volatile characteristics of peralkaline rhyolites from kenya an ion microprobe infrared spectroscopic and hydrogen isotope study peralkaline rhyolites of the greater olkaria volcanic complex kenya rift valley were derived from separated though closely related magma chambers ion microprobe analyses of glass inclusions in quartz phenocrysts show pre eruptive water contents of up to 3 4 wt contrasting with previous estimates that the magmas were anhydrous the values approach predicted solubility levels corresponding to water saturation at low crustal pressures 1 kbar the glass matrices of the rhyolites have low water contents ranging from 0 07 to 0 46 wt suggesting significant degassing during or prior to eruption infrared measurements of the matrix glasses show variation in the relative proportions of the two hydrous species dissolved in the glasses the amount of molecular water determined semi quantitatively apparently increases with increased fluorine content and peralkalinity this suggests a competition between hydroxyl groups and fluoride ions for similar sites within the melt structure the mechanism of degassing has been investigated using hydrogen isotopes the range of d values in most rocks can be produced by varied degrees of open system degassing of rhyolite melt initially in equilibrium with water of a fixed or limited d value there is evidence to suggest that closed system degassing may also have been a significant component in some rhyolites the exact mechanisms of degassing remain uncertain particular problems include the relative contribution of open and closed system degassing during eruption and the initial vapour compositions and solubility relationships

hydrogen and oxygen isotope geochemistry of ascension island lavas and granites variation with crystal fractionation and interaction with sea water lavas and pyroclastics on ascension island contain plutonic blocks that include fluid inclusion bearing peralkaline granite 18o 16o ratios f and cl have been analysed on whole rocks and or minerals for lavas and granites and d h ratios and h2o for comenditic obsidians and granites whole rock 18o 16o ratios of fresh alkali basalt hawaiite trachyandesite trachyte and comendite range from 6 0 to 6 9 with 18o tending to increase with increase in sio2 the 18o values of the granites are from 0 0 to 0 3 depleted in 18o relative to the comendites comenditic obsidians have d 80 4 and h2o 0 3 wt while amphiboles from the granites have d 56 2 the o isotope trend of the lavas is consistent with a crystal fractionation model fresh igneous rocks with 18o values greater than 7 involve processes in addition to crystal fractionation of a basaltic magma the d h ratios and cl contents 3 000 ppm of the h2o poor comenditic obsidians represent undegassed primary magmatic values the h isotope compositions and low h2o and cl 167 ppm contents of the granites are consistent with the major degassing loss of 90 of initial h2o of an h2osaturated magma derived from the interaction of sea or possibly meteoric water with the h2o undersaturated comenditic melt it is proposed that associated with caldera subsidence and stoping water was sucked in around the residual magma before the system had time to be sealed up the h2o undersaturated magma consumed this h2o with possibly some minor partial dehydration and dewatering of the hydrated volcanic roof blocks at a pressure of about 1 5 kb the granites are the plutonic equivalents of rhyolitic pyroclastics and not directly of the comendites granites from oceanic islands may in general be a result of generating an h2o saturated acid melt by such direct or indirect crustal water magma interaction processes

predatorial borings in late precambrian mineralized exoskeletons the late precambrian tube forming cloudina the earliest known animal to produce a mineralized exoskeleton shows evidence of having been attacked by shell boring organisms of more than 500 tubes from shaanxi province china 2 7 have rounded holes 40 to 400 micrometers in diameter the relation between the size of the holes and the width of the bored tubes suggests that the attacking organism was a predator selecting its prey for size if true this would be the oldest case of predation in the fossil record and would support the hypothesis that selection pressures from predation was a significant factor in the evolution of animal skeletons around the precambrian cambrian boundary

new material of the biomineralizing tubular fossil sinotubulites from the late ediacaran dengying formation south china abstract sinotubulites is a late ediacaran biomineralizing tubular fossil with a probable animal affinity it is characterized by millimeter to centimeter sized and multi layered tubes open at both ends the tube consists of two morphologically different walls a multi layered inner wall with weak ornamentations and a multi layered outer wall with transverse or oblique corrugations and sometimes longitudinal ridges the majority of previously published sinotubulites species are considered as synonymous with the type species s baimatuoensis three new species s triangularis n sp s pentacarinalis n sp and s hexagonus n sp are reported from the late ediacaran beiwan member of the dengying formation in southern shaanxi province south china the three new species are similar to the type species in having nested multilayered inner and outer tube walls however they are different in their polygonal cross sections and longitudinal ridges s baimatuoensis is more or less circular in cross section and lack longitudinal ridges on the outer tube wall whereas s triangularis s pentacarinalis and s hexagonus are respectively triangular pentagonal and hexagonal in cross section with three five and six longitudinal ridges on the exterior surface of the outer wall the new material adds to the diversity of late ediacaran biomineralizing animals the triradial pentaradial and hexaradial tubes of s triangularis s pentacarinalis and s hexagonus share some intriguing similarities in body symmetry with several early cambrian tubular fossils although these cambrian tubes are not open at both ends still it would be interesting to explore the tantalizing possibility of evolutionary continuity of triradial pentaradial and hexaradial tubular animals across the precambrian cambrian boundary

the terminal ediacaran tubular fossil cloudina in the yangtze gorges area of south china abstract the advent of biomineralizing metazoans in the terminal ediacaran age ca 550 539 ma represents a remarkable biological innovation in the history of life as a poster child of this evolutionary episode cloudina is widely regarded as a weakly biomineralizing tubular fossil with a global distribution therefore cloudina can both inform the evolution of animal biomineralization and facilitate terminal ediacaran stratigraphic correlation however this key taxon has not been fully described from the yangtze gorges area of south china where classical terminal ediacaran strata have been investigated extensively by paleontologists stratigraphers and geochemists here we document an assemblage of three dimensionally silicified tubular fossils from siliceous dolostone of the terminal ediacaran baimatuo member of the dengying formation in the yangtze gorges area western hubei province south china the baimatuo assemblage consists of sinotubulites which has been previously known in the baimatuo member as well as cloudina including c ningqiangensis c hartmannae c cf carinata and c sp indet and other unnamed tubular forms this discovery adds to the diversity of early biomineralizing metazoans in the yangtze gorges area and facilitates the biostratigraphic correlation of the cloudina sinotubulites co occurrence assemblage in terminal ediacaran strata

magmatic model for the mount st helens blast of may 18 1980 analytical and numerical solutions to the hydrodynamic equations of motion constrained by physical properties of juvenile ejecta in the mount st helens blast deposit were used to investigate magmatic conditions required to produce the initial devastating blast phase of the eruption of may 18 1980 evidence that the blast was magmatic includes equivalence in volume of juvenile blast ejecta to preemption inflation of the cone substantial vesicularity of this ejecta and continued vesiculation of large juvenile clasts after eruption observed or inferred ejecta velocities of 100 to 250 m s are shown to require 0 2 to 0 7 wt water vapor preexisting in magma unloaded by a landslide 200 to 900 m thick these conditions imply total magmatic water contents of 0 7 to 1 7 wt respectively such low required water content suggests that volcanic blasts may be regarded as a normal consequence of magma intrusion into an unstable edifice

transitional ediacaran cambrian small skeletal fossil assemblages from south china and kazakhstan implications for chronostratigraphy and metazoan evolution abstract the ediacaran cambrian transition records distinct evolutionary changes of metazoans the ediacaran fossils i e ediacara type biota and tubular fossils are contrasting with the diverse small skeletal fossils ssfs from the early cambrian the apparent dissimilarities hindered studies deciphering their evolutionary relationships this also led to a popular assumption that there exists a mass extinction of the ediacara biota and cloudinids at the precambrian cambrian pc c boundary here we report for the first time a transitional fauna which consists of typical elements of ediacaran i e cloudinids and related tubicolous organisms together with cambrian ssfs including protoconodonts anabaritids and siphogonuchitids from south china and maly karatau kazakhstan the sediments yielding the transitional fauna are characterized by siliceous rocks in both regions and were previously considered to be unfossiliferous their chronostratigraphic assignment in south china has been debated for years based on the new fossil assemblage the siliceous strata of the daibu member northeast yunnan south china and the basal kuanchuanpu formation south shaanxi south china can be assigned to the earliest cambrian ssf biozone anabarites trisulcatus protohertzina anabarica assemblage zone and thus can be considered of early cambrian in age a new subzone of the earliest ssf zone in eastern yunnan south china is proposed herein defined as ganloudina symmetrica rugatotheca typica interval subzone the new fauna demonstrates that the cloudinids originally confined to the late ediacaran persisted into the cambrian fortunian and no major extinction event occurred at the pc c transition

origin and impact of the oldest metazoan bioclastic sediments the emergence of soft bodied metazoans and the radiation of the earliest skeletal organisms substantially changed the ecological dynamics of ediacaran environments leading to the genesis of biogenic hard part deposits for the first time in earth s history the impact of bioclast origin on sedimentary processes is analyzed herein focusing on the sedimentology and taphonomy of shell concentrations dominated by the ediacaran index fossil cloudina from the itapucumi group paraguay skeletal concentrations include both dense accumulations of parautochthonous disarticulated specimens type 1 deposits and in situ specimens preserved as loosely packed assemblages type 2 deposits at that time cloudina was the critical source of durable biomineralized hard parts in an environment nearly free of other bioclasts the simple fabric and geometry of these accumulations are typical of cambrian style shell beds despite their precambrian age these deposits indicate that the establishment of the phanerozoic style of marine substrates and preservation in early shell beds was determined more by the acquisition of hard parts than by environmental changes

the probable occurrence of interstitial al in hydrous f bearing and f free aluminosilicate melts experiments carried out on the system sio2 naalsi3o8 kalsi3o8 qz ab or at 1 kbar in the presence of h2o and f show that the quartz alkali feldspar field boundary is progressively displaced towards the feldspar join as f contents increase from 0 to 4 wt f increasing p h 2 o in the absence of f has already been shown to have a similar effect tuttle and bowen 1958 luth jahns and tuttle 1964 the increased size of the quartz field in the f bearing system compared to the hydrous system is believed to be caused by progressive removal of al from the tetrahedral network of the melt by complexing with f the residual network in the melt is thus enriched in si and this stabilizes precipitation of quartz rather than feldspar for certain bulk compositions the common presence of quench cryolite na3alf6 in certain experiments carried out with 4 wt f supports this interpretation and indicates that some al in the melt may be present in six fold coordination with f the effect of h2o in the absence of f may be similar with al being progressively removed from four fold coordination as more h2o is dissolved in the melt although a proportion of al in hydrous melts may occur in six fold coordination dry melts predominantly contain al in four fold coordination this major difference in al complexing may be one of the main causes for differences in the high pressure phase relations of wet and dry albearing silicate systems

controls on fossil pyritization redox conditions sedimentary organic matter content and chuaria preservation in the ediacaran lantian biota abstract pyritization represents a major taphonomic pathway for exceptional preservation of soft tissues although various ediacaran lagerstatten contain pyritized fossils the controls on this taphonomic pathway prior to the precambrian phanerozoic transition have only recently received significant attention and no studies have yet thoroughly investigated environmental conditions facilitating exceptional preservation via pyritization in the early ediacaran here we investigate the preservational environments of macroscopic fossil chuaria in the early ediacaran lantian formation of south china using petrographic electron microscopic and geochemical data chuaria occur as pervasively pyritized spheroid shaped globose and non pervasively pyritized disk shaped subglobose fossils in different stratigraphic intervals although these different stratigraphic intervals are similar in terms of total sulfur ts 5 subglobose chuaria shales have greater total organic carbon toc 7 9 contents than globose chuaria shales 3 6 additionally petrographic observations and toc ts cross plots suggest that whereas globose fossils were preserved under suboxic bottom waters the subglobose fossils were preserved in euxinic bottom waters overall these results affirm that pyritization in the precambrian was generally favored in organic poor reactive iron rich sulfate rich environments probably with slow sedimentation rates when organics were scarce and reactive iron was abundant bacterial sulfate reduction bsr created hydrogen sulfide and reactive iron concentration gradients around fossils which kinetically and thermodynamically favored rapid and pervasive pyritization as observed in globose chuaria conversely when organics were abundant and reactive iron sulfate availabilities were limited by widespread bsr fossils were relatively rapidly buried beneath the bsr metabolic zone of sediment where they were principally preserved as aluminosilicified carbonaceous fossils that were compacted into subglobose forms thus preservational variations among pyritized fossils in the ediacaran may reflect differences in sedimentary organic matter content and or bottom water redox chemistry

a new modular palaeopascichnid fossil curviacus ediacaranus new genus and species from the ediacaran dengying formation in the yangtze gorges area of south china non biomineralizing ediacaran macrofossils are rare in carbonate facies but they offer valuable information about their three dimensional internal anatomy and can broaden our view about their taphonomy and palaeoecology in this study we report a new ediacaran fossil curviacus ediacaranus new genus and species from bituminous limestone of the shibantan member of the dengying formation in the yangtze gorges area of south china curviacus is reconstructed as a benthic modular organism consisting of serially arranged and crescent shaped chambers the chambers are confined by chamber walls that are replicated by calcispars and are filled by micritic sediments such modular body construction is broadly similar to the co occurring yangtziramulus zhangii and other ediacaran modular fossils such as palaeopascichnus the preservation style of curviacus is similar to yangtziramulus although the phylogenetic affinities of both genera remain unresolved the new fossil adds to the diversity of ediacaran modular organisms

ion microprobe analyses of trace elements in anorthoclase hedenbergite aenigmatite quartz apatite and glass in pantellerite evidence for high water contents in pantellerite melt

pre eruptive water content of rhyolitic magmas as determined by ion microprobe analyses of melt inclusions in phenocrysts abstract the ion microprobe was used to analyze trapped melt inclusions in phenocrysts from two rhyolitic eruptions for h2o f and incompatible trace elements eleven melt inclusions from phenocrysts in air fall tephra from obsidian dome near long valley usa gave an average of 4 1 1 2 wt h2o and showed large variations in f and incompatible trace elements reflecting a complex history reported to involve mixing of at least two magma types eight inclusions in phenocrysts from the taupo ultraplinian event new zealand gave 4 5 0 8 wt h2o with small variations in the other elements analyzed measured water contents are similar to earlier estimates of the pre eruptive water contents of these and similar rhyolites and major and trace element analyses of inclusions compare closely with bulk analyses suggesting that the analyses of inclusions represent the chemistry of the magma at the time of entrapment

ion microprobe studies of water in silicate melts concentration dependent water diffusion in obsidian abstract the temperature dependence of water diffusivity in rhyolite melts over the range 650 950 c and pt h2o 700 bars is evaluated from water concentration distance profiles measured in glass with an ion microprobe diffusivities are exponentially dependent on concentration over this temperature range and vary from about 10 8 cm2 s at 650 c to about 10 7 cm2 s at 950 c at 2 wt water water solubility also varies with temperature at a rate of 0 14 wt per 100 c increase the avtivation energy ea appears to be constant at 19 1kal mole for 1 2 and 3 wt h2o comparison of these data with results for cation diffusion indicates that this value is a minimum ea for diffusion of any species in a rhyolite melt compensation plots of log10d0 the frequency factor versus ea indicate that hydrous rhyolite melts follow the same trend as anhydrous basalts d0 increases for h2o and ca2 1 as ea decreases this suggests that these molecules may diffuse by different mechanisms than do monovalent cations and that hydration of the melt affects diffusion of ca2 and h2o differently than it does monovalent cation diffusion the results imply that dramatic increases in cation diffusivities by hydration 1 may occur with additions of less than 1 wt h2o

hydrogen isotope systematics of submarine basalts the d h ratios and water contents in fresh submarine basalts from the mid atlantic ridge the east pacific rise and hawaii indicate that the primary d h ratios of many submarine lavas have been altered by processes including 1 outgassing 2 addition of seawater at magmatic temperature and 3 low temperature hydration of glass decreases in d and h2o from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between d and h2o in basalts from the galapagos rise and the famous area are attributed to outgassing of ch4 and h2 a good correlation between d values and h2o is observed in a suite of submarine tholeiites dredged from the kilauea east rift zone where seawater added directly to the magma affected only the isotopic compositions of hydrogen and argon analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having d values as low as 100 d values vary with h2o contents of subaerial transitional basalts from molokai hawaii and subaerial alkali basalts from the society islands indicating that the primary d values were similar to those of submarine lavas extrapolations to possible unaltered d values and h2o contents indicate that the primary d values of most thoteiite and alkali basalts are near 80 5 the weight percentages of water are variable 0 15 0 35 for mor tholeiites about 0 25 for hawaiian tholeiites and up to 1 1 for alkali basalts the primary d values of 80 for most basalts are comparable to those measured for deep seated phlogopites these results indicate that hydrogen in marked contrast to other elements such as sr nd pb and o has a uniform isotopic composition in the mantle this uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the earth

stable carbon isotopes of sedimentary kerogens and carbonaceous macrofossils from the ediacaran miaohe member in south china implications for stratigraphic correlation and sources of sedimentary organic carbon abstract ediacaran stratigraphic correlation depends on the integration of paleontological and 13ccarb data however these data often come from different lithologies e g shales vs carbonates making it difficult to directly link biostratigraphic and chemostratigraphic markers ediacaran successions in south china consist of mixed carbonates and shales offering an opportunity for integrated biostratigraphic and chemostratigraphic investigations black shales of the ediacaran miaohe member in the yangtze gorges area contain macroalgae and ediacara type macrofossils both preserved as carbonaceous compressions the miaohe member has been traditionally regarded as an equivalent to member iv of the uppermost doushantuo formation that records part of the shuram negative 13ccarb excursion in south china thus the miaohe member may have the potential to clarify the stratigraphic relationship between ediacara type fossils and the shuram excursion recently however the miaohe member has been partially correlated with the shibantan member of the dengying formation the a correlation or partially with member iv the z correlation these correlations have different implications for the relationship between the miaohe member and the shuram excursion here we test these two correlations using 13c data of sedimentary kerogen or total organic carbon 13corgtoc taking advantage of the distinct 13corgtoc values between the shibantan member and member iv our data show that 13corgtoc of the miaohe member is more similar to that of member iv than to shibantan member providing tentative support for the z correlation in addition we also analyzed 13corg of individual carbonaceous compression fossils 13corgtaxon which have lower carbon isotope values than toc the data suggest that oxygenic photosynthesizers e g macroalgae cyanobacteria and eukaryotic phytoplankton were not the only contributors to sedimentary toc methylotrophs photoautotrophs and chemoautotrophs living in anoxic waters or at the oxycline may have fixed recycled co2 derived from organic carbon remineralization thus contributing a significant amount of 13c depleted organic carbon to sedimentary kerogen in the miaohe member

evolution of the aveiro cretaceous aquifer nw portugal during the late pleistocene and present day evidence from chemical and isotopic data

tectono depositional pattern and evolution of the middle yangtze platform south china during the late ediacaran abstract the depositional environments and evolution of the yangtze block during the ediacaran cambrian transition remain uncertain preventing a thorough understanding of the coeval ocean stratification redox heterogeneity and organism distribution in this study detailed facies and sequence stratigraphic analyses were carried out on the dengying formation and its correlatives to better constrain the tectono depositional pattern and evolution of the middle yangtze area in the late ediacaran twenty five lithofacies were identified in nineteen sections and grouped into six depositional facies intrashelf basin intrashelf shoal restricted platform interior platform margin upper middle slope and lower siliceous slope basin the spatiotemporal distributions of these depositional facies reveal an isolated rimmed middle yangtze carbonate platform embayed with a westward opening intrashelf basin this intrashelf basin gradually became wider and deeper westwards linking to a nne trending basinal seaway that separated the middle yangtze platform from its upper yangtze counterpart the enclosure of the marginal shoal reef complex with an intrashelf shoal chain formed a large scale restricted lagoon and associated tidal flat in the platform interior based on key sequence boundaries and cycle stacking patterns two and a half transgressive regressive sequences were distinguished in the dengying formation and its correlatives in the twelve representative sections furthermore these sequences can be correlated well with those identified in other upper ediacaran successions elsewhere in upper yangtze namibia india and oman indicating a eustatic origin for these sequences however the complex platform basin configuration with significant facies and thickness variations reflects a transtensional tectonic regime in the middle yangtze area during the late ediacaran the abrupt changes in lithofacies and thickness across the southern principal platform margin and those from sandy shoals banks to storm influenced shelf along the northern portion of the intrashelf basin in the three gorges western hubei suggest synthetic extensional faulting across the margins during deposition the two synthetic extensional fault zones might have been induced by nne trending transtensional faulting along the eastern margin of the upper yangtze platform this structural pattern fundamentally controlled the depositional differentiation of the middle yangtze platform during the late ediacaran and also laid the foundation for the subsequent facies distribution and depositional evolution during the early cambrian terreneuvian epoch

cloudina microbial reef resilience to substrate instability in a cadomian retro arc basin of the iberian peninsula abstract the navalpino anticline of the central iberian zone provides a unique case study of well exposed late ediacaran reefs drone images show their spatial arrangement as tens of shelly microbial patch biohermal and complex reefs encased in a dominantly shaly succession the latter was deposited on a perturbed block platform located in a cadomian retro arc basin fringing west gondwana reefs display the traditional core flank inter reef subdivision and developed on generally low energy clear water substrates between normal and storm wave base cores consist of millimetre sized irregular mesoclots of dense microbial microsparite locally interrupted by laminated stromatolitic fabrics containing abundant sinotubulites and cloudina shells the last with upright preservation basement breakdown perturbations related to fissuring hydrothermal influence and slope apron breccia deposition led to nucleation and development of patch reefs on unstable substrates their ecological succession is characterized by stabilization pioneer to colonization phases the latter represented by cloudina shells preserved upright and encased in microbial crusts pointing to their active role as pioneer frame building taxa

petrogenesis of coexisting high silica aluminous and peralkaline rhyolites from yunshan yongtai southeastern china abstract the late cretaceous bimodal yunshan yongtai volcanics in fujian province contain peralkaline rhyolites the only presence of such rhyolites in southeastern china whole rock and mineral chemical compositions are analyzed for the coexisting aluminous metaluminous to weakly peraluminous peralkaline high silica rhyolites from the yunshan volcanics they are sparsely porphyritic and contain k feldspar ferromagnesian minerals quartz magnetite and titanomagnetite phenocrysts as well as accessory minerals such as fayalite chevkinite apatite and zircon the mineral assemblage indicates an oxidizing pre eruption condition these rhyolites exhibit diagnostic geochemical features of a type granites such as elevated 10 4 ga al mostly greater than 2 6 and feo t feo t mgo ratios enrichment in high field strength elements hfse such as zr 400 ppm and nb and strong depletion in al 2 o 3 2 o poor meta granitic igneous rocks in the deep crust and cannot be fractionated from the coeval yunshan mafic magmas geochemical variations of major and trace elements indicate the possible fractionation of k feldspar calcium rich pyroxene fe ti oxides and minor chevkinite during the magma evolution in peralkaline rhyolites we found that the pre existing fe ti oxide and hedenbergite phenocrysts had been transformed into aegirine oxide and aegirine oxide fluorite assemblages respectively these mineral assemblages are the products of the subsolidus reaction of pre existing phenocrysts and extraneous na f rich fluids such na f rich fluids may be derived from the degassing of the subvolcanic rocks the reactions indicate that the yunshan peralkaline rhyolites could be generated through the reaction of highly fractionated aluminous silica magmas and na f rich fluids

new data on magmatic h 2 o contents of pantellerites with implications for petrogenesis and eruptive dynamics at pantelleria infrared spectroscopic analyses of melt inclusions in quartz phenocrysts from pantellerites erupted at pantelleria italy show that the magmas contained moderate pre eruptive h2o contents ranging from 1 4 to 2 1 wt melt h2o concentrations increase linearly with incompatible elements demonstrating that h2o contents were not buffered significantly during fractionation by any crystalline or vapor phase the relatively low h2o contents of pantellerites are consistent with an origin by partial melting of alkali gabbros rather than fractional crystallization of basalt preeruptive h2o concentrations do not correlate with the volume or explosivity of pantellerite eruptions decompression history is critical in determining the style of pantellerite and other eruptions

integrated correlation of the vendian to cambrian arroyo del soldado and corumba groups uruguay and brazil palaeogeographic palaeoclimatic and palaeobiologic implications abstract the corumba group of sw brazil and the arroyo del soldado group asg of uruguay are correlated on the basis of litho bio and chemostratigraphy both units represent marine sedimentation with alternating siliciclastics and carbonates developed on a stable continental shelf in the corumba basin sedimentation began in the varangerian represented by the glaciomarine puga formation a series of sea level fluctuations coupled with climatic changes are recorded up section while uppermost deposits of the asg are of lowermost cambrian age sedimentation ceased in the latest vendian in the corumba basin an assemblage of six species of organic walled microfossils dominated by bavlinella faveolata and soldadophycus bossii three species of vendotaenids and two species of skeletal fossils cloudina and titanotheca is described from the corumba group the vendotaenid eoholynia corumbensis sp nov is described from siltstones of the guaicurus formation an important diversity of skeletal fossils in the corumba arroyo del soldado and nama groups points to favourable vendian palaeoclimatic conditions in sw gondwana preliminary carbon isotopic data show a series of alternating positive and negative excursions corroborating the upper vendian age indicated by fossils for both units previously reported strontium isotopic data are also consistent with this age it is postulated that the corumba and asgs were deposited onto the same shelf which opened to the east the rio de la plata superterrane craton extends farther to the north than previously expected or it was already amalgamated with the amazonian craton by vendian times collision of the platform with the parana block caused closure of the basin during the cambrian early ordovician finally models of neoproterozoic glaciations based on enhanced bioproductivity driven by high nutrient availability are discussed

proterozoic modular biomineralized metazoan from the nama group namibia we describe a proterozoic fully biomineralized metazoan from the omkyk member 549 million years before the present of the northern nama group namibia namapoikia rietoogensis gen et sp nov is up to 1 meter in diameter and bears a complex and robust biomineralized skeleton it probably represents a cnidarian or poriferan namapoikia encrusts perpendicular to the walls of vertical synsedimentary fissures in microbial reefs this finding implies that large modular metazoans with biologically controlled mineralization appeared some 15 million years earlier than previously documented

the puzzle assembled ediacaran guide fossil cloudina reveals an old proto gondwana seaway abstractduring the ediacaran the clymene ocean separated the lau rentia amazonia and rio apa cratons from several landmasses to the west forming the proto gondwana supercontinent however no clear evidence about the existence of ediacaran epeiric seas over those landmasses has been found here we report and discuss the discovery of the ediacaran guide fossil cloudina sp associated with other metazoan body and trace fossils in the bambui group central eastern brazil the ediacaran age of the bambui group and the paleogeographic position of cloudina bearing successions in brazil antarctica namibia and argentina suggest a scenario of ocean con nectivity among coeval intracratonic basins of south america africa and antarctica at the end of neoproterozoic time the new nding epitomizes one of the most important paleontological discoveries ever made in south america helping to solve an old paleogeographic puz zle of the gondwana supercontinent introduction the bambui group crops out in central eastern brazil and is one of the most studied neoproterozoic sedimentary successions in south america it comprises an extensive at lying cratonic cover in erosional contact with paleoproterozoic and archean basement rocks of the sao francisco craton in its eastern and westernmost occurrences the bam bui group is deformed and metamorphosed by the brasilia and aracuai mobile belts respectively fig 1a the regional tectonostratigraphy has been interpreted as re ecting deposition in a foreland basin related to gondwanan collisions involving the sao francisco craton pimentel et al 2011 the bambui group bg overlies rocks of the macaubas group and jequitai formation and the carrancas conglomerate units supposedly de posited during sturtian glaciation the unit is 700 1000 m thick misi et al 2007 and comprises limestones and dolomites of the sete lagoas for mation at the base that grade upward to shales and siltstones of the serra de santa helena formation followed by marls siltstones limestones and sandstones of the lagoa do jacare serra da saudade and tres marias formations fig 1b the precise age of the bg is not well established precluding accurate correlation with other supposedly coeval successions such as the corumba group brazil itapucumi paraguay arroyo del soldado uruguay si erras bayas argentina and nama group namibia the presence of

the solubility of h_ 2 o in melts in the system sio_ 2 al_ 2 o_ 3 na_ 2 o k_ 2 o at 1 to 2 kbars the solubilities of water in six melts in the system k 2 o na 2 o al 2 o 3 sio 2 were determined at 970 1630 bars and 800 c melts were synthesized hydrothermally in pt capsules in cold seal vessels and then quenched isobarically the solubilities were determined by micromanometric measurement of h 2 o evolved during vacuum fusion of vesicle free glass wafers and are reproducible to 2 8 2 s d of the concentration the solubilities at 970 bars for the granitic and phonolitic minimum melts are 2 88 pm 0 10 wt and 5 01 pm 0 14 wt respectively both peralkaline and peraluminous granitic melts have higher h 2 o solubilities than the 1 kbar p h 2 o minimum melt and indicate the existence of minima in solubilities at molar na k al 1 along joins of constant sio 2 two melts at constant sio 2 and na k al ratio but varying in na na k 0 57 0 69 molar have the same solubility 2 88 pm 0 10 2 74 pm 0 10 wt the compositional depend

a tale of three taphonomic modes the ediacaran fossil flabellophyton preserved in limestone black shale and sandstone abstract ediacaran macrofossils are typically preserved in three taphonomic modes casts molds in siliciclastic rocks casts molds in carbonate rocks and carbonaceous compressions in black shales only a few taxa are known to be preserved in more than one of these taphonomic modes flabellophyton is a genus that has been previously reported from lower ediacaran black shale of the lantian formation 635 551 ma in south china and upper ediacaran sandstone of the ediacara member 560 550 ma in south australia here we report flabellophyton from upper ediacaran limestone of the shibantan member of the dengying formation 551 539 ma in south china making flabellophyton the only genus that occurs in all three taphonomic modes we also provide a systematic description of flabellophyton based on material from the lantian and dengying formations in south china recognizing three morphospecies f lantianense f typicum sp nov and f obesum sp nov flabellophyton is reconstructed as an erect epibenthic marine organism attached to sandy carbonate and muddy substrates its phylogenetic affinity remains ambiguous though it was historically interpreted as an algal fossil the wide environmental and stratigraphic distribution of flabellophyton allow comparative taphonomic and paleoecological analysis taphonomic analysis of flabellophyton indicates that multiple taphonomic pathways can facilitate the preservation of ediacaran macrofossils as a window into ediacaran paleoecology flabellophyton and other ediacaran fossils played a crucial role in the construction of epibenthic communities in ediacaran oceans and helps to understand the ecological migration and evolutionary expansion from deeper to shallower oceans during the ediacaran period

late ediacaran skeletal body fossil assemblage from the navalpino anticline central spain mineralized fossils have been described from late ediacaran rocks all over the world and have been interpreted as the first metazoans capable of secreting a mineralized skeleton among these early biomineralizers the tubular cloudina has a cosmopolitan distribution it often occurs in association with other mineralizing organisms such as sinotubulites and namacalathus which show a more restricted and seemingly non overlapping distribution many aspects of the biology of these organisms remain poorly known in particular the structure and composition of the ecosystems they formed part of here we report a diverse fossil assemblage from the ibor group of villarta de los montes of the central iberian zone spain the assemblage includes the first material of sinotubulites baimatuoensis from spain which is found in association with cloudina hartmanae and cloudina carinata also found are small tubular fossils and a flask shaped form comparable to protolagena among previously described assemblages of late ediacaran skeletal fossils the villarta assemblage unique in europe most closely compares to that from the dengying formation of south china

extremely fluid behavior of hydrous peralkaline rhyolites the viscosities of a series of water bearing peralkaline rhyolitic melts have been experimentally determined the dry melt compositions are composed of a series of additions of na2o to a metaluminous base composition the melts initially hydrated at high pressures and quenched isobarically have been prepared by cutting and polishing then reheating across the glass transition at 1 atm where they are annealed to a relaxed metastable state and then investigated dilatometrically using micropenetration methods the measurements have been performed in the viscosity range of 108 5 1011 5 pa s which corresponds to temperatures in the range of 675 220 c for these compositions despite the relatively low viscosities of dry peralkaline melts in comparison with metaluminous melts of similar sio2 content the viscosities of peralkaline rhyolitic melts also decrease strongly and non linearly with the addition of water the resulting viscosity temperature relationships for water bearing peralkaline rhyolitic melts are shifted to much lower temperatures such that glass transition temperatures for moderate cooling rates correspond to extraordinarily low temperatures a model is presented for the calculation of melt viscosities in the range of 108 5 1011 5 pa s for peralkaline rhyolites with up to 7 wt h2o the very fluid nature of these peralkaline rhyolites over a wide range of water contents may facilitate a very efficient degassing history of glassy peralkaline rhyolites in nature efficient degassing might explain the apparent contradiction of the presence of common water rich melt inclusions in phenocryst phases hosted in water free glassy rhyolites versus the absence of vesicular layers or textural evidence for a vesicular past for the glassy rocks