constraints on the nature and evolution of the magma plumbing system of mt etna volcano 1991 2008 from a combined thermodynamic and kinetic modelling of the compositional record of minerals maren kahl sumit chakraborty massimo pompilio and fidel costa school of earth and environment university of leeds leeds ls2 9jt uk institut fur geologie mineralogie und geophysik ruhr universit at bochum bochum 44780 germany istituto nazionale di geofisica e vulcanologia sezione di pisa pisa 56126 italy and earth observatory of singapore nanyang technological university singapore 639798 singapore

coesite eclogites from the roberts victor kimberlite south africa abstract coesite or pseudomorphic quartz is more common than previously recognized in the eclogite xenolith suite from the roberts victor mine south africa all coesite eclogites in this study 18 samples are classified as group i and have mineral compositions that span virtually the entire compositional range of group i eclogites described from this locality most of the roberts victor group i eclogite suite may be in equilibrium with free silica thus weakening the case for these rocks representing residues of partial melting oxygen isotope compositions of garnets from 15 samples are in the range 5 32 6 95 similar to other roberts victor group i eclogites these observations are consistent with a model in which protoliths of these rocks were oceanic basalts and intrusive rocks that underwent exchange with seawater at relatively low temperatures to variable extents prior to subduction and metamorphism to eclogite facies without suffering significant partial melting during or following subduction

scaling behavior and source mechanism of tremor recorded at erebus volcano ross island antarctica abstract erebus is an intraplate stratovolcano of phonolitic composition located at the southern end of terror rift within the west antarctica rift system the volcanic edifice of erebus exhibits a summit plateau that hosts the main crater and a smaller inner crater that contains a persistent lava lake volcano seismic signals are recorded at erebus by a permanent network of short period and broadband seismometers installed around the volcano edifice we investigate the duration amplitude scaling of tremor recorded by the closest 0 6 2 km broadband stations to the inner crater during may 2002 april 2004 in order to elucidate the physical processes taking place at its source after screening the data for signals related to non volcanic sources such as iceberg calving we obtained 72 tremor events spectral analysis reveals that these events were either harmonic exhibiting several overtones or broadband but there were also mixed events that contained both of these two states all tremor episodes considered in this study follow an exponential duration amplitude distribution that implies a scale bound source process this precludes the possibility that any of these signals is related to iceberg calving near ross island or that rock fracturing processes play any role in tremor generation the large viscosity of phonolitic magma 105 3 pa s suggests that source mechanisms which require high flow speeds in order to excite tremor are unlikely to operate at erebus on the other hand oscillations of co2 bubble clouds formed at localized asperities within the magma chamber represent a scale bound source that is also consistent with the observed oxidation state of ascending gas and with the deep 5 7 km location of the tremor source these results are important for understanding tremor processes at volcanoes which similar to erebus exhibit vigorous convection in their shallow plumbing system

stratigraphy and remanence acquisition of a palaeomagnetic reversal in alluvial siwalik rocks of pakistan a high resolution record of a palaeomagnetic reversal is documented in miocene alluvial rocks of pakistan we examined lateral variability of lithostratigraphy and palaeomagnetic stratigraphy through the same palaeomagnetic reversal in six correlated sections each section contains one or more palaeomagnetic sites with directions between fully reversed and fully normal the position of the reversal illustrates local relief in the study area and the time transgressive nature of certain stratigraphic units variability in the thickness of the transitional interval indicates contemporaneous variability in sediment accumulation rates different characteristics of palaeomagnetic remanence are associated with the depositional and postdepositional history of these sediments variability in patterns of remanence behaviour is the basis for inferences about post depositional processes we discuss two magnetic parameters that express coherency of palaeomagnetic samples the maximum angle of deviation and the circular standard deviation of particular interest are samples with incoherent palaeomagnetic signals the incoherency of samples is inversely correlated with the thickness of the transitional interval a low rate of sediment accumulation suggested by a thin transitional interval may facilitate a prolonged period of remagnetization through pedogenic or hydrological processes alternatively transition intervals defined by coherent magnetization may be thin as a result of pedogenically induced incoherency

a model of the geochemical and physical fluctuations of the lava lake at erebus volcano antarctica abstract erebus volcano antarctica exhibits periodical surface fluctuations of both geochemical and physical nature modeling the physics driving the lake oscillation is a challenge even with a relatively simple theoretical framework we present a quantitative analysis that aims to reconcile both lake level and gas geochemical cycles our model is based on the assumption that the periodicity is caused by the regular release of magma batches and or core annular flow that have a fixed volume of melt and ascend and degas in equilibrium results suggest that cycles are not caused by the mixing between magma residing in the lake and a deep magma but by two distinct deep sources that rise separately these sources of bubbly magma come from at most 2 3 km depth and rise buoyantly individual batches detach from the rising magmas at depths of 20 250 m the two batch types can coexist in a single conduit up to a depth of 30 m above which they rise alternately to release respectively 19 and 23 kg s of gas at the lake surface every 10 min the temperature of the descending flow is between 890 and 950 c which is roughly 100 c colder than the ascending currents batch pairs have shapes likely constrained by the conduit width regardless of their shapes the pairs reach very high porosities near the surface and have diameters of 4 14 m that are consistent with video observations showing spreading waves at the lake surface the alternating arrival of these large batches suggests a lava lake mostly filled with gas rich magma

inferred oxygen isotope profile of archaean oceanic crust onverwacht group south africa whole rock oxygen isotope data from a suite of mafic and ultramafic samples from the onverwacht group the basal unit of the archaean greenstone succession of the barberton mountain land south africa show a range of values from 18o 3 to 18o 14 a range which coincides with those of phanerozoic ophiolites and oceanic crust when the samples are arranged in an inferred ophiolitic pseudostratigraphy from basal serpentinized ultramafic cumulates through altered mafic extrusives they have an oxygen isotope distribution profile which is indistinguishable from that of phanerozoic ophiolites the distinctive isotopic profiles and secondary mineral assemblages in phanerozoic ophiolites are caused by hydrothermal interaction between seawater 18o 0 and oceanic crust 18o 5 8 the existence of a similar isotopic and metamorphic profile in the onverwacht group argues strongly that these rocks were hydrothermally altered by an archaean ocean whose isotopic composition was indistinguishable from modern sea water

compositionally zoned crystals and real time degassing data reveal changes in magma transfer dynamics during the 2006 summit eruptive episodes of mt etna one of the major objectives of volcanology remains relating variations in surface monitoring signals to the magmatic processes at depth that cause these variations we present a method that enables compositional and temporal information stored in zoning of minerals olivine in this case to be linked to observations of real time degassing data the integrated record may reveal details of the dynamics of gradual evolution of a plumbing system during eruption we illustrate our approach using the 2006 summit eruptive episodes of mt etna we find that the history tracked by olivine crystals and hence most likely the magma pathways within the shallow plumbing system of mt etna differed considerably between the july and october eruptions the compositional and temporal record preserved in the olivine zoning patterns reveal two mafic recharge events within months of each other june and september 2006 and each of these magma supplies may have triggered the initiation of different eruptive cycles july 14 24 and august 31 december 14 correlation of these observations with gas monitoring data shows that the systematic rise of the co2 so2 gas values is associated with the gradual pre and syn eruptive supply of batches of gas rich mafic magma into segments of etna s shallow plumbing system where mixing with pre existing and more evolved magma occurred

chemical sedimentology and paleoenvironmental history of lake olduvai a pliocene lake in northern tanzania lake olduvai was a saline highly alkaline closed basin lake in which lacustrine sediments of bed i and lower bed ii of olduvai gorge were deposited between ca 1 92 and 1 70 ma the lake consisted of a lacustrine area the central basin cb surrounded by lake margin terrain that was alternately flooded and dried the lacustrine deposits consist largely of authigenic clay and carbonate minerals zeolites k feldspar chert and pyrite claystones of the cb and most of the eastern lake margin elm terrain consist chiefly of mg rich authigenic illite smectite and interlayered illite smectite i s these clays were formed penecontemporaneously with detrital clay deposition by interaction of the detrital clays with the saline highly alkaline water the lake olduvai clays average 65 70 mg 2 sio 2 and alkali ions from lake water and the remaining 30 35 is detrital clay lacustrine clays of the elm indicate that this area was flooded by saline water for substantial periods of time hominids occupied the elm at times of lower lake levels when the clays were dried calcite is the most abundant carbonate mineral and occurs chiefly as sand size euhedral crystals dispersed in claystone and less commonly as calcite crystal limestones and nodules the sand size calcite crystals were formed diagenetically at shallow depths possibly from a gaylussite and or micritic calcium carbonate precursor dolomite occurs chiefly in the form of dolostones some of which are probably of replacement origin the oxygen isotope composition of lake water and pore fluid of the cb varied over 11 as determined from calculated delta 18 o h 2 o values of authigenic minerals clay minerals were formed chiefly in the more saline waters sand size calcite crystals were generated mostly from less saline pore fluids and limestone nodules were formed chiefly from dilute pore fluid dolostones were formed from fluids of varying salinity hydrogeology of the central basin was complex and postdepositional fluid movements are documented by differing delta 18 o h 2 o values for different authigenic minerals of the same samples pore fluids moved within the lake deposits for a considerable period of time after deposition as shown by 18 o values of quartz and k feldspar formed at different times downward flow of lacustrine fluids is indicated by zeolitic alteration of the underlying tuffaceous alluvial deposits lake olduvai became overall shallower during the deposition of bed i and the subaerial exposure increased leading to occasional playa conditions these changes very likely resulted from increasing aridity and tectonic activity the lake that deposited lower bed ii was larger than that responsible for upper bed i and episodes of wetter climate alternated with drier periods in which salinity and probably temperatures were higher than in the bed i lake

oxygen and carbon isotope evidence for seawater hydrothermal alteration of the macquarie island ophiolite abstract rocks of the miocene macquarie island ophiolite south of new zealand have oxygen and carbon isotopic compositions comparable to those of seafloor rocks basalt glass and weathered basalts have 18o values at 5 8 6 0 and 7 9 9 5 respectively similar to drilled seafloor rocks including samples from the leg 29 dsdp holes near macquarie island compared to the basalt glass the greenschist to amphibolite facies metaintrusives are depleted in18o 18o 3 2 5 9 similar to dredged seafloor samples whereas the metabasalts are enriched 18o 7 1 9 7 although the gabbros are only slightly altered in thin section they have exchanged oxygen with a hydrothermal fluid to a depth of at least 4 5 km there is an approximate balance between18o depletion and enrichment in the exposed ophiolite section the carbon isotopic composition of calcite in the weathered basalts 13c 1 0 2 0 is similar to those of drilled basalts but the metamorphosed rocks have low 13c values 14 6 to 0 9 these data are compatible with two seawater circulation regimes in the upper regime basalts were weathered by cold seawater in a circulation system with high water rock ratios 1 0 based on calcite compositions weathering temperatures were less than 20 c and the carbon was derived from a predominantly inorganic marine source as previously suggested for the samail ophiolite it is postulated that the lower hydrothermal regime consisted of two coupled parts at the deeper levels seawater circulating at low water rock ratios 0 2 0 3 and high temperatures 300 600 c gave rise to18o depleted gabbro and sheeted dikes via open system exchange reactions during reaction the seawater underwent a shift in oxygen isotopic composition 18o 1 0 5 0 and subsequently caused18o enrichment of the overlying metabasalts in the shallower part of the hydrothermal regime the metabasalts were altered at relatively high water rock ratios 1 0 10 0 and temperatures in the range 200 300 c the relatively low water rock ratios in the hydrothermal regime are supported by the low 13c values of calcite interpreted as evidence of juvenile carbon in contrast to the inorganic marine carbon found in the weathered basalts

sustaining persistent lava lakes observations from high resolution gas measurements at villarrica volcano chile active lava lakes as the exposed upper part of magmatic columns are prime locations to investigate the conduit flow processes operating at active degassing volcanoes persistent lava lakes require a constant influx of heat to sustain a molten state at the earth s surface several mechanisms have been proposed to explain how such heat transfer can operate efficiently these models make contrasting predictions with respect to the flow dynamics in volcanic conduits and should result in dissimilar volatile emissions at the surface here we look at high frequency so2 fluxes plume composition thermal emissions and aerial video footage from the villarrica lava lake in order to determine the mechanism sustaining its activity we found that while fluctuations are apparent in all datasets none shows a stable periodic behaviour these observations suggest a continuous influx of volatiles and magma to the villarrica lava lake we suggest that ascending volatile rich and descending degassed magmas are efficiently mixed within the volcanic conduit resulting in no clear periodic oscillations in the plume composition and flux we compare our findings to those of other lava lakes where equivalent gas emission time series have been acquired and suggest that gas flux magma viscosity and conduit geometry are key parameters determining which flow mechanism operates in a given volcanic conduit the range of conduit flow regimes inferred from the few studied lava lakes gives a glimpse of the potentially wide spectrum of conduit flow dynamics operating at active volcanoes

eclogites with oceanic crustal and mantle signatures from the bellsbank kimberlite south africa part 2 sr nd and o isotope geochemistry the sr nd and o isotopic compositions of garnet and clinopyroxene mineral separates from nine eclogite xenoliths from the bellsbank kimberlite erupted at 120 ma define three groups group a eclogites considered to be mantle cumulates are characterized by 6180 and 87sr srsr values typical of mantle derived materials 5 1 to 5 6 o and 0 7042 0 7046 respectively and very low sm nd ratios apparent rb sr ratios and nallz01 values 0 057 0 078 0 00005 0 00136 and 14 to 16 respectively the ree and isotopic data for these eclogites can be modelled in terms of crystallization from a group ii kimberlite magma at 1 1 5 ga group b and c eclogites believed to be the metamorphosed products of ancient subducted oceanic crust are characterized by low 3180 2 9 to 4 7 extremely high nd a20 40 to 219 and radiogenic 87sr s6sr ratios 0 708 0 710 the sm nd ratios of the group b eclogites are very high up to 1 6 the data for group b and c eclogites define a linear correlation on sm nd and 1 nd vs nal 201 diagrams these relationships are consistent with mixing of the bellsbank kimberlite cno 1201 10 sm nd 0 10 with a depleted eclogite end member end 120 219 sm nd 1 6 during a cryptic metasomatic event the sr isotopic variations in group b and c eclogites cannot be generated by simple two component mixing the st nd and o isotope data for group b and c eclogites probably reflect a complex sequence of depletion and enrichment events in both crust and mantle settings enrichments which possibly affected the group b and c eclogites include seawater alteration of a morb like protolith which lowered the 6180 and raised the svsr 86sr ratio but left the nd isotopic compositions unchanged and cryptic metasomatism by the magmatism that produced the bellsbank kimberlite the high sm nd ratio of the depleted eclogite end member cannot be generated by extraction of a melt from a modern morb composition rather it is argued that such high sm nd ratios are produced as a result of partitioning during the recrystallization of a morb like component to eclogite during subduction nd model ages suggest that this process occurred 2 3 2 4 ga

volatile transport in a convecting magma column implications for porphyry mo mineralization we propose that convection in a column of silicic magma allows transport of volatile components from large 50 km3 magma chambers to the sites of shallow 3 km porphyry type mo deposits using constraints from the henderson and pine grove systems we show that even at temperatures as low as 700 c a granitic magma with 30 vol phenocrysts can flow through a magma column with a 150 m radius at a rate 10 km3 yr enough magma to contribute the mo necessary to form an ore deposit in a geologically reasonable time frame this process requires an efficient mechanism for bubble separation at the top of the magma column to produce degassed magma that can descend down through the column it also requires slow rates of crystallization in the silicic magma consistent with experimental studies hydrothermal fluids released from the convecting magma column can explain many geologic features of the deposits

conduit convection magma mixing and melt inclusion trends at persistently degassing volcanoes abstract magmas progressively exsolve volatiles as they ascend towards the earth s surface such that their volatile content is a function of pressure water and carbon dioxide concentrations measured in melt inclusions from degassing volcanoes rarely coincide with modelled degassing trends i show that observed melt inclusion trends can be reproduced through mixing of magmas either during convection within the volcanic conduit or within a subterranean magma reservoir no fluxing gas phase or post entrapment loss of water need be invoked a permeable network allowing gas transport is still required to avoid fragmentation of magma at shallow depths

carbon and oxygen isotope variations in diamond and graphite eclogites from orapa botswana and the nitrogen content of their diamonds abstract in eclogite xenoliths from orapa botswana 13 c of graphite varies between 4 6 and 7 8 vs pdb while that of diamonds ranges from 4 0 to 22 3 in graphite eclogites 18 o values of garnets fall between 5 2 and 7 vs smow and those of clinopyroxenes between 4 6 and 6 5 for diamond eclogites the respective ranges are wider i e 3 9 to 9 2 and 5 2 to 8 3 the 18 o enrichment above normal mantle oxygen isotopic compositions 5 to 6 5 and the wide 13 c range appear to occur in parts of the mantle in which diamond is stable eclogites with 13 c values in the range 4 to 8 and 13 o values in the normal mantle range tend to have positive garnet clinopyroxene 18 o fractionations and lower fe ca and fe mg ratios than eclogites which have simultaneously 13 c values lower than 8 and 18 o values higher than 6 5 in these eclogites the garnet clinopyroxene fractionations tend to be negative positive garnet clinopyroxene 18 o fractionations increase slightly in the sequence diamond eclogites 0 36 0 15 n 6 graphite diamond eclogites 0 46 0 36 n 3 and graphite eclogites 0 53 0 15 n 3 neither igneous fractionation processes subduction nor metasomatic alteration can satisfactorily explain all observed chemical and isotopic trends

oxygen isotope systematics of eclogite mineral phases from south africa abstract eleven eclogite xenoliths from the bobbejaan and roberts victor mines south africa have been characterized by a variety of geochemical techniques the samples include nine bimineralic eclogites and two grospydites that span the range of mantle eclogite major element compositions primary phase end member compositions range from py 75 al 16 gr 09 jd 18 di 82 caes 0 to py 28 al 21 gr 51 jd 49 di 36 caes 15 oxygen isotope analyses of mineral separates were performed for clinopyroxene and garnet isotopic values 18 o range from 4 0 to 7 8 for clinopyroxene and 3 6 to 7 2 for garnet differences in isotopic values between the primary phases clinopyroxene minus garnet range in value from 0 6 to 0 6 although oxygen isotopes are not expected to fractionate at mantle temperatures the anion electrostatic site potential values determined by smyth 1989 are consistent with small degrees of fractionation in the mantle oxygen isotope fractionation between minerals may be calculated by the equation of smyth and clayton f a 10 6 t 2 where a 0 7911 ev fractionation values at 1300 k range from 0 6 jd pyr to 0 03 cats grs while at 1600 k these values are between 0 39 and 0 02 respectively the re equilibration temperatures of the studied eclogite suite are 1050 to 1250 c at 30 kbar therefore the obtained differences between the mineral phases are consistent with the calculated isotope fractionation values the data obtained for eclogite oxygen isotopes suggest that 18 o may fractionate to a small extent at mantle pt conditions

stable isotope characteristics of eclogites from the ultra high pressure metamorphic terrain east central china abstract a stable isotope study was made for the ultra high pressure uhp metamorphic rocks in the dabie and sulu terrains east central china 18 o values of minerals in eclogites cover a wide range from 4 to 6 the o isotope fractionations between minerals i e omphacite phengite and or amphibole and garnet in eclogites are mostly not in isotopic equilibrium the large range of o isotope composition of garnet may approximate the 18 o variations of eclogites during uhp metamorphism and must have been largely inherited from their pre subduction basaltic precursors which might have interacted with meteoric waters under variable temperature conditions and water rock ratios before uhp metamorphism the o isotope disequilibrium between minerals and garnet in eclogites was caused by retrograde hydration reactions the fluids required for these hydration reactions must have low or even negative o isotope compositions it is postulated that such fluids were either originated from meteoric deep infiltration or were derived from low 18 o para gneisses

bi directional flows in constrained systems we consider the exchange flow of relatively dense viscous fluid in a container connected by a vertical pipe to a container beneath it initially full of relatively light fluid a non dimensional value for the flux of dense fluid down the tube is determined experimentally as a function of the ratio of the two viscosities and the reynolds number the experimental data are satisfactorily collapsed using dimensional analysis and balancing buoyancy inertial and viscous forces as appropriate a theoretical analysis assuming steady axisymmetric motion captures a considerable part but not all of the processes involved the paper discusses quantitative applications of the results to the movement of magma in volcanic conduits the concepts indicate how bi directional convection in the conduit between a lava lake and a magma reservoir deep in the crust is the essential ingredient in the explanation of the long standing problem that the amount of degassing of sulphur dioxide from a lava lake in a volcanic crater can exceed by many orders of magnitude that consistent with the amount of lava solidified in the crater movies are available with the online version of the paper

archean mantle heterogeneity and the origin of diamondiferous eclogites siberia evidence from stable isotopes and hydroxyl in garnet data are presented for the o isotopic composition of clinopyroxene and garnet the c isotopic composition of diamond and the oh content of garnet from eclogite xenoliths brought to the surface by the udachnaya kimberlite pipe yakutia siberia radiogenic isotopic data suggest that the eclogites could have been derived from an ultradepleted mantle at approximately 2 9 ga pearson et al 1995 snyder et al in preparation o isotopic compositions generally show equilibration between the eclogitic minerals cpx grt 0 11 0 41 and have 18 o smow for both garnet and clinopyroxene that lie near the range of accepted mantle values of 5 7 0 7 however several eclogites indicate significant deviations from this range at higher values of 6 8 7 0 also two eclogites lie at the lower end of the mantle range at values of 4 8 and 5 0 all in clinopyroxene c isotopic compositions of diamonds all have 13 c pdb in the range of 1 to 7 and are centered at approximately 5 also within the range of accepted mantle values the oh contents of the garnet are generally between 0 and 22 ppm as h 2 0 although two samples exceed 70 ppm this range of oh is similar to eclogitic garnet from the kaapvaal craton of southern africa the mantle c isotopic values of associated diamonds the majority of o isotopic data and the low oh content of the minerals although suggesting a general lack of crustal participation in the formation of the udachnaya eclogites do not rule out the participation of some ancient crustal material that these eclogites include both 18 o enriched and 18 o depleted types suggests that the protoliths may have been overprinted by both low and high temperature hydrothermal events cf jacob et ai 1994 a positive correlation between 18 o and 87 sr 86 sr allows the interpretation of these eclogites as representing a cross section of an archean ophiolite however the lack of a single coherent grouping on a plot of 18 o vs 87 sr 86 sr suggests that if the udachnaya eclogites were derived from oceanic crust they cannot be cogenetic and must represent at least two separate ophiolite sequences conversely if the eclogites are found to be cogenetic then a totally different process affected their formation and a probable metasomatic mechanism was operative because of the total lack of correlation of 18 o with other geochemical parameters we find no compelling evidence that all eclogites are derived ultimately from oceanic crust

principles of thermal expansion in the feldspar system following the recent thermal expansion work of hovis et al 2008 on alsi 3 feldspars we have investigated the thermal expansion of plagioclase ba k and ca k feldspar crystalline solutions x ray powder diffraction data were collected between room temperature and 925 c on six natural plagioclase specimens ranging in composition from anorthite to oligoclase and as well on their k exchanged equivalents and five synthetic ba k feldspars with compositions from 25 to 99 mol baal 2 si 2 o 8 the resulting thermal expansion coefficients for volume v have been combined with earlier results for end member na and k feldspars unlike alsi 3 feldspars al 2 si 2 feldspars including anorthite and celsian from the present study and sr and pb feldspar from other workers show essentially constant and relatively limited thermal expansion in the context of structures where the lowenstein rule requires al and si to alternate among tetrahedra the proximity of bridging al o si o atoms to divalent neighbors ranging from 0 to 2 results in ca o or ba o bonds that are especially short it is suggested that short bonds such as these have a partly covalent character resulting from the requirement for local charge balance this in turn stiffens the structure for feldspar series with coupled substitution the change away from a purely divalent m site occupant gives the substituting less strongly bonded monovalent cations increasingly greater influence on thermal expansion overall thermal expansion in the feldspar system is well represented on a plot of v against room temperature volume v rt where one sees a quadrilateral bounded by data for 1 alsi 3 feldspars whose expansion behavior is governed largely by the size of the monovalent alkali site occupant 2 al 2 si 2 feldspars whose expansion is uniformly limited by divalent cations having stronger partially covalent bonds to bridging al o si o atoms and 3 4 plagioclase and ba k feldspars where expansion behavior across the series transitions from one control to the other overall the coefficient of thermal expansion in any binary feldspar series is a linear function of v rt between the pertinent end members this makes it possible to predict the thermal expansion behavior of any feldspar simply from knowledge of its chemical system and room temperature volume

3 d x ray tomography of diamondiferous mantle eclogite xenoliths siberia a review abstract diamonds form over billions of years hundreds of kilometers beneath the earth s surface and in combination with inclusions trapped within provide important constraints on the evolution of the mantle over geological time diamonds are generally studied as individual crystals sourced from highly explosive kimberlite pipes which entrain and subsequently disaggregate mantle fragments xenoliths en route to the surface this has resulted in a general absence of robust textural descriptions of diamonds relative to their hosting mantle protolith the textural associations of diamonds within their mantle host rocks are reviewed here on the basis of a compilation of x ray tomographic data for 17 diamondiferous eclogite xenoliths from siberian kimberlites this review represents a comprehensive description of diamonds relative to their host silicates the lack of such descriptions in previous studies is largely due to the rarity of these xenoliths the difficulty in preparing petrographic thin sections containing diamonds and their high monetary value high resolution computed x ray tomography hrcxt produces up to 1200 sequential 2 d slices through individual xenoliths each of which represents a pseudo thin section with a resolution on the order of 5 20 m the improved resolution of x ray imaging in recent studies allows for the identification of not only primary minerals but metasomatic minerals assemblages including spongy textured clinopyroxene phlogopite k richterite and hercynitic spinel allowing for the delineation of distinct metasomatic pathways through the xenoliths and their relationship to diamonds diamonds are observed in three distinct textural settings potentially representing several temporally distinct diamond growth events these setting includes 1 diamonds completely enclosed in garnet 2 diamonds associated with highly embayed silicate grain boundaries and 3 diamonds contained within distinct metasomatic plumbing systems diamonds observed completely enclosed in garnets suggest an early diamond forming event prior to major re crystallization and eclogite formation during subduction the occurrence of diamond in association with embayed garnets suggests that diamond grew at the expense of the hosting silicate protolith in addition the spatial relationships of diamonds with metasomatic pathways which are generally interpreted to result from late stage proto kimberlitic fluid percolation indicate a period of diamond growth occurring close to but prior to the time of kimberlite emplacement furthermore the paragenesis of sulfides within eclogite xenoliths are described using 3 d models for entire xenoliths volumes providing important constraints of the timing of sulfide mobilization within the mantle three d animations created using x ray tomography data for ten of the xenoliths can be viewed at the following link http eps utk edu faculty taylor tomography php

a co2 gas precursor to the march 2015 villarrica volcano eruption we present here the first volcanic gas compositional time series taken prior to a paroxysmal eruption of villarrica volcano chile our gas plume observations were obtained using a fully autonomous multi component gas analyser system multi gas in the 3 month long phase of escalating volcanic activity that culminated into the march 3 2015 paroxysm the largest since 1985 our results demonstrate a temporal evolution of volcanic plume composition from low co2 so2 ratios 0 65 2 7 during november 2014 january 2015 to co2 so2 ratios up to 9 then after the h2o co2 ratio simultaneously declined to 38 in the same temporal interval we use results of volatile saturation models to demonstrate that this evolution toward co2 enriched gas was likely caused by unusual supply of deeply sourced gas bubbles we propose that separate ascent of over pressured gas bubbles originating from at least 20 35 mpa pressures was the driver for activity escalation toward the march 3 climax

gas driven lava lake fluctuations at erta ale volcano ethiopia revealed by modis measurements the long lived lava lake of erta ale volcano ethiopia is remotely monitored by moderate resolution imaging spectroradiometers modis installed on satellites the normalised thermal index nti wright et al remote sens environ 82 135 155 2002 is shown to be proportional to the volume of the lava lake based on visual observations the lava lake s variable level can be plausibly related to a stable foam i e a mixture composed of densely packed non coalescing bubbles in suspension within a liquid this foam is trapped at the top of the magma reservoir and its thickness changes in response to the gas flux feeding the foam being successively turned on and off the temporal evolution of the foam thickness and the resulting variation of the volume of the lava lake is calculated numerically by assuming that the gas flux feeding the foam initially constant and homogeneous since december 9 2002 is suddenly stopped on december 13 2002 and not restarted before may 2003 the best fit between the theoretical foam thickness and the level of the lava lake deduced from the nti provides an estimate of both the reservoir radius 155 170 m and the gas flux feeding the foam 5 5 10 3 7 2 10 3 m 3 s 1 when existing this is in agreement with previous estimates from acoustic measurements bouche et al earth planet sci lett 295 37 48 2010 the very good agreement between the theoretical foam thickness and that deduced from modis data shows for the first time the existence of a regime based on the behaviour of a stable foam whose spreading towards the conduit wide conduit condition can explain the long lived activity our predictive model which links the gas flux at the vent to the foam spreading could potentially be used on any volcano with a long lived activity the underlying gas flux and the horizontal surface area of the magma reservoir can then be deduced by combining modelling to continuous measurements of gas flux the lava lake when high often shows regular rise and fall of its level we have recognised a minimum of 26 very well marked cycles between january 2001 and december 13 2002 corresponding to a typical return time of 10 8 2 3 days and a gas volume of 8 3 105 2 0 105 m 3 this corresponds to a gas volume fraction in the reservoir equal to 0 023 0 063 the yearly gas flux estimated between december 13 2002 and september 27 2004 varies between 2 3 10 6 and 5 9 10 6 m 3 s 1 at the depth of the reservoir the long time series provided by infra red sensors mounted on satellites could be used on any persistent volcano to detect potential periodic variations in the level of lava lakes or lava columns providing that the vent has a funnel shape as often and is sufficiently large

eclogites pyroxene geotherm and layered mantle convection temperatures of equilibration for the majority 81 percent of the eclogite xenoliths of the roberts victor kimberlite pipe in south africa range between 1000 and 1250 c falling essentially on the gap of the lower limb of the subcontinental inflected geotherm derived from garnet peridotite xenoliths in view of the archean age 2 6 x 109 years of these eclogites and their stratigraphic position on the geotherm it is proposed that the inflected part of the geotherm represents the convective boundary layer beneath the conductive lid of the lithospheric plate the gradient of 8 celsius degrees per kilometer for the inflection is characteristic of a double thermal boundary layer and suggests layered convection rather than whole mantle convection for the earth

in situ oxygen isotope major and trace element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from roberts victor kaapvaal craton a subducted oceanic crustal origin for most eclogite xenoliths in kimberlites has long been a cornerstone of tectonic models for craton development however eclogite xenoliths often have protracted and complex histories involving multiple metasomatic events that could overprint some of the key geochemical indicators typically taken as evidence of a subducted origin e g garnet 18o values and mineral 87sr 86sr compositions to assess the potential for disturbance of oxygen isotopic compositions in mantle eclogites via diamond forming and other possible metasomatic fluids we have conducted a multi technique in situ study of a diamondiferous eclogite xenolith from the roberts victor kimberlite s africa using sims we provide the first texturally controlled in situ measurements of 18o values in eclogitic garnet in close proximity to diamond garnet and clinopyroxene modal proportions are heterogeneous in the xenolith and garnet compositions vary from mg 75 8 79 2 grossular proportions 8 05 10 14 mol and omphacitic pyroxene has jd13 24 and mg 86 6 90 0 rare earth element patterns of minerals across the xenolith including grains close to diamond are typical lree depleted garnets and markedly lree enriched pyroxenes these silicate minerals also record detectable intra and inter grain lree abundance variations clinopyroxenes of the studied xenoliths show hfse and sr abundance variations that are decoupled from lree contents and major element variations mineralogical constraints and bulk rock reconstructions indicate that the studied sample likely experienced selective incompatible element enrichment during small volume 0 03 wt infiltration of metasomatic fluid s potentially linked to ancient diamond evolution intra grain major element lree and hfse variations in clinopyroxene resulted from late stage metasomatism oxygen isotope compositions in garnet are decoupled from all major and trace element variations with garnet 18o values being uniform across the xenolith in a wide variety of textural settings garnet 18o values of 6 5 0 2 are higher than the mean 5 19 0 26 of the mantle garnet range 4 8 5 5 modelling of the buffering effect of mantle peridotite on co2 rich and h2o rich metasomatic fluids at temperatures within the diamond stability field indicates that the likelihood of a metasomatic fluid with exotic oxygen isotopic composition arriving at a mantle eclogite body with its isotopic composition unmodified after percolative flow through dominantly peridotitic mantle at great depth is very low as we find no evidence of metasomatically induced garnet oxygen isotope variations in the studied diamondiferous eclogite xenolith we conclude that the most likely origin for the elevated garnet 18o values is via inheritance from a crustal protolith altered at relatively low temperatures these results have broader relevance and support the hypothesis of a low pressure protolith for mantle eclogite xenoliths demonstrating the robust nature of garnet oxygen isotope compositions even in diamond bearing eclogites

magma evolution and open system processes at shiveluch volcano insights from phenocryst zoning phenocryst zoning patterns are used to identify open system magmatic processes in the products of the 2001 eruption of shiveluch volcano kamchatka the lavas and pumices studied are hornblende plagioclase andesites with average pre eruptive temperatures of 840c and fo2 of 1 5 2 1 log units above nickel nickel oxide nno plagioclase zoning includes oscillatory and patchy zonation and sieve textures hornblendes are commonly unzoned but some show simple multiple or patchy zoning apatite microphenocrysts display normal and reverse zoning of sulphur the textural similarity of patchy hornblende and plagioclase together with ba sr systematics in patchy plagioclase indicate that the cores of these crystals were derived from cumulate material plagioclase liquid equilibria suggest that the patchy texture develops by resorption during h2o undersaturated decompression when h2o saturated crystallization recommences at lower pressure reduced ph2o results in lower xan in plagioclase causing more al rich hornblende to crystallize plagioclase cores with diffuse oscillatory zoning and unzoned hornblende crystals probably represent a population of crystals resident in the magma chamber for long periods of time in contrast oscillatory zoning in the rims of plagioclase phenocrysts may reflect eruption dynamics during decompression crystallization increasing fe al in oscillatory zoned rims suggests oxidation as a result of degassing of h2o during decompression a general lack of textural overlap between phenocryst types suggests that different phenocryst populations were spatially or temporally isolated during crystallization we present evidence that the host andesite has mixed with both more felsic and more mafic magmas olivine and orthopyroxene xenocrysts with reaction or overgrowth rims and strong normal zoning indicate mixing with basalt sieve textured plagioclase resulted from mixing of a more felsic magma with the host andesite the mineralogy and mineral compositions of a mafic andesite enclave are identical to those of the host magma which implies efficient thermal quenching and thus small volumes of intruding magma mixing of this magma with the host andesite results in phenocryst zoning because of differences in dissolved volatile contents we suggest that small magma pulses differentiated at depth and ascended intermittently into the growing magma chamber producing incremental variations in whole rock compositions

unusually low δ18o ultra high pressure metamorphic rocks from the sulu terrain eastern china eclogite and quartzite interlayers containing ultra high pressure uhp metamorphic mineral assemblages occur at qinglongshan in the sulu terrain eastern china these rocks exhibit some of the most negative oxygen isotope compositions that have ever been reported for metamorphic rocks the 18 value of minerals in eclogite ranges from 10 4 to 9 0 o and in quartzite ranges from 10 2 to 7 3 near isotopic equilibrium between minerals and rocks suggests that these rocks must have acquired such low oxygen isotope compositions before the uhp metamorphism through near surface meteoric water rock interactions the rocks were then subducted to a great depth during continent continent collision forming uhp minerals with concomitant local oxygen isotope redistribution it is significant that the rocks have retained such negative oxygen isotope compositions despite subduction into the upper mantle

oxygen isotopes in subducted oceanic crust a new perspective from siberian diamondiferous eclogites 1 there are three methods of investigating the oxygen isotope composition of oceanic crust 1 data from drilling obtained samples 2 study of obducted ophiolites and 3 from crustal derived xenoliths however each approach has limitations drilling capabilities are limited to the upper 1 3 km of the oceanic crust ophiolites are segments of oceanic crust and underlying mantle exposed at the earth s surface but their origin and emplacement are still debated nonetheless the study of ophiolite sequences has enabled many of the fundamental observations regarding ancient oceanic crust and the oxygen isotope signature of the upper mantle analyses of ophiolite sequences e g samail reveal 18o variations ranging from isotopically enriched in relation to morb 18o morb to isotopically depleted low 18o morb interpreted to show the effects of seawater interaction at different temperatures mantle derived xenoliths also display compelling geochemical evidence suggesting they are remnants of subducted ocean crust and may in fact be more representative samples some kimberlite hosted diamondiferous eclogite xenoliths with crustal protoliths reveal a marked oxygen isotope disparity between oceanic crust and the upper mantle i e a bimodal distribution of both mantle like 18o 5 6 and isotopically enriched oxygen 18o 6 8 the isotopically high 18o values are often observed in eclogite xenoliths with ultramafic protoliths which form beyond the low temperature alteration zone this finding is in contrast to traditional oxygen isotope versus depth curves of ophiolite sequences here we present a compiled data set from siberian eclogites and pyroxenites that illustrates this bimodality using magnesium number mg as a depth proxy and investigate the conditions that could potentially result in the observed abnormal oxygen isotope variations

oxygen isotope composition of syngenetic inclusions in diamond from the finsch mine rsa oxygen isotope data have been obtained for silicate inclusions in diamonds and similar associated minerals in peridotitic and eclogitic xenoliths from the finsch kimberlite by laser fluorination oxygen isotope analyses of syngenetic inclusions weighing 20 400 g have been obtained by laser heating in the presence of clf3 18o 16o ratios are determined on oxygen converted to co2 over hot graphite and for samples weighing less than 750 g producing 12 moles o2 enhanced co production in the graphite reactor causes a systematic shift in both 13c and 18o that varies as a function of sample weight a pressure effect correction procedure based on the magnitude of 13c co2 depletion relative to 13c graphite is used to obtain corrected 18o values for inclusions with an accuracy estimated to be 0 3 for samples weighing 40 g syngenetic inclusions in host diamonds with similar 13c values 8 4 to 2 7 have oxygen isotope compositions that vary significantly with a clear distinction between inclusions of peridotitic 4 6 to 5 6 and eclogitic paragenesis 5 7 to 8 0 the mean 18o composition of olivine inclusions is indistinguishable from that of typical peridotitic mantle 5 25 0 22 whereas syngenetic purple garnet inclusions possess relatively low 18o values 5 00 0 33 reversed oxygen isotope fractionation between olivine and garnet in both diamond inclusions and diamondiferous peridotite xenoliths suggests that garnet preserves subtle isotopic disequilibrium related to genesis of cr rich garnet and or exchange with the diamond forming fluid garnet in eclogite xenoliths in kimberlite show a range of 18o values from 2 3 to 7 3 but garnets in diamondiferous eclogites and as inclusions in diamond all have values 4 7

degassing dynamics of basaltic lava lake at a top ranking volatile emitter ambrym volcano vanuatu arc persistent lava lakes are rare on earth and provide volcanologists with a remarkable opportunity to directly investigate magma dynamics and degassing at the open air ambrym volcano in vanuatu is one of the very few basaltic arc volcanoes displaying such an activity and voluminous gas emission but whose study has long remained hampered by challenging accessibility here we report the first high temporal resolution every 5 s measurements of vigorous lava lake degassing inside its 300 m deep benbow crater using op ftir spectroscopy our results reveal a highly dynamic degassing pattern involving i recurrent 100 200 s short period oscillations of the volcanic gas composition and temperature correlating with pulsated gas emission and sourced in the upper part of the lava lake ii a continuous long period 8 min modulation probably due to the influx of fresh magma at the bottom of the lake and iii discrete co2 spike events occurring in coincidence with the sequential bursting of meter sized bubbles which indicates the separate ascent of large gas bubbles or slugs in a feeder conduit with estimated diameter of 6 1 m this complex degassing pattern measured with unprecedented detail and involving both coupled and decoupled magma gas ascent over short time scales markedly differs from that of quieter lava lakes at erebus and kilauea it can be accounted for by a modest size of benbow lava lake and its very high basalt supply rate 20 m3 s 1 favouring its rapid overturn and renewal we verify a typical basaltic arc signature for ambrym volcanic gas and based on contemporaneous so2 flux measurements we evaluate huge emission rates of 160 gg d 1 of h2o 10 gg d 1 of co2 and 8 gg d 1 of total acid gas so2 hcl and hf during medium activity of the volcano in 2008 such rates make ambrym one of the three most powerful volcanic gas emitters at global scale whose atmospheric impact at local and regional scale may be considerable

a crustal mineral in a mantle diamond recent studies of diamonds inclusions in diamonds and rock fragments from the earth s mantle have shown the presence of minerals and rocks with a chemical composition that suggests a previous origin in the crust presumably this crustal material was transported into the mantle at a subduction zone the chemical evidence of previous residence of mantle materials in the crust has come principally from determinations of the proportions of the isotopes of oxygen1 2 sulphur3 4 lead4 5 and carbon6 which typically show a much wider range of composition in the crust than in material of purely mantle origin here we report a less subtle crustal signature in a mantle derived diamond an inclusion of the mineral staurolite staurolite has never previously been found in mantle diamonds or rocks but is a common mineral in metamorphic rocks in the crust the characteristics of the staurolite suggest not only the probability that it formed in rocks with a crustal bulk composition but also the possibility that it formed by metamorphism in the crust or in a subduction zone before being incorporated in the mantle and eventually in a growing diamond

a global synthesis of lava lake dynamics abstract active lava lakes represent a variety of open vent volcanism in which a sizable body of lava accumulates at the top of the magma column constrained by the vent and or crater geometry the longevity of lava lakes reflects a balancing of cooling and outgassing occurring at the surface and input of hot and gas rich magma from below due to their longevity and relative accessibility lava lakes provide a natural laboratory for studying fundamental volcanic processes such as degassing convection and cooling this article examines all seven lakes that existed at the time of writing in 2018 located in the pacific antarctica africa and south and central america these lakes span all tectonic environments and a range of magma compositions we focus on analysis of the lake surface motion using image velocimetry which reveals both similarities and contrasts in outgassing and lake dynamics when comparing the different lakes we identify two categories of lake behavior organized erta ale nyiragongo k lauea after 2011 and erebus and chaotic villarrica masaya marum this division does not map directly to lake size viscosity gas emission rate or temperature instead when examined together we find that the lakes follow a linear relationship between average surface speed and the ratio of total gas flux to lake surface area this relationship points to the combined importance of both flux and lake size in addition to the total volume of gas emission and suggests that a shared deep mechanism controls the supply of heat and gas to all lakes on the other hand the differences between chaotic and organized lakes highlight the important role of the geometry of the conduit lake transition which superimposes a shallow signal on that of the deep circulation the spatial patterns of surface motion we document suggest that the release of gas bubbles at chaotic lakes is more efficient i e bubbles are less likely to be retained and recycled compared with organized lakes in addition the data presented here indicate that the solidified crust of organized lakes plays a role in regulating convection and outgassing in lava lakes

experimental quantification of plagioclase crystal size distribution during cooling of a basaltic liquid an experimental study has been conducted to constrain how thermal history controls crystal size distribution csd of plagioclase in cooling basalts data from all experiments are dominated by a log linear segment of decreasing number density with increasing crystal size consistent with observations in many natural rocks the slope of the csd is found to be a function of cooling rate faster cooling leading to greater slopes at constant cooling rate the csds flatten as temperature decreases and are sometimes kinked characteristics consistent with independent textural observations that crystal agglomeration contributes significantly to crystal growth a downturn is observed toward small size which suggests ostwald ripening furthermore we find that thermal history above the liquidus has a major influence on csds and on the temperatures of phase appearance we conclude that near the liquidus heterogeneous nucleation typically dominates

eclogite facies fluid infiltration constraints from δ 18 o zoning in garnet in situ analysis reveals that eclogite facies garnets are zoned in 18o with lower values in the core and rims that are 1 5 to 2 5 higher this pattern is present in 9 out of 12 garnets analyzed by sims from four orogenic eclogite terranes and correlates with an increase in the mole fraction of pyrope and mg fe ratio from core to rim indicating prograde garnet growth at the maximum temperatures and the time scales experienced by these garnets calculated intragranular diffusion distances for oxygen are small 5 from two kimberlites are homogeneous in 18o possibly due to diffusive exchange which is possible for prolonged periods at higher mantle temperatures homogeneity of 18o in garnets outside the normal mantle range 5 6 may be common in kimberlitic samples

a type magmatism in a syn collisional setting the case of the pan african hook batholith in central zambia abstract the pan african hook batholith formed during the assembly of the gondwana supercontinent as a result of syn collisional stage interaction between the congo and kalahari cratons the bimodal magmatism mafic to predominantly felsic is characterized by both an alkali calcic and an alkalic suite with typical a type metaluminous high fe mg and k na geochemical signature occasionally sodic granitoids have been documented compositions were driven to more differentiated products by fractional crystallization while sr nd isotopes exclude crustal assimilation during crystallization recent new u pb age data constrain most of the felsic magmatism between 550 and 540 ma scattered outcrops of gabbroic rocks both tholeiitic and alkaline testify to periodic input of mantle material and in some cases to interaction with metasomatizing fluids crystallization ages on mafic rocks span from 570 to 520 ma thus indicating that they were contemporaneous with the major granitic intrusion which was the result of a number of successive felsic batches eventually forming a coalescing batholith highly radiogenic pb isotopic values attest to the radiogenic character of the rocks such an anomalous signature was acquired during or soon after magma emplacement perhaps as result of metasomatizing fluids enrichment in th u of large portions of the crust along this part of the margin of the congo craton is suggested geochemical and isotopic evidence support the interaction between mantle components and portions of the deep crust at pressure of

oxygen isotopes in coexisting garnets clinopyroxenes and phlogopites of roberts victor eclogites implications for petrogenesis and mantle metasomatism abstract 18 o values of coexisting garnet clinopyroxene and phlogopite for twelve compositionally and texturally diverse roberts victor eclogite xenoliths range from 3 8 to 7 1 4 0 to 7 4 and 5 9 to 7 4 respectively differences between the 18 o values of coexisting garnets and clinopyroxenes are normally zero however there is some variation in the 18 o values of different fractions of the same mineral in four samples which suggests the presence of isotopic zonation and inhomogeneity possibly resulting from the introduction of a secondary fluid which metasomatized the eclogites and resulted in the formation of phlogopite amphibole and celsian the 18 o value of the metasomatic fluid is generally buffered by the isotopic composition of the primary garnet and clinopyroxene as indicated by a correlation between the isotopic composition of phlogopite and the primary pyroxene and garnet the large range in 18 o values of the eclogites and the similarity in the isotopic composition of coexisting pyroxene and garnet support the interpretation that the roberts victor eclogites represent metamorphosed altered basalts the eclogites were subjected to infiltration metasomatism in the mantle prior to their incorporation in the kimberlite and the source of this fluid was probably unrelated to the eclogite

megacrystals track magma convection between reservoir and surface abstract active volcanoes are typically fed by magmatic reservoirs situated within the upper crust the development of thermal and or compositional gradients in such magma chambers may lead to vigorous convection as inferred from theoretical models and evidence for magma mixing recorded in volcanic rocks bi directional flow is also inferred to prevail in the conduits of numerous persistently active volcanoes based on observed gas and thermal emissions at the surface as well as experiments with analogue models however more direct evidence for such exchange flows has hitherto been lacking here we analyse the remarkable oscillatory zoning of anorthoclase feldspar megacrystals erupted from the lava lake of erebus volcano antarctica a comprehensive approach combining phase equilibria solubility experiments and melt inclusion and textural analyses shows that the chemical profiles are best explained as a result of multiple episodes of magma transport between a deeper reservoir and the lava lake at the surface individual crystals have repeatedly travelled up and down the plumbing system over distances of up to several kilometers presumably as a consequence of entrainment in the bulk magma flow our findings thus corroborate the model of bi directional flow in magmatic conduits they also imply contrasting flow regimes in reservoir and conduit with vigorous convection in the former regular convective cycles of 150 days at a speed of 0 5 mm s 1 and more complex cycles of exchange flow and re entrainment in the latter we estimate that typical 1 cm wide crystals should be at least 14 years old and can record several from 1 to 3 complete cycles between the reservoir and the lava lake via the conduit this persistent recycling of phonolitic magma is likely sustained by co 2 fluxing suggesting that accumulation of mafic magma in the lower crust is volumetrically more significant than that of evolved magma within the edifice

oxygen isotopes in indian plate eclogites kaghan valley pakistan negative δ18o values from a high latitude protolith reset by himalayan metamorphism abstract oxygen isotope compositions are reported for the first time for the himalayan metabasites of the kaghan valley pakistan in this study the highest metamorphic grades are recorded in the north of the valley near the india asia collision boundary in the form of high pressure hp group i and ultrahigh pressure uhp group ii eclogites the rocks show a step wise decrease in grade from the uhp to hp eclogites and amphibolites the protoliths of these metabasites were the permian panjal trap basalts ca 267 2 4 ma which were emplaced along the northern margin of india when it was part of gondwana after the break up of gondwana india drifted northward subducted beneath asia and underwent uhp metamorphism during the eocene ca 45 1 2 ma at the regional scale amphibolites group i and ii eclogites yielded 18o values of 5 84 and 5 91 1 66 to 4 24 and 2 25 to 0 76 respectively relative to vsmow on a more local scale within a single eclogite body the 18o values were the lowest 2 25 to 1 44 in the central the best preserved least retrograded parts and show a systematic increase outward into more retrograded rocks reaching up to 0 12 these values are significantly lower than the typical mantle values for basalts of 5 7 0 3 the unusually low or negative 18o values in group ii eclogites potentially resulted from hydrothermal alteration of the protoliths by interactions with meteoric water when the indian plate was at southern high latitudes 60 s the stepwise increase in 18o values among different eclogite bodies in general and at single outcrop scales in particular reflects differing degrees of resetting of the oxygen isotope compositions during exhumation related retrogression

potassium argon dating of bed i olduvai gorge 1961 1972 abstract potassium argon dating of bed i in olduvai gorge was done or supervised by g h curtis and j f evernden in the department of geology and geophysics at the university of california at berkeley this work proceeded in three phases the first of which was the dating of a small number of tuff samples by evernden and curtis the dates ranged from 1 57 to 1 89 ma averaging 1 73 ma for tuffs interstratified with hominid bearing deposits this dating was controversial because of the long span indicated for the pleistocene and for the antiquity of early tool making hominids koenigswald et al 1962 obtained a date of 1 3 0 1 ma for a sample of basalt from below the dated tuffs of bed i and argued that the dates from tuffs were in error evernden and curtis then dated the basalts at 4 4 ma which was revised to 1 92 ma supporting the earlier dates on tuffs the stratigraphy of bed i was worked out over the period of dating and some dates that made no sense early in the stratigraphic work fell into place later the second phase of dating was primarily intended to check the earlier date of 1 75 ma for hominid remains and stone artifacts of bed i this work was based on a large number of tuff samples and yielded a wide scatter of ages contamination by older materials was recognized in some samples and inferred in others and criteria were established for accepting and rejecting dates using these criteria 15 dates were considered as valid from the total of 41 dates obtained from tuffs above the basaltic lavas the mean age of the valid dates was 1 74 0 05 ma confirming the antiquity of the early hominids the 15 valid dates were however difficult to interpret in terms of the stratigraphy and further selection was made among the valid dates to estimate a time span of 350 000 years for the interval between the basalts 1 92 0 06 ma and the highest tuff dated 1 57 0 05 ma the third phase involved dating samples from previously undated horizons and re evaluating all work done previously on bed i samples the highest priority for new dating was tuff if at the top of bed i but the results were unsatisfactory apparently because of undetected contamination by older volcanic materials remeasurement of the earlier mass spectograms led to changes in some of the published dates and the re evaluated dates except for those of tuff ib still showed a wide scatter indicating that most of the sampled tuffs were unsuited for accurate k ar dating only tuff ib with a mean age of 1 786 0 029 ma gave results with a precision comparable to laboratory standards the re evaluated bed i dates showed no trend toward younger ages upward suggesting a relatively short time span for bed i recent 40 ar 39 ar dates based on analyses of single crystals mark a great improvement over the k ar dating of bed i dates on all of the marker tuffs fall in the correct stratigraphic order and the age span of bed i is 0 27 ma from 2 02 to 1 75 ma components yielding two or more ages were identified in several tuffs thus accounting for some and perhaps most of the variability in the k ar dates

the jurassic danba hypozonal orogenic gold deposit western china indirect derivation from fertile mantle lithosphere metasomatized during neoproterozoic subduction the genesis of hypozonal orogenic gold deposits is highly controversial with both crustal metamorphic fluids and variously derived sub crustal fluids suggested as fluid and metal sources the lower jurassic danba gold deposit a rare phanerozoic hypozonal orogenic gold deposit on the western margin of the yangtze craton china provides an opportunity to resolve these controversies danba is characterized by biotite amphibole alteration and pyrrhotite dominant ore assemblages formed at 500 600 c the 34s values of ore related pyrrhotite range from 3 1 to 9 9 in sharp contrast to those of syn sedimentary pyrrhotite from devonian host rocks which are between 6 8 and 9 5 the pb isotope compositions of ore related pyrrhotite with 206pb 204pb 17 85 18 25 and 207pb 204pb 15 48 15 67 are less radiogenic and more variable than those of unaltered host rocks these differences combined with the hypozonal feature of the deposit indicate that ore metals and sulfur were derived from a sub crustal rather than a local crustal source the pge patterns of ore related pyrrhotites are similar to those of sulfides in baltic mantle lithosphere that was metasomatized by aqueous fluids in terms of marked enrichment of pd and ru but significantly differ from those of magmatic intrusions and related magmatic hydrothermal deposits this suggests that the metals that formed the danba deposit were transported via aqueous fluid from metasomatized mantle lithosphere rather than from a magmatic source the 18o of mantle fluid calculated from hydrothermal quartz and biotite ranges from 10 to 12 the anomalously high s and o isotope ratios and variable pb isotope ratios together with pge data support a model in which the inferred mantle lithosphere source was significantly hydrated metasomatized and fertilized by fluid derived from subducted oceanic sediments with elevated 34s and 18o values geological evidence suggests that this subduction event was neoproterozoic in age this study defines a model in which hydrated and metasomatized mantle lithosphere was formed during early subduction but it was over 500 million years later during later lower jurassic asthenosphere upwelling that metal and sulfur rich fluid was released into the crust to form the danba gold deposit

chronostratigraphy and age modeling of pleistocene drill cores from the olduvai basin tanzania olduvai gorge coring project abstract the olduvai gorge coring project drilled a total of 611 72 m of core 575 48 m recovered of mostly fluvio lacustrine and fan delta volcaniclastic pleistocene strata at three sites in the olduvai basin tanzania in 2014 we have developed a chronostratigraphic framework for three of the cores based on 40ar 39ar dating of core and outcrop volcanic and volcaniclastic units core paleomagnetic stratigraphy and tephrochemical correlation between cores and from core to outcrop this framework is then used to constrain bayesian stratigraphic age models which permit age estimates for desired core levels with realistic confidence intervals the age models reveal that the deepest core level reached at 245 mbs is 2 24 ma 210 kyr older than the oldest strata exposed at olduvai gorge strata net accretion rates in this early phase of basin history were relatively rapid 57 69 cm kyr but decreased within 250 kyr to 15 cm kyr in lower bed i rates rebounded partially in upper bed i but subsequently declined to

petrotectonic evolution of the high to ultrahigh pressure maksyutov complex karayanova area south ural mountains structural and oxygen isotope constraints the maksyutov complex consists of three fault bounded lithologic units a quartzofeldspathic gneiss containing mafic eclogite boudins unit a1 a metasedimentary blueschist facies yumaguzinskaya unit and a meta ophiolitic melange unit a2 the geologic history of the high to ultrahigh pressure hp uhp assembly of the maksyutov complex is complicated by several stages of prolonged retrograde metamorphism and deformation the sakmara river exposes all three units near the former village of karayanova a structuralrpetrologic cross section through the area yields new quantitative data for the complex and regionally for the south urals analysis of the karayanova area has identified the major structures regional folding within the complex is parallel to the dominant foliation trending northeast southwest stereonet data show that during exhumation this large scale folding was refolded about axes trending southeast unit a1 and the yumaguzinskaya are tectonically and petrologically distinct units juxtaposed by west vergent thrusting and recrystallization within the same subduction zone a shear zone developed later between unit a2 and the unit a1q yumaguzinskaya tectonic package accompanying exhumation field relations and petrofabric demonstrate that blueschist facies recrystalliza tion overprinted an earlier eclogite facies metamorphism thermobarometric measurements yield p t values of 594 6378c 1 5 1 7 gpa for eclogite but these conditions may reflect annealing during the early stage exhumation at 375 ma cuboid graphite aggregates testify to precursor conditions for unit a1 within the diamond stability field if such textures are correctly interpreted measured 18 or 16 o partitioning between pairs of coexisting phases yield three main recrystallization temperature ranges 1 678 838c attending unit a1 eclogite facies metamorphism 2 453 178c during transitional blueschistrgreenschist facies metamorphism for the amalgamated unit a1q yumaguzinskayaq unit a2 assembly and 3 250 688c reflecting late stage hydrothermal alteration and exhumation oxygen isotope data for units a1 and a2 indicate that garnet blue amphibole and pyroxene crystallized in isotopic equilibrium validating previous thermobarometric calculations for a unit a1 retrograde metamorphic event variations in d 18 o values for phengites suggest the possibility of late metamorphic fluid infiltration retrograde recrystallization at high pressure in the presence of fluids and a calculated

the application of c isotope measurements to the identification of the sources of c in diamonds a review abstract in the past decade the isotopic compositions of c in 600 inclusion bearing diamonds have been determined such analyses have revealed the following isotopic characteristics 1 peridotitic diamonds which typically contain garnet chromite olivine and or orthopyroxene inclusions with refractory compositions high mg cr have 13 c values predominantly between 10 and 1 with a sharp peak in the distribution near 5 2 eclogitic diamonds which commonly contain inclusions of omphacitic clinopyroxene cr poor pyrope and or eclogitic accessory minerals such as rutile kyanite coesite or sanidine have 13 c values between 34 and 3 with a smaller peak near 5 3 the isotopic compositions obtained for suites of diamonds from individual occurrences are in general unique and do not resemble the range and distribution obtained by amalgamating the diamond isotope data from a number of localities 4 isotopic zoning patterns and heterogeneities are found in some diamonds cores of coated diamonds tend to be depleted in 13 c relative to the rims and within single octahedral diamonds 13 c variations of nearly 6 have been reported because expected c isotope fractionations at mantle temperatures are small attempts to model the full range of diamond isotope values through fractionating a homogeneous mantle c source have been unsuccessful nevertheless fractionation is probably responsible for some of the observed variation in 13 c values two other models have also been proposed to account for the diamond characteristics outlined above the primordial model suggests that the range and distribution of c isotope compositions are inherited from primordial c in the mantle which has an inhomogeneous isotopic composition such as that found in meteorites the subduction model suggests that subducted crustal c is the source of c in diamonds as organic and inorganic c compounds in the crust exhibit a range of 13 c values similar to that observed in diamonds this paper reviews the c isotope characteristics of diamonds and compares the models which have been proposed to explain the origins of these characteristics

the impact of degassing on the oxidation state of basaltic magmas a case study of kīlauea volcano volcanic emissions link the oxidation state of the earth s mantle to the composition of the atmosphere whether the oxidation state of an ascending magma follows a redox buffer hence preserving mantle conditions or deviates as a consequence of degassing remains under debate thus further progress is required before erupted basalts can be used to infer the redox state of the upper mantle or the composition of their co emitted gases to the atmosphere here we present the results of x ray absorption near edge structure xanes spectroscopy at the iron k edge carried out for a series of melt inclusions and matrix glasses from ejecta associated with three eruptions of k lauea volcano hawai i we show that the oxidation state of these melts is strongly correlated with their volatile content particularly in respect of water and sulfur contents we argue that sulfur degassing has played a major role in the observed reduction of iron in the melt while the degassing of h2o and co2 appears to have had a negligible effect on the melt oxidation state under the conditions investigated using gas melt equilibrium degassing models we relate the oxidation state of the melt to the composition of the gases emitted at k lauea our measurements and modelling yield a lower constraint on the oxygen fugacity of the mantle source beneath k lauea volcano which we infer to be near the nickel nickel oxide nno buffer our findings should be widely applicable to other basaltic systems and we predict that the oxidation state of the mantle underneath most hotspot volcanoes is more oxidised than that of the associated lavas we also suggest that whether the oxidation states of a basalt in particular morb reflects that of its source is primarily determined by the extent of sulfur degassing

magma genesis in an ongoing rifting zone the tadjoura gulf afar area basalts from the tadjoura gulf afar area northeastern africa have been analyzed for chemical and isotopic sr nd o compositions in order to understand processes of magma genesis in relation to the active rifting in this region we use these data to characterize the mantle source of the basalts and to examine the potential role of lithospheric mantle in their genesis the isotopic compositions of sr nd and o show considerable variation with 87sr86sr 0 703 0 707 nd 8 1 to 0 5 and 18o 4 6 to 8 4 combined with major and trace element data these isotopic results suggest that the basalts were essentially produced by partial melting of the mantle sources comparable to those of southern red sea basalts namely both depleted mantle and somewhat enriched plume sources are responsible for the generation of the magmas the continental lithospheric mantle seems to have played only a very minor role in the petrogenesis prior to the opening of the tadjoura gulf and the asal rift some lavas penetrated through continental crust resulting in variable degrees of crustal contamination at present the crust of the tadjoura gulf and the asal rift is dominantly basaltic some lavas of the recent series show very unusual chemical and isotopic characteristics high tath ratios positive eu anomalies and low 18o values these features can be best explained by a contamination process involving assimilation of hydrothermally altered gabbros by some basaltic magmas

eruptive behavior of the marum mbwelesu lava lake vanuatu and comparisons with lava lakes on earth and io abstract observations from field remote sensing of the morphology kinematics and temperature of the marum mbwelesu lava lake in the vanuatu archipelago in 2014 reveal a highly active vigorously erupting lava lake active degassing and fountaining observed at the 50 m lava lake led to large areas of fully exposed lavas and rapid 5 m s movement of lava from the centers of upwelling outwards to the lake margins these rapid lava speeds precluded the formation of thick crust there was never more than 30 non translucent crust the lava lake was observed with several portable handheld low cost near infrared imagers all of which measured temperatures near 1000 c and one as high as 1022 c consistent with basaltic temperatures fine scale structure in the lava fountains and cooled crust was visible in the near infrared at 5 cm pixel from 300 m above the lake surface the temperature distribution across the lake surface is much broader than at more quiescent lava lakes peaking 850 c and is attributed to the highly exposed nature of the rapidly circulating lake this lava lake has many characteristics in common with other active lava lakes such as erta ale in ethiopia being confined persistent and high temperature however it was much more active than is typical for erta ale which often has 90 crust furthermore it is a good analogue for the persistent high temperature lava lakes contained within volcanic depressions on jupiter s moon io such as pele also believed from spacecraft and ground based observations to exhibit similar behavior of gas emission rapid overturn and fountaining

ophiolites as faithful records of the oxygen isotope ratio of ancient seawater the solund stavfjord ophiolite complex as a late ordovician example

fluid inclusion and mineral isotopic compositions hco in eclogitic rocks as tracers of local fluid migration during high pressure metamorphism eclogite facies metagabbros from the monviso ophiolitic complex italian western alps provide a unique opportunity to trace fluid migration processes in a portion of the oceanic crust that has undergone subduction at a depth of 40 km we have determined the omphacites 18o and the abundance d and 13c of hydrous and carbonaceous compounds present in whole rocks which are believed to trace the residual phases of what was mobilized in the original rocks during subduction prograde dehydration reactions and eclogitization of hydrothermally altered oceanic metagabbros was accompanied by approximately 90 fluid loss the remaining fluid was trapped as primary water rich fluid inclusions in omphacite megacrysts that developed at the expense of the precursor magmatic pyroxene deformation of the eclogitic rocks resulted in continuous recycling of fluid between mylonites and omphacite veins without further fluid loss from the host ductile shear zone oxygen isotopes of omphacite omp and hydrogen isotopes of water in the fluid inclusions fi analyzed in low strain rocks mylonites and undeformed deformed veins show marked variations 18oomp and dfi values ranging from 3 0 to 5 3 and from 31 to 93 respectively detailed isotopic analysis of several individual vein wallrock pairs show that the scale of isotopic equilibration is of the order of one centimeter therefore the eclogitic minerals and fluids filling the veins are concluded to be locally derived our results argue against recent models which suggest large scale mass flushing of isotopically homogeneous fluids during subduction zone metamorphism on average the h2o contents and dfi value are within the upper mantle range the carbon has been inherited from the metamorphic transformation of the original carbon present in the oceanic crust and has a mean 13c value of 24 2 1 2 two carbonaceous components can be recognized condensed carbon which represents the major carbon species and carbonate daughter crystals present in fluid inclusions 18oomp values are significantly lower than those reported for mantle minerals and are similar to pyroxene and whole rock values from hydrothermally altered oceanic crust it is suggested that the isotopic imprint of the monviso eclogitic minerals and fluids represents the signature of mid ocean ridge hydrothermal alteration and subduction zone eclogitization processes the rocks studied could be the missing link between altered oceanic gabbros and eclogitic xenoliths in addition the range in 18oomp values recorded in the different microstructural domains covers most of the 18o values reported in type a b and c eclogitic xenoliths implying that caution must be exercised when using oxygen isotopes as an indicator of the origin of eclogitic xenoliths

counter current convection in a volcanic conduit abstract volcanoes of strombolian type are able to maintain their semi permanent eruptive states through the constant convective recycling of magma within the conduit leading from the magma chamber in this paper we study the form of this convection using an analytic model of degassing two phase flow in a vertical channel we provide solutions for the flow at small grashof and large prandtl numbers and we suggest that permanent steady state counter current convection is only possible if an initial bubbly counter current flow undergoes a regime transition to a churn turbulent flow we also suggest that the magma in the chamber must be under pressured in order for the flow to be maintained and that this compromises the assumed form of the flow

deciphering textural and chemical zoning of k feldspar megacrysts from mt amiata volcano southern tuscany italy insights into the petrogenesis and abnormal crystal growth abstract this study reports the complex textural and chemical features of k feldspar megacrysts up to 5 cm long hosted in trachydacitic lava flows lava domes and coulees from mt amiata volcano tuscan magmatic province central italy backscattering and cathodoluminescence imaging coupled with core to rim major and trace elements patterns reveal complex zoning and resorption surfaces associated with sharp chemical variations e g sr and ba these zoning patterns originated by disequilibrium and re equilibration events related to the repeated influx of mafic magmas or convective motions in the trachydacitic magma reservoir multiple mafic magma refilling occurences are also supported by the field record of abundant microgranular magmatic enclaves in the studied products our results highlight that the abnormal dimension of the studied k feldspars originates by the interplay between petrological and kinetic processes involving i extensive dissolution ii heterogeneous nucleation iii alternation of spasmodic growth events in disequilibrium and near equilibrium crystallization the repeated influx of hotter magmas and reheating can determine the thermal condition to the growth of few large k feldspar megacrysts also the strong textural and chemical similarities observed in the k feldspar megacrysts from mt amiata volcanic rocks and mt capanne monzogranite elba island central italy support the hypothesis of a phenocrystic origin of intrusive k feldspar megacrysts

textural constraints on the kinetics of crystallization of igneous rocks natural systems provide time controlled experiments on scales unreachable in the laboratory and with complexities unapproachable by computer experiments since temperature undercooling cannot be directly scaled with time in situ crystallization experiments provide information unavailable by other means the kinetics of crystallization in simple silicate systems have been studied extensively for glass ceramic systems e g uhlmann 1982 james 1982 but predictive models for nucleation behavior in particular remain inadequate part of the problem in creating models of nucleation lies in the difficulty of experimentally isolating the nucleation process crystal growth measurements are more accessible experimentally and for this reason crystal growth is better understood e g dowty 1980 kirkpatrick 1981 james 1982 baronnet 1984 however predictive models of crystal growth e g lasaga 1982 require accurate models for the temperature and compositional dependence of both the thermodynamic and kinetic driving forces for crystallization information that is currently limited the cse parameterization of the position of ymax as a function of t tl and t suggested by dearnley 1983 provides a potentially useful model that requires further testing similarly application of the adiabatic nucleation model of meyer 1986 to simple systems suggests that the position of tmax relative to tl has an approximately constant value of 0 55 0 60 exceptions are low dsf minerals such as albite and quartz additionally while experimental crystallization rate data is a necessary link between processes in natural melts and numerical models experimental growth rate data is confined to simple systems and conditions of moderately large undercooling while growth rate estimates in complex natural systems suggest that under most conditions undercoolings remain very small quantitative textural studies of dikes suggest a pronounced dependence of growth rate on total crystallization time e g ikeda 1977 which is in turn related to effective undercooling a function of both total dike width and distance from the dike margin by analogy minimum crystallization rates may be estimated from programmed cooling experiments if a temperature interval of crystallization is assumed growth rates determined in this way show the same relationship between growth rate and total crystallization time seen in the dike studies reflecting a systematic relationship between growth rate and cooling rate effective undercooling for plagioclase in basaltic systems while the data are not available for other mineral systems there is no reason to believe that similar patterns would not exist for other silicate minerals i e the similarity in dendritic olivine growth rates estimated by fowler et al 1989 using fractal analysis to the growth rates of plagioclase under conditions of rapid cooling estimated from programmed cooling rate experiments analysis of measured crystal size frequencies provides a means of extracting effective bulk kinetic information from geologic systems that can help to constrain the dynamic feedback between physical models of magmatic systems and rates of chemical change

stable and radiogenic isotope study of eclogite xenoliths from the orapa kimberlite botswana eclogite xenoliths from orapa can be accurately classified as group i or group ii on the basis of na2o in garnet and k2o in clinopyroxene group i xenoliths are commonly diamondiferous while group ii xenoliths are diamond free both xenolith varieties may contain graphite isotopic character is to some degree correlated with major and trace element chemistry group ii samples with ca poor garnet have clinopyroxenes with radiogenic 87sr86sr 0 705 0 709 and the least radiogenic 143nd144nd 0 5122 0 5125 group i eclogites with higher ca and fe in garnets have less radiogenic 87sr86sr 0 702 0 7066 and bulk earth or higher 143nd144nd ratios group i eclogites have more radiogenic 206pb204pb 18 6 19 than group ii xenoliths 16 5 18 6 in contrast group ii xenoliths have more variable and in some cases more radiogenic 206pb204pb 36 6 39 3 than group i xenoliths 38 3 38 4 the sr sm nd and pb concentrations of minerals in orapa group i eclogite xenoliths are much lower than in group ii samples all the group ii xenoliths are inferred to be enriched in light rare earth elements while group i xenoliths are probably characterised in many cases by light rare earth element depletion constituent garnet and clinopyroxene in both group i and ii eclogite xenoliths are essentially in isotopic equilibrium at the time of pipe emplacement mineral as well as calculated whole rock 143nd144nd compositions of most of the group i eclogites are too close to bulk earth and depleted mantle estimates in order to obtain useful model age information depleted mantle model ages derived from the much lower 143nd144nd compositions of the group ii eclogite xenoliths range from 661 to 1248 my with an average clinopyroxene model age of 908 my and an average whole rock model age of 1016 my on the basis of an observed covariation of o and sr isotopic compositions the entire orapa group i eclogite xenolith suite can be modelled as mixtures of oceanic basalt with or without a few percent of ocean floor sediment the group ii xenoliths might have crystallised from a melt which derives from a protolith with time averaged lree depletion their radiogenic sr isotope character could be due to interaction of the melt with metasomatised lithosphere or might be a superimposed metasomatic signature

deposition models for chronological records abstract long term sedimentary sequences provide a wealth of useful information for research into the palaeo environment especially in relation to past climate change shorter records provide similar information in many archaeological contexts however if such records are to be used to provide precise timing of events and more critically the relative timing between different records methods are needed to provide accurate and precise age depth models for these sequences given the imprecision of individual calibrated radiocarbon determinations it is necessary to use the information we have about the deposition process to refine our chronologies and also to provide interpolation between dated levels in any sequence even with layer counted chronologies the uncertainties are sometimes hard to quantify this paper outlines a range of bayesian models for deposition which have been implemented in the computer programme oxcal these models can be used to combine information from the sediments themselves with radiocarbon or other direct dating information such models have the potential to integrate information between different records and provide a coherent chronology on which to base environmental or archaeological research