abstract there are numerous mountain ranges in africa which carry evidence of quaternary glaciation though few have glaciers now and these are rapidly disappearing the best studied sites are in east africa and ethiopia in east africa the three highest mountains kilimanjaro kenya and the rwenzori each have evidence of 3 5 major glaciations dating back to an estimated 400 000 bp one of which is dated to the last glacial maximum lgm by radiocarbon or cosmogenic radionucleide crn methods two other mountains have similar large moraine systems inferred to date to the lgm in ethiopia three mountains have clear evidence of glaciation and on two of them radiocarbon dates give minimum ages which suggest that these are lgm features in north africa the atlas and other groups of mountains have evidence of glaciation but in the absence of any dating it is unclear whether these sites were glaciated at the lgm although geomorphic features resembling moraines and protalus ramparts have been described from the drakensberg mountains south africa it is still unclear whether these features indicate glaciation and can be attributed to the lgm thus although circumstances for fieldwork on african mountains are very difficult in many cases investigations for more than a century provide an extensive picture of quaternary glaciation in some cases there has been an increasingly detailed mapping of field evidence and the first crn dates were carried out recently thus improving the global picture considerably particularly in east africa and ethiopia there is sufficiently reliable field information to make useful estimates of equilibrium line altitude changes at the provisionally identified lgm but this age needs confirmation by further crn dating by making some hypothetical assumptions inferences can be drawn that indicate a temperature lowering of 5 6 c during the lgm elsewhere in africa further field surveys are required

dynamic control on serpentine crystallization in veins constraints on hydration processes in oceanic peridotites deformation and hydration processes are intimately linked in the oceanic lithosphere but the feedbacks between them are still poorly understood especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and or mass transfer serpentinization is accompanied by abundant veining marked by different generations of vein filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation we use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow spreading ridges we have selected a representative set of veins from ocean floor serpentinites of the mid atlantic ridge near kane transform fault 23 n and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy sem tem with electron microprobe analyses four main veining episodes v1 to v4 accompany the serpentinization the first episode identified as vein generation v1 is interpreted as the tectonically controlled penetration of early seawater dominated fluid within peridotites enhancing thermal cracking and mesh texture initiation at 3 4 km up to 8 km depth and at t 300 350 c the two following vein stages v2 and v3 formed in a closed diffusive system and accommodate volume expansion required to reach almost 50 serpentinization of the protolith the cracks exploited by these veins were caused by the progressive unroofing at depths of 4 to 2 km along a detachment fault degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization and this capacity is in turn linked to the exhumation rate and local tectonics during this period water consumed by hydration may prevent the establishment of convective hydrothermal cells the onset of an open hydrothermal system in the shallow lithosphere 2 km where brittle fracturing and advective transfer dominate and enable the completion of serpentinization is marked by the last vein generation v4 these results show a complete history of alteration with the crystallization of different types of serpentine recording different tectonic events chemical conditions and modes of hydrothermal alteration of the lithosphere

authigenesis of carbonate minerals in modern and devonian ocean floor hard rocks abstract petrographic and stable isotope 13c 18o patterns of carbonates from the logatchev hydrothermal field lhf the gakkel ridge gr and a late devonian outcrop from the frankenwald germany were compared in an attempt to understand the genesis of carbonate minerals in marine volcanic rocks specifically were the carbonate samples from modern sea floor settings and the devonian analog of hydrothermal origin low temperature abiogenic origin as inferred for aragonite in serpentinites from elsewhere on the mid atlantic ridge or biogenic origin aragonite is the most abundant carbonate mineral in serpentinites from the two modern spreading ridges and occurs within massive sulfides of the lhf the precipitation and preservation of aragonite suggests high mg2 and sulfate concentrations in fluids values of 18opdb as high as 5 3 for serpentinite hosted aragonite and as high as 4 2 for sulfide hosted aragonite are consistent with precipitation from cold seawater most of the corresponding

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

geochemistry of high h2 and ch4 vent fluids issuing from ultramafic rocks at the rainbow hydrothermal field 36 14 n mar the flores diving cruise was part of the mast iii amores 1995 1998 program funded by the european union one of the major achievements of the flores cruise was the discovery of the rainbow hydrothermal field hosted in ultramafic rocks south of the amar segment on the mid atlantic ridge mar the rainbow hydrothermal fluids exhibit temperatures of 365 c ph of 2 8 high chlorinity 750 mmol kg and low silica 6 9 mmol kg the uniformity in endmember major minor trace element concentrations and gas contents suggests that all rainbow fluids originate from the same deep source although h2s content is relatively low 1 20 mmol kg all vent fluids show extraordinary high h2 16 mmol kg ch4 2 5 mmol kg and co 5 mol kg endmember concentrations compared to fluids collected from other vent sites along the mar hydrogen represents more than 40 of the total gas volume extracted from the fluids at rainbow h2 production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization given that exposures of ultramafic rock may be common particularly along slow spreading ridges the production of h2 may have important implications for microbial activity at and beneath the seafloor

hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro exposed by detachment faulting at the mid atlantic ridge 15 20 n odp leg 209 a sulfur and oxygen isotope study whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15 20 n fracture zone on the mid atlantic ridge were analyzed to investigate hydrothermal alteration processes and test for a subsurface biosphere in oceanic basement three processes are identified 1 high temperature hydrothermal alteration 250 350 c at sites 1268 and 1271 is characterized by 18o depletion 2 6 4 4 elevated sulfide s and high 34s up to 2 wt and 4 4 10 8 fluids were derived from high temperature 350 c reaction of seawater with gabbro at depth these cores contain gabbroic rocks suggesting that associated heat may influence serpentinization 2 low temperature 150 c serpentinization at sites 1272 and 1274 is characterized by elevated 18o up to 8 1 high sulfide s up to 3000 ppm and negative 34s to 32 1 that reflect microbial reduction of seawater sulfate these holes penetrate faults at depth suggesting links between faulting and temperatures of serpentinization 3 late low temperature oxidation of sulfide minerals caused loss of sulfur from rocks close to the seafloor sulfate at all sites contains a component of oxidized sulfide minerals low 34s of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized low 34s sulfide derived from microbial sulfate reduction results show that peridotite alteration may be commonly affected by fluids heat derived from mafic intrusions and that microbial sulfate reduction is widespread in mantle exposed at the seafloor

hydrothermal circulation and the dike gabbro transition in the detachment mode of slow seafloor spreading one of the most ubiquitous boundaries within our planet is between sheeted dikes and gabbros in fast spreading ocean crust this boundary marks the brittle ductile transition at the ridge crest and is localized by a decametric conductive boundary layer between hydrothermal circulation in the sheeted dike layer and a shallow quasi steady state melt lens in contrast at slow spreading ridges the crustal structure appears chaotic with no consistent sheeted dike layer and widespread occurrences of gabbro and serpentinized peridotite on the seafloor recent work suggests that as much as 50 of the atlantic ocean crust formed by a detachment mode of seafloor spreading including the formation of oceanic core complexes capped by long lived convex upward detachment faults these detachment faults are often associated with large hydrothermal systems in which the location of any magmatic heat source is uncertain here we show that detachment faults can act as thermal boundaries between gabbroic melt in the fault footwall and hydrothermal circulation in the fault zone and hanging wall thus explaining the link between faulting and black smoker systems we suggest that interaction between magmatism and hydrothermal circulation means that detachment faults can act as the dike gabbro transition in the detachment mode of spreading inevitably leading to exposure of gabbros on the seafloor through continued faulting this concept provides a means of unifying apparently contrasting processes and crustal structures at different spreading rates

chronology of quaternary glaciations in east africa abstract a new glacial chronology for equatorial east africa is developed using in situ cosmogenic 36 cl measured in 122 boulders from moraines on mount kenya and kilimanjaro the oldest deposits sampled on kilimanjaro yield a limiting 36 cl age of 360 calendar kyr all 36 cl ages are in calendar years cal kyr or cal yr on mount kenya the oldest moraines give ages of 355 420 kyr liki i and 255 285 kyr teleki given the uncertainty in our 36 cl ages the liki i moraine may correspond to either marine isotope stage 10 or 12 whereas the teleki moraine correlates with stage 8 there is no evidence for stage 6 on either mountain the liki ii moraines on mt kenya and moraines of the fourth glaciation on kilimanjaro give ages of 28 3 kyr and 20 1 kyr respectively they represent the last glacial maximum lgm and correlate with stage 2 of the marine isotope record a series of smaller moraines above the lgm deposits record several readvances that occurred during the late glacial on mt kenya these deposits date to 14 6 1 2 kyr liki iia 10 2 0 5 kyr liki iii 8 6 0 2 kyr liki iiia and 200 yr lewis the corresponding deposits on kilimanjaro have mean ages of 17 3 2 9 kyr fourth glaciation saddle 15 8 2 5 kyr little glaciation saddle and 13 8 2 3 kyr fourth glaciation kibo these data indicate that the climate of the tropics was extremely variable at the end of the last glacial cycle

kinematics of crystal growth in syntectonic fibrous veins abstract detailed observations of a set of fibrous antitaxial calcite veins in a slate reveal that some of the calcite fibres do not connect material markers on both sides of the vein and can therefore not have tracked the full opening trajectory during vein growth this calls for a better understanding of the mechanisms of fibre formation and reliable criteria to test the tracking hypothesis based on surface roughness characteristics of the vein wall we develop a simple model for shape growth of crystals in a crack seal environment which can account for both tracking and non tracking behaviours and propose a set of tracking criteria for antitaxial veins finally we discuss ways by which the model can be tested against natural examples

abrupt younger dryas cooling in the northern tropics recorded in lake sediments from the venezuelan andes a radiocarbon dated sediment record from laguna de los anteojos a cirque lake in the merida andes of venezuela indicates that warmer and wetter atmospheric conditions occurred in the northern tropics at the onset of the bolling 14 600 cal yr bp and abruptly colder and drier conditions around the time of the younger dryas yd geochemical and clastic sediment analyses from los anteojos show that glaciers advanced at 12 850 cal yr bp reached their yd maximum extent at 12 650 cal yr bp and then retreated until complete deglaciation of the watershed at 11 750 cal yr bp the onset of warmer conditions that ended the coldest phase of the yd occurred several hundred years earlier at los anteojos than in the high latitudes of the northern hemisphere during the peak yd glacial advance glacier equilibrium line altitudes in the region were 360 to 480 m lower and temperature was 2 2 to 2 9 c colder than modern independent palynological evidence from the los anteojos sediment core indicates that the northern andes were more arid and at least 2 3 c colder during the yd the direction and timing of glacial fluctuations in venezuela are consistent with observations of marine sediment records from the cariaco basin that suggest abrupt cooling occurred at 12 850 cal yr bp followed by a shift to higher temperature after 12 300 cal yr bp the timing and pattern of climatic changes in northern south america are also consistent with paleoclimate records from the southern tropical andes that suggest a southward shift in the position of the intertropical convergence zone occurred at the start of the cooling event followed by a return to wetter conditions in northern south america during the late stages of the yd the early warming of the tropical atmosphere and invigoration of the hydrologic cycle likely contributed to the shift to increased temperature in the higher latitudes of the northern hemisphere at the end of the late glacial stage

contrasting fracture patterns induced by volume increasing and decreasing reactions implications for the progress of metamorphic reactions abstract hydration and dehydration reactions during metamorphism cause drastic changes in porosity and fluid pressure in a rock and some of these reactions proceed with fracturing for this paper we have developed a model for coupled hydraulic chemical mechanical processes using the distinct element method in order to understand the relationships among chemical reactions fluid flow and fracturing during metamorphism the model considers fluid advection along fractures and the dependence of reaction rates on fluid pressure with the help of the model volume decreasing dehydration reactions and volume increasing hydration reactions are investigated in detail as analogs of prograde and retrograde reactions respectively both types of reaction proceed inwards from drained boundaries or fractures but they show contrasting fracture patterns a volume decreasing dehydration reaction produces tree type fractures with the new fractures generated as branches of a pre existing fracture a volume increasing hydration reaction produces a polygonal network of fractures where new fractures nucleate at sites far from the pre existing fracture and extend to form t junctions these contrasting fracture patterns are essentially controlled by solid volume changes during reaction rather than by the fluid pressure gradient in the case of the tree type fractures the fractures are continuously generated at the reaction front and the reaction proceeds smoothly by positive feedbacks between reaction fracturing and fluid flow in contrast in a volume increasing hydration reaction fracturing initially occurs in response to the irregular boundary shape but as the reaction progresses a compressive stress field is generated which inhibits further fracture generation the compressive stress field also prevents fluid flow by closing the pre existing fractures which slows down the reaction these contrasting feedback systems between volume decreasing dehydration and volume increasing hydration reactions help to explain why prograde metamorphic reactions proceed pervasively and why the progress of a retrograde hydration reaction tends to be localized along fractures so that relics of peak metamorphism are commonly preserved

geologic implications of seawater circulation through peridotite exposed at slow spreading mid ocean ridges peridotite denuded by tectonic extension and exposed at the seafloor adjacent to slow spreading centers hosts hydrothermal circulation of seawater the reaction of seawater with peridotite causes serpentinization which generates a high ph strongly reducing fluid rich in methane and hydrogen and is accompanied by as much as 40 volume expansion complete serpentinization of peridotite requires tectonic activity to open fluid paths sealed by volume expansion diffuse venting of serpentinization fluids causes lithification of calcareous ooze on the seafloor to chalk like limestone this may be the main mechanism of deposition of ophicarbonates common in ophiolites the degree of induration is a function of the fluid flux through the sediment column calcareous ooze infiltrates faults and fractures and can be deformed following lithification focused venting of serpentinization fluids may lead to deposition of large chimneys composed of calcite aragonite and brucite such as those in the recently documented lost city vent field 30 n mid atlantic ridge geophysical implications of serpentinization include 1 creation of magnetic anomalies due to growth of magnetite in serpentinite and 2 lowered seismic velocity integrated studies of geologic and geophysical effects of serpentinization may aid in a more complete understanding of the structure of oceanic lithosphere and the mechanisms that expose mantle peridotite at the seafloor

fluid mixing and the deep biosphere of a fossil lost city type hydrothermal system at the iberia margin subseafloor mixing of reduced hydrothermal fluids with seawater is believed to provide the energy and substrates needed to support deep chemolithoautotrophic life in the hydrated oceanic mantle i e serpentinite however geosphere biosphere interactions in serpentinite hosted subseafloor mixing zones remain poorly constrained here we examine fossil microbial communities and fluid mixing processes in the subseafloor of a cretaceous lost city type hydrothermal system at the magma poor passive iberia margin ocean drilling program leg 149 hole 897d brucite calcite mineral assemblages precipitated from mixed fluids ca 65 m below the cretaceous paleo seafloor at temperatures of 31 7 4 3 c within steep chemical gradients between weathered carbonate rich serpentinite breccia and serpentinite mixing of oxidized seawater and strongly reducing hydrothermal fluid at moderate temperatures created conditions capable of supporting microbial activity dense microbial colonies are fossilized in brucite calcite veins that are strongly enriched in organic carbon up to 0 5 wt of the total carbon but depleted in 13c 13ctoc 19 4 we detected a combination of bacterial diether lipid biomarkers archaeol and archaeal tetraethers analogous to those found in carbonate chimneys at the active lost city hydrothermal field the exposure of mantle rocks to seawater during the breakup of pangaea fueled chemolithoautotrophic microbial communities at the iberia margin possibly before the onset of seafloor spreading lost city type serpentinization systems have been discovered at midocean ridges in forearc settings of subduction zones and at continental margins it appears that wherever they occur they can support microbial life even in deep subseafloor environments

thermal cracking and the deep hydration of oceanic lithosphere a key to the generation of plate tectonics 1 the development of transient thermal stress in suboceanic mantle is investigated on the basis of two dimensional thermoviscoelastic models incorporating composite rheology appropriate for dry oceanic lithosphere thermal stress is shown to be sufficiently high to deeply fracture the coldest part of lithosphere e g to the depth of at least 30 km and possibly down to 50 km in 100 ma old lithosphere the release of thermal stress by tension cracking is limited to the vicinity of cracks and the cascade crack system is suggested to be required given the finite fracture strength of mantle materials possible physical and chemical consequences of deep thermal cracking are also discussed the rheological evolution of oceanic lithosphere is likely to be affected by thermal cracking and subsequent serpentinization which introduces the localized zones of weakness in the otherwise stiffest part of lithosphere this localized weakening may help to explain why plate tectonic convection not stagnant lid convection operates in earth s mantle

hydrothermal activity in a peculiar oceanic ridge oxygen and hydrogen isotope evidence in the xigaze ophiolite tibet china the xigaze ophiolite locally displays a 7 km thick continuous sequence which probably represents an oceanic crust formed at a slow spreading ridge it consists from top to bottom of 1 1 pillow lavas with 18o between 9 and 16 and d around 90 vs smow 2 2 rare trondjhemites 9 7 18o 13 4 and d around 75 3 3 dolerite sills and dikes with 18o between 7 0 and 9 5 and d between 77 and 40 4 4 isotropic gabbros occurring as screens of the sills and dikes and rare layered gabbros these gabbros have 18o between 4 8 and 11 0 and d around 65 and bear high 18o albitized plagioclases 18o 10 which coexist with clinopyroxenes of mantle values 18o 5 5 or having values being slightly depleted down to 3 0 5 5 ultramafic rocks are either fresh or serpentinized they have 18o mainly around mantle values and d s are between 250 and 100 for the mafic part of the ophiolite the isotopic compositions correspond to hydrothermal interaction with seawater derived fluids under zeolite to greenschist facies but the trondjhemites were additionally affected by fluids poorer in d in particular the gabbros reacted with seawater derived fluids under amphibolite facies conditions as seen from their secondary mineralogy and their 18o depleted pyroxenes and subsequently were pervasively reacted under greenschist zeolite facies conditions during which the high 18o albitized plagioclase formed this occurrence of amphibolite facies conditions followed by greenschist zeolite facies conditions can be interpreted either to be a result of slow cooling of the oceanic crust at the ridge or to result from a fundamentally lower temperature hydrothermal interaction if compared with other ophiolites in contrast the d s of the serpentine 100 from the ultramafics are attributed to fluids of meteoric origin at high elevation nevertheless the particular location of the serpentinized harzburgites just beneath the gabbros and the high d dolerite sills and dikes 60 d 40 enclosed in these serpentinized harzburgites suggests an oceanic origin and therefore the possibility of hydrogen isotope exchange between serpentine and meteoric fluids after the obduction the xigaze ophiolite sequence displays very little 18o depleted rocks relative to mantle oxygen composition they cannot balance the amount of 18o enriched rocks consequently the result of the hydrothermal interaction between the xigaze ophiolite sequence and the seawater is an 18o depletion of the seawater however the contribution of xigaze type ridges to the buffering of 18o in cretaceous seawater was probably small