resolution of the earliest metazoan record differential taphonomy of ediacaran and paleozoic fossil molds and casts abstract much of our knowledge of early metazoan evolution is derived from unmineralized death mask or endorelief mold and cast fossils in ediacaran clastic sedimentary rocks this record is often regarded as a unique ediacaran taphonomic window however the prevalence of soft bodied molds and casts in paleozoic clastic rocks has been increasing begging an extension or modification to our understanding of this preservational motif chief amongst such fossils are eldonids a non biomineralized group of stem deuterostomes because eldonids are also preserved as compressed or flattened fossils from deposits like the burgess shale chengjiang and kaili they offer a comparative case study for evaluating the taphonomic fidelity of mold cast style preservation during this interval eds and raman microspectroscopic analysis of ordovician and devonian eldonid molds and casts and comparison with burgess shale eldonids suggests the mold cast taphonomic style produces significantly lower fidelity of fossil preservation we propose that eldonid mold cast fossils are preserved by the adsorption of reduced iron ions onto tissues composed primarily of high molecular weight hmw biopolymers which require enzymatic degradation prior to decay nucleation and growth of aluminosilicates and or sulfides around these adsorbed ions forms a fossilizable surface veneer preserving a death mask mold more labile tissues could not be fossilized in this mold and cast style ediacaran mold and cast fossils from south australia the white sea region of russia namibia and newfoundland exhibit preservational characteristics consistent with this new proposed model analysis of their preservational mode suggests that the first metazoans which would have lacked hmw biopolymeric tissues could not have been fossilized in this particular style thus understanding the origin and earliest evolution of the metazoa requires a focus on alternative modes of fossilization

generation and evolution of siliceous high magnesium basaltic magmas in the formation of the permian huangshandong intrusion xinjiang nw china abstract numerous permian orthopyroxene rich mafic ultramafic intrusions in the southern margin of the central asian orogenic belt caob have been emplaced in a post orogenic environment the huangshandong mafic ultramafic intrusion is the largest in the eastern tianshan orogenic belt of the caob and consists of a layered unit intruded by a massive unit the layered unit consists of lherzolite troctolite olivine gabbro hornblende gabbro gabbrodiorite and diorite whereas the massive unit is made of gabbronorite and olivine gabbronorite olivine from the layered unit has fo values of 75 to 84 and contains low nio and mno rocks from this unit have initial 87sr 86sr ratios 0 7031 0 7043 and end values 3 4 7 1 while plagioclase has initial 87sr 86sr ratios ranging from 0 70010 to 0 71569 olivine from the massive unit has relatively low fo values of 59 to 70 and high nio and mno rocks from this unit have relatively low and uniform initial 87sr 86sr ratios 0 7030 and high end values 6 0 7 5 and contain plagioclase with relatively low initial 87sr 86sr ratios 0 70042 to 0 70520 olivines from both units have low cao and cr2o3 both units formed from siliceous high magnesium basaltic magmas were derived from a hydrous depleted mantle source magma forming the layered unit experienced earlier removal of sulfides before emplacement with the earlier sulfide saturation caused by the addition of crustal material the magma that formed the massive unit has undergone fractional crystallization of olivine cr spinel siliceous high magnesium basaltic magmatism produced by melting of a hydrous depleted mantle source may be an important feature of orogenic belts

ordovician trilobites from the tafilalt lagerstatte new data and reappraisal of the bou nemrou assemblage

high palaeolatitude hodh mauritania recovery of graptolite faunas after the hirnantian end ordovician extinction event marine shales directly overlying lower hirnantian uppermost ordovician glacially related sediments in mauritania northwest africa have produced a rich graptolite fauna spanning the ordovician silurian boundary in an area of high palaeolatitude the lowermost transgressive sandy shales are barren of graptolites but overlying shales show a sudden appearance of a diverse fauna indicative of the terminal ordovician persculptus zone suggesting that with improving conditions colonisation by a relatively cold tolerant fauna was possible this fauna is replaced by a low diversity assemblage dominated by long ranging taxa probably representing the basal silurian acuminatus and atavus zones with the extinction of the persculptus zone fauna conditions were still hostile to warm water silurian graptolites and a normalograptus fauna was again established a sudden influx of fairly diverse taxa marks the base of the acinaces zone and the establishment of a typical lower silurian fauna with the establishment of warmer water conditions

helium 3 anomalies and crust mantle interaction in italy abstract the distribution of primordial 3he in the crust of italy has been investigated through a survey of groundwaters hot springs exploration and production geothermal wells the mantle derived component varies from 0 to 60 and reaches a maximum at vulcano in the eolian islands 3he anomalies occur where the conductive heat flow is relatively high but the relationships between heat flow and anomaly are uncertain the distribution of 3he in the main reservoir and cap rock to the larderello geothermal system shows that the mantle derived 3he varies from 5 to 40 of total he and reaches a maximum in fluids extracted from a depth of ca 3 km within the basement bomb produced tritiogenic 3he appears to make a near negligible contribution well 107 at larderello has been monitored over a period of 17 months and the 3 he 4 he ratio r varies from 1 4 to 1 6 times the atmospheric ratio ra whilst he ne varies from 9 to 160 a model is proposed for the larderello geothermal system whereby mantle derived heat and volatiles are advected to shallow 6 8 km depths in association with melts and released into the main geothermal reservoir at larderello in a non uniform manner possibly associated with fracturing isotopic and elemental equilibrium of gaseous species c o h appears to have taken place in the basement to the main reservoir from where fluid with r ra 3 2 has been extracted

the fezouata fossils of morocco an extraordinary record of marine life in the early ordovician the discovery of the fezouata biota in the latest tremadocian of southeastern morocco has significantly changed our understanding of the early phanerozoic radiation the shelly fossil record shows a well recognized pattern of macroevolutionary stasis between the cambrian explosion and the great ordovician biodiversification event but the rich soft bodied fezouata biota paints a different evolutionary picture the fezouata assemblage includes a considerable component of cambrian holdovers alongside a surprising number of crown group taxa previously unknown to have evolved by the early ordovician study of the fezouata biota is in its early stages and future discoveries will continue to enrich our view of the dynamics of the early phanerozoic radiation and of the nature of the fossil record supplementary material a complete faunal list is available at http www geolsoc org uk sup18843

the role of komatiitic and picritic magmatism and s saturation in the formation of ore deposits abstract the introduction of high temperature high magnesium komatiitic and picritic magmas into the earth s upper crust has given rise either directly or indirectly to many of the world s major ore deposits there is a direct association between komatiitic magmatism and kambalda style ni cu pge deposits the link between magma type and mineralization is not as clear cut in the case of the noril sk cu ni pge deposits but nevertheless there is an association between picritic magmatism and ore deposit development komatiitic and or picritic magmatism may also have contributed to the formation of many types of hydrothermal ore deposits recent nd isotope data generated by kent et al 1994 provide strong evidence for the hypothesis proposed by keays in 1983 that the au in archaean au deposits was derived either directly or indirectly from komatiites through the action of metamorphic fluids the importance of komatiitic and picritic magmas in ore forming processes arises from the fact that they are among the few mafic ultramafic magma types that are s undersaturated at the time of magma formation s saturation of magmas leads to marked depletion in the chalcophile metals for example czamanske and moore 1977 estimated that a third of the original cu and ni contents of morb were lost from the magmas due to extraction into magmatic sulphides the depletion in au and the pge is much greater due to their much larger partition coefficients keays and scott 1976 komatiitic and picritic magmas are s undersaturated because they are high temperature magmas produced by large degrees of partial melting of upper mantle sourc regions that were already depleted in s through earlier partial melting events most mafic magmas produced by less than 25 partial melting are s saturated and hence depleted in the ore forming chalcophile elements in contrast komatiitic and picritic magmas have their full complement of chalcophile metals when they do become s saturated they form sulphides that are strongly enriched in ni cu au pge and other strongly chalcophile metals these may accumulate to form ore deposits directly e g magmatic ni cu pge sulphides or be dispersed in komatiitic picritic rocks or their fractionation products making these source rocks for hydrothermal au or vms deposits

isotopes of the light noble gases in mineral waters in the eastern part of the balkan peninsula bulgaria the concentration and isotopic composition for the light noble gases in spontaneous gas from mineral waters in the eastern part of balkan peninsula are presented in this paper the isotopic ratio 3he4he in dissolved gases varies within the range 0 15 1 1 10 6 and the concentration of helium in these gases is between 0 002 and 3 14 vol it is determined that 86 7 98 8 0 4 9 2 and 1 of 4he is derived from the crust mantle and atmosphere respectively the main source of 3he is the earth s mantle the isotopic composition of neon is nearly the same as that in the atmosphere several samples show a slight excess of light isotopes probably due to the mass fractionation of atmospheric neon the isotopic composition of argon is also quite near to that in the atmosphere in three cases argon contains 18 1 12 3 and 7 9 of the radiogenic component the arn2 a ratios as a rule vary between 0 012 air and 0 027 dissolved air so that n2 in most of the samples is also assumed to be of atmospheric origin the ratio c3he in many sites is as low as 107 but this fact cannot be used to reveal unambiguously the origin of c containing gases

continental mafic dyke swarms as tectonic indicators an example from the vestfold hills east antarctica abstract the two main types of magma that were emplaced as mafic dyke swarms in the vestfold hills are high mg tholeiite hmt and fe rich tholeiite frt the former magma type is less dense than the vestfold hills felsic crust whereas the latter is more dense at deep to intermediate crustal levels therefore frt dyke emplacement requires a higher deviatoric extensional stress as is reflected in a more constant orientation palaeoproterozoic hmt frt bimodal dyke swarms were emplaced around 2 4 ga and 2240 ma the geometry and orientation of 2240 ma hmt dykes imply a low deviatoric extensional stress field inconsistent with a mantle plume origin for magmatism current petrogenetic models for hmt magmatism suggest generation at active convergent plate boundaries although this is broadly consistent with emplacement of the bimodal dyke swarms subsequent to a crustal thickening event at 2 5 ga the exact mechanisms for melting and the generation of crustal extensional stress are unknown a possible scenario involves the sinking of gravitationally unstable oceanic crust a major mesoproterozoic magmatic extensional event in the vestfold hills started with normal faulting followed by lamprophyric dyke emplacement and by two distinct episodes of frt dyke emplacement at 1380 and 1245 ma a consistent direction of the stress field during this event suggests control of a linear belt of crustal uplift by comparison with other mafic dyke swarms this is interpreted to be due to a sheet of upwelling asthenosphere elongated plume around 1 4 ga rather than a low angle detachment this upwelling led to thermal erosion of the base of the lithosphere that channelled a subsequent magma pulse at 1245 ma erosion during a long period of uplift prior to 1380 ma was followed by sedimentation during thermal subsidence as suggested by segmentation of 1245 ma dykes the lithospheric structure inherited from the mesoproterozoic magmatic extensional event was probably reactivated during two high grade tectono metamorphic events around 1 0 and 0 5 ga the vestfold hills formed part of a relatively stable craton peripheral to the mobile belt that was formed during these events

the geochemistry and petrogenesis of the agnew intrusion canada a product of s undersaturated high al and low ti tholeiitic magmas

archaean basic volcanism in the eastern goldfields province yilgarn block western australia abstract petrographic petrological and geochemical studies have demonstrated the presence of three distinctive basic volcanic suites in the eastern goldfields province yilgarn block western australia these are termed the high magnesian series basalts hmsb the low magnesian series basalts lmsb and the siliceous high magnesian series basalts shmsb the hmsb and shmsb constitute differentiation series which contain both high mgo 9 5 14 wt and low mgo the hmsb and lmsb were derived from source mantle regions which were variably depleted in the incompatible elements archaean komatiites were derived from similarly depleted source regions and it is argued that the main petrogenetic difference between these three volcanic suites was the degree of partial melting from which they were derived the depleted nature of the source regions may have been induced by earlier small degree source mantle regions of the shmsb were variably enriched in the incompatible elements and water and parental magmas of the shmsb were derived from moderately hydrous conditions of partial melting the relative proportions of each basalt suite varies considerably between the layered successions examined for example the basic volcanics overlying the komatiites at kambalda are shmsb while the footwall volcanics consist predominantly of hmsb with subordinate lmsb however the norseman succession where no ultramafic volcanics are known to occur is comprised mainly of lmsb with a smaller proportion of hmsb

rare gas isotopic compositions in natural gases of japan isotopic and elemental compositions of rare gases in various types of gas samples collected in the japanese islands were investigated excess3he was found in most samples many samples showed a regionally uniform high3he 4he ratio of about 7 times the atmospheric ratio the he concentrations varied from 0 6 to 1800 ppm and they were low in co2 rich gases and high in n2 rich gases ne isotopic deviations from the atmospheric ne were detected in most volcanic gases the deviations and the elemental abundance patterns in volcanic gases can be explained by a mixing between two components one is mass fractionated rare gases and the other is isotopically atmospheric and is enriched in heavy rare gas elements ar was a mixture of mass fractionated ar atmospheric ar and radiogenic ar and the contribution of radiogenic40ar was small in all samples except for he elemental abundance patterns were progressively enriched in the heavier rare gases relative to the atmosphere several samples were highly enriched in kr and xe relative to the abundance pattern of dissolution equilibrium of atmospheric rare gases in water the component which is highly enriched in heavy rare gases may be released from sedimentary materials in the crust

helium isotopic evidence for recent subcrustal volcanism in eastern australia a component with approx 6 mantle helium sup 3 he sup 4 he 5 9 x 10 sup 7 has been identified in borewaters from the great artesian basin australia that is isotopically distinct from previously identified in situ production sup 3 he sup 4 he 1 6 x 10 sup 8 and a crustal degassing flux sup 3 he sup 4 he 6 75 x 10 sup 8 the occurrence of this component is limited to three identified localities approx 20 5 sup 0 s 144 sup 0 e approx 25 5 sup 0 s 146 sup 0 e approx 27 sup 0 s 144 5 sup 0 e all within the region of high reduced heat flow in eastern australia no evidence for a mantle component was apparent in central australia this component is most probably the result of continued intrusive volcanism 1 m y associated with the cainozoic lava flow fields in the region although the volcanic activity has no present day surface manifestation its mantle helium was probably carried to the near surface by large scale fluid transport processes in the crust copyright american geophysical union 1987

sulfur saturation and second stage melts application to the bushveld platinum metal deposits a model is presented for the formation of platinum group element ores by precipitation from sulfur deficient platinum group element rich second stage magmas numerous investigations indicate that basaltic magmas originating from undepleted or mildly depleted source regions are sulfur saturated at the time of eruption mantle derived peridotites residual from midocean ridge basalt magma genesis have pd abundances that are uniformly too high for it to have originated solely from silicate melt retained in the source region at the time of segregation they imply the presence of an accessory immiscible sulfide melt to host the platinum group elements the very high sulfide melt silicate melt partition coefficients of the platinum group elements would result in strong fractionation of these metals into the accessory sulfide component during the melting event the extreme platinum group element enrichment of this accessory sulfide component is evidenced by the discovery ofplatinoid minerals in residual mantle nodules containing pd and pt abundances at the low ppb level keays et al 1981 magmas generated by low to moderate degrees of partial melting of undepleted mantle are therefore sulfur saturated at the time of segregation these first stage magmas become impoverished in platinum group elements during the early stages of fractional crystallization because of coprecipitation of an immiscible sulfide phase e g midocean ridge basalt glasses average

platinum group elements pge in basalts of the seaward dipping reflector sequence se greenland coast the rift related seaward dipping reflector sequence sdrs se of introduction greenland consists of basaltic lavas that exhibit variable degrees of seaward dipping reflector sequences sdrss are charmagmatic differentiation derived from a heterogeneous mantle source acteristic features of volcanic rifted margins the sdrs platinum group elements pge are used to provide insights into se of greenland formed during the initial break up of the petrogenetic evolution of the sdrs and to characterize the the north atlantic in the late paleocene early eocene magma sources noble metal concentrations correlate well with duncan et al 1996 and spans the whole period from indicators for magmatic differentiation mg number mgo excontinental break up to seafloor spreading brooks hibiting two distinct trends concentrations of ir ru and rh tend nielsen 1982 saunders et al 1997 large volumes of to decrease with progressive differentiation indicating compatible lava erupted in a short period of time under subaerial behaviour of these elements during fractional crystallization processes or shallow water conditions along the continental margins the variation of pt and pd shows segmented trends in primitive of norway and greenland the high magma production magmas pt and pd are incompatible and become enriched in the rates which led to the formation of magmatic sequences melt the primitive magma is s undersaturated despite derivation up to 6 km thick are inferred to require the presence of from a depleted mid ocean ridge basalt source at a moderate degree a mantle plume white mckenzie 1989 of melting reflecting enhanced s solubility in the melt caused by the aims of ocean drilling program odp legs 152 high fe content and elevated temperature in the more evolved lavas and 163 were to investigate the origin and nature of the pt and pd decrease with decreasing mgo and mg number this sdrs with regard to the influence of a mantle plume indicates that s saturation had occurred with pt and pd being larsen et al 1994a duncan et al 1996 nine sites were incorporated in sulphides which probably segregated during ascent drilled in two transects distal to the ancient plume track bulk partition coefficients for the pge during partial melting are at 63 n and 66 n fig 1 calculated based on data from a primitive basaltic unit with mgo it has been shown that platinum group element pge 20 wt representing a near primary magma composition the systematics can provide important information about determined bulk partition coefficients for an s undersaturated melt the petrogenesis of basaltic magmas such as magmatic are about 2 ir 4 ru 1 2 rh 0 5 pt and 0 4 pd this differentiation sulphide segregation and mantle source indicates that ir ru and rh are compatible during partial melting characteristics e g brugmann et al 1987 greenough whereas pt and pd are incompatible owen 1992 barnes picard 1993 seitz keays

geology geochemistry and origin of platinum group element chromitite occurrences in the heazlewood river complex tasmania three distinctive chromitite occurrences are recognized from the cambrian mafic ultramafic heazlewood river complex tasmania type i chromitite represents very thin schlieren comprising cr rich spinel and is hosted by primitive olivine rich cumulates type ii chromitite occurs as thicker schlieren containing more aluminous spinel and is hosted by an interlayered sequence of cumulate dunite and peridotite type iii chromitite is more massive occurring as irregular stringers pods and layers containing chromite having intermediate cr al ratios it is spatially associated with gabbroic dikes that intrude dunite harzburgite orthopyroxenite cyclic units and is interlayered with or enclosed by an unusual xenolith bearing gabbronorite both type i and ii chromitites are interpreted to have precipitated directly from a primitive magma and may reflect periods of magma replenishment type iii chromitite was formed in response to mixing between ultramafic magma and volatile rich intercumulus liquids producing hybrid magmas having chromite as the only liquidus phase all of the chromitites display enrichment in platinum group elements relative to the cumulate ultramafic sequences the maximum total platinum group element tenor in the chromitites is 6 ppm with average grades between 0 2 and 1 ppm the chromitites are richest in ruthenium platinum and rhodium reflecting the presence of discrete platinum group minerals laurite rus 2 is the most abundant platinum group mineral occurring as isolated inclusions in chromite a wide variety of pt and rh minerals are contained in the chromitites these include alloys arsenides and sulfarsenides the majority of which occur as altered anhedral grains in the silicate matrix the platinum group mineral assemblages recognized in this investigation are similar to bimodal platinum group mineral suites documented in chromitites from ophiolites legendre and auge 1986 and stratiform intrusions von gruenewaldt et al 1986 these include early crystallizing os ir and ru ir group elements phases and later formed rh pt and pd minerals pd group elements barnes et al 1985 the ir group elements display a preference for octahedrally coordinated sites in magmas similar to cr murck and campbell 1986 and like cr have a low solubility in primitive magmas the unusually low ir tenor of type i chromitites from the heazlewood river complex may reflect the prior removal of os ir alloys from the parental liquids which would account for the known occurrence of osmiridium in the complex localized magma mixing associated with the development of the type iii chromitites led to increasing a sub s 2 and a sub a s but not necessarily to s saturation in the resultant hybrid liquid under these conditions laurite was precipitated as an early formed cumulus phase and the more soluble pd group elements crystallized as intercumulus sulfarsenides and arsenides

upper ordovician planktic and benthic graptolites and a possible hydroid from the tafilalt biota southeastern morocco

gas geochemistry and seismotectonics a review abstract publications on soil and spring gases have been examined regarding their relationships with both tectonic and seismic activities the main sources behaviours and uses of species detected in soils and springs are displayed and their mode of sampling and analysing briefly described the main patterns of degassing in soils are described and we outline the wide range of geochemical signatures as the result of both permeability and mineralogical contrasts because thermomineral waters have been in contact with great volumes of crustal rocks at various depths spring gases might be more representative of the local environment than soil gases moreover gas signature comparisons show that spring gases are much more enriched with deep gases and slightly contaminated by atmospheric gases therefore they can be considered as better samples for identifying precursors of earthquakes environmental perturbations are examined and it is shown from divergent cases that pressure temperature soil moisture or earth tides may generate very high perturbations of the degassing process such effects demonstrate that no systematic correction law can be proposed and that removing external contributions from gas concentrations must be performed case by case this demonstrates therefore the need for the simultaneous measurement of external parameters during gas monitoring a qualitative examination of about 150 claimed precursors proposed in the literature has been reviewed as noted by previous authors anomalies appear at distances sometimes much greater than typical source dimensions and occur in the field of strain higher than 10 9 most of them being in the field of strain higher than 10 8 taking into account the very high heterogeneity of such a set of data we can suggest that amplitudes of gas anomalies are independent of both magnitudes and epicentral distances of related earthquakes suggesting local conditions to control amplitudes on the contrary precursory time and duration of anomalies seem to increase both with magnitudes and epicentral distances abundant evidence demonstrates the major role of crustal fluids in the earthquake cycle many works have outlined the fact that crustal instabilities can appear as the result of low stress strain perturbations during loading it has been suggested that motion of fluids may occur at various scales from microcrack fluid transfer up to changes of hydraulic levels of water tables the study of subsequent anomalies is expected to supply a tool for earthquake prediction following previous authors we outline the need for further methodological improvements including the setting up of multiparameter station networks and the simultaneous recording of the main external parameters atmospheric pressure water and air temperature soil moisture for signal processing

book review helium isotopes in nature developments in geochemistry 3 by b a mamyrin and i n tolstikhin elsevier amsterdam 1984 xiv 273 pp price u s 53 75 u s a and canada dfl 140 00 rest of world hardback

u pb zircon dating of mafic dykes and its application to the proterozoic geological history of the vestfold hills east antarctica the vestfold hills one of several archaean cratonic blocks within the east antarctic shield comprises a high grade metamorphic basement complex intruded by at least nine generations of early to middle proterozoic mafic dykes extensive u pb ion microprobe shrimp analyses of zircons derived predominantly from late stage felsic differentiates of the mafic dykes provide precise crystallisation ages for several dyke generations these new ages enable constraints to be placed on both the history of mafic magmatism in the vestfold hills and the timing of the various interspersed proterozoic deformation events in addition to demonstrating the utility of zircons derived from felsic late stage differentiates for the dating of co genetic mafic dykes this study also places doubt on previous wholerock rb sr dating of mafic dyke suites in this and other areas of east antarctica the 207pb 206pb zircon ages of 2241 4 ma and 2238 7 ma for the homogeneous and mottled norites respectively provide a younger emplacement age for associated group 2 high mg tholeiite dykes than the whole rock rb sr date 2424 72 ma originally interpreted as the age of all high mg intrusives in the vestfold hills zircon ages of 1754 16 ma and 1832 72 ma confirm the previously defined rb sr age of the group 2 fe rich tholeiites two later dyke generations the group 3 and 4 fe rich tholeiites are distinguished on the basis of field orientations and cross cutting relationships and yield zircon emplacement ages of 1380 7 ma and 1241 5 ma which also define minimum ages for two suites of lamprophyre dykes xenocrystic zircons within both felsic segregations and mafic dykes yield zircon ages of 2478 5 ma to 2740 ma indicating the presence of archaean crustal source rocks of this antiquity beneath the vestfold hills

biostratigraphy and palaeoecology of lower ordovician graptolites from the fezouata shale moroccan anti atlas abstract the fezouata shale fezouata formation has produced the most complete lower ordovician graptolite succession known from the african continent which was studied from 32 sections located along the western central and eastern moroccan anti atlas a composite biostratigraphic and chronostratigraphic scheme is presented for the entire formation that in ascending order comprises the anisograptus matanensis rhabdinopora flabelliformis anglica zones uppermost lower tremadocian adelograptus tenellus and aorograptus victoriae zones middle tremadocian araneograptus murrayi and hunnegraptus copiosus zones upper tremadocian cymatograptus protobalticus zone lower floian baltograptus jacksoni zone middle floian and the baltograptus minutus zone plus an azygograptus interval upper floian most of these zones or biozones are recorded for the first time in africa together with important graptolite species such as ancoragraptus bulmani spjeldnaes cym protobalticus monsen b jacksoni rushton clonograptus multiplex nicholson b minutus tornquist and azygograptus eivionicus elles a correlation with the graptolite sucession from the algerian sahara is also suggested reinforced by the common record of the minute anisograptid choristograptus louhai legrand providing its second world occurrence in the tremadocian of morocco the prior late tremadocian dating of the fezouata lagerstatte is here extended to the middle floian as revealed by the graptolite biochronology the repeated record along the lower and middle parts of the fezouata formation of deep water mesopelagic species of the genera araneograptus with mass occurrences of conical colonies together with large horizontal rhabdosomes of paratemnograptus paradelograptus clonograptus tetragraptus and holograptus schizograptus do not fit the sedimentological interpretation of deposition of these strata in much shallower inshore to mid shelf environments this inconsistency has implications when inferring the living conditions of the fezouata soft bodied assemblages

emplacement and cooling of komatiite lavas we propose that komatiite lavas were emplaced as turbulent flows accompanied by vigorous forced convection with cooling rates often in excess of hundreds of c h 1 they melted and assimilated up to 10 of the ground over which they flowed forming deep channels nickel sulphide mineralization may have resulted from incorporation of sulphur rich sediment after emplacement high cooling rates persisted resulting in spinifex textures due to compositional convection

the lower ordovician fezouata konservat lagerstatte from morocco age environment and evolutionary perspectives abstract the lower ordovician fezouata konservat lagerstatte from southern morocco has been one of the major palaeontological discoveries of the last decade it provides a unique insight into one of the most critical periods in the evolution of marine life the cambrian ordovician transition however its potential for deciphering key trends in animal diversification was hitherto largely limited by major uncertainties concerning its stratigraphic position age and environmental setting based on extensive fieldwork fossil evidence and facies recognition our study provides clarification on these three crucial issues exceptional preservation is limited to two intervals within the fezouata shale graptolites indicate a late tremadocian age for the fezouata konservat lagerstatte as a whole which is supported by biostratigraphical evidence provided by acritarchs sedimentological features and reconstructed patterns of relative sea level changes indicate relatively shallow water environmental conditions under distal storm influence in an offshore to lower shoreface siliciclastic ramp setting the fezouata biota represents a unique and exceptional window into the palaeobiodiversity in open marine conditions thus contrasting with the other ordovician konservat lagerstatten presently known in our analyses of this new set of data we pave the way for accurate temporal faunal and environmental comparisons with other lower palaeozoic konservat lagerstatten and unlock the full potential of the fezouata biota to better understand the processes and scenarios of early animal radiations

the proterozoic deformation of the vestfold hills block east antarctica implications for the tectonic development of adjacent granulite belts abstract the archaean vestfold block occurs at the margin of the 1000 ma mobile belt of east antarctica and shows a prolonged and very complicated proterozoic deformational history in which at least seven extensional and compressional deformational episodes alternated with emplacement of at least fourteen different dyke swarms these swarms can be grouped into five major suites each bracketed by predominantly compressional deformational events the most voluminous dyke suites 4 and 5 were emplaced around 1380 ma and 1250 ma respectively the former is associated with a progressive sequence of extensional structures that evolved from pseudotachylites to ductile normal shears as heat flow increased in response to extensional processes and dyke emplacement similar increased heat flows probably accompanied the emplacement of suite 5 the subsequent 1000 ma compressional events and associated recrystallisation of earlier structures and dykes at amphibolite facies grade show a marked intensity increase towards the sw vestfold hills where a second pegmatite suite was associated with rehydration of the granulites in the vestfold hills for a long period during the mesoproterozoic the compressional and extensional stages alternated along an e w axis in the rauer group this direction parallels the strain gradient associated with 1000 ma events and is approximately normal to the outcrop trace of the envelope to the complexly folded archaean proterozoic boundary it suggests that mesoproterozoic extensional and compressional events including the formation of the 1000 ma mobile belt occurred along similar structural trends in the ne half of the rauer group a large number of mafic dykes truncating older gneiss foliations can be recognized going w across the rauer group the number of dyke types decreases and the older dykes are increasingly reworked by the 1000 ma events most of these dykes can be correlated with the multiple dyke suites in the vestfold hills and therefore some of the granulite events that have been described as part of the 1000 ma events but which pre date the emplacement of most dykes probably correlate with the extensional events that formed between 1250 and 1400 ma in the vestfold hills this has major implications for the interpretation of the described decompressional p t path in the rauer group which connects an upper p t section that corresponds to the pre to syn dyke granulite events with a lower p t section representing post dyke granulite events dated at 1000 ma comparison with the vestfold block suggests that the path may be the aggregate result of several unrelated events within an extensional compressional tectonic cycle the vestfold hills block serves as an example that much information about high grade mobile belts may be obtained from adjacent stable terrains and that extreme care is required in interpreting the simple decompressional p t t deformation paths which are generally described in the mobile belt of east antarctica

ductile reactivation of proterozoic brittle fault rocks an example from the vestfold hills east antarctica abstract brittle fault rocks usually in the form of minor pseudotachylyte veins have been reported from most high grade metamorphic archaean and proterozoic terrains in analyses of local crustal evolution such brittle structures are often ignored brittle faults commonly reflect the very last phases of local deformation during final uplift of the basement to shallow crustal levels and as such they are thought to be of minor or no importance in large scale archaean and proterozoic tectonics new data from archaean and proterozoic high grade terrains in australia and antarctica indicate that minor fault veins may in some cases represent significant early low grade brittle events which have been overprinted and largely erased by subsequent phases of ductile deformation this paper describes such a brittle event in the vestfold hills east antarctica which may represent an important phase of mid proterozoic uplift and crustal extension

geochemistry of the j m howland reef of the stillwater complex minneapolis adit area i sulfide chemistry and sulfide olivine equilibrium platinum mineralization in the stillwater complex occurs within a complex sequence of olivine bearing plagioclase rich cumulates about 400 m above the base of the banded zone the reef consists of a 2 m thick layer of disseminated sulfides extending over the entire strike length of the complex in the area of this study the host rock to the reef sulfides shows a lateral transition over a strike length of 700 m from a heterogeneous olivine plagioclase cumulate mixed rock to a plagioclase olivine cumulate containing a high proportion of postcumulus bronzite with olivine preserved only as rare resorbed cores on bronzite grains platinum group elements within the reef correlate strongly with abundances of sulfides which show convincing textural evidence for having originated as immiscible sulfide liquid strong correlations are also observed between all six platinum group elements ni and cu despite the gradation in host rock mineralogy the sulfide fraction of the reef displays a constant bulk composition over 700 m of strike length containing 9 3 percent ni 6 9 percent cu and approximately 0 7 percent total platinum group elements principally pd these platinum group element concentrations are two to three orders of magnitude higher than those found in typical magmatic sulfide deposits and are attributed to equilibration of sulfide liquid with a very large volume of silicate melt olivines from the reef show a range of compositions from fo 71 to fo 85 and from 2 400 to 4 500 ppm ni a positive correlation is observed between nio and feo in olivines from sulfide bearing samples indicating ni fe exchange between olivine and sulfides on a local scale such that ni contents of olivines were buffered by that of the sulfide phase a value of 9 70 or 1 49 is obtained for the ni fe distribution coefficient between sulfide and olivine in agreement with values obtained at magmatic temperatures by boctor 1982 these observations may be interpreted in the framework of the plume model of campbell et al 1983 a pulse of replenishing magma enters the stillwater chamber as a buoyant plume and subsequently spreads to form a layer of hybrid melt sulfide liquation and olivine crystallization occur within the plume and the hybrid layer in response to mixing and subsequent cooling sulfide liquid attains high platinum group element concentrations during turbulent ascent and spreading of the plume crystallization of olivine from the hybrid layer gives rise to olivine sulfide boulders which settle to the floor of the chamber reaction between the boulders and the evolved liquid at the bottom of the chamber causes olivine to become more iron rich subsequent reequilibration between olivine and sulfide grains gives rise to the observed ni fe distribution

geochemistry and geochronology of proterozoic tholeiite dykes of east antarctica evidence for mantle metasomatism two episodes of tholeiite dyke emplacement have been identified in archaean high grade metamorphics of the napier complex in enderby land middle proterozoic amundsen dykes are typical continental tholeiites and most of the chemical variation in individual suites can be explained in terms of different degrees of partial melting and low pressure crystal fractionation group i amundsen tholeiites were derived from a relatively homogeneous source region 1 190 200 m y ago whereas that of the group ii amundsen tholeiites was chemically and isotopically heterogeneous group ii dykes have various degrees of enrichment in incompatible elements and commonly show normalised trace element abundance patterns with negative nb anomalies these features imply variable metasomatism of the source region by a volatile rich fluid phase rather than a melt of any observed igneous composition enriched in k rb ba th and possibly la and ce early proterozoic 2 350 48 m y tholeiites were emplaced at considerable depths in the crust during the waning stages of granulite facies metamorphism and include a high mg suite of possible komatiitic affinity ranging in composition from hypersthene rich tholeiite norite to quartz rich tholeiite they tend to have higher ratios of highly to moderately incompatible elements e g k zr k ce and larger nb anomalies i e higher k nb compared with middle proterozoic tholeiites suggesting derivation from more enriched source regions isotopic data are not compatible with significant crustal contamination but constrain source metasomatism to a time immediately before emplacement metasomatism of the source region of the much younger group i tholeiites may have been contemporaneous with that of the high mg suite

principles of mobilization dissolution of metals in mafic and ultramafic rocks the role of immiscible magmatic sulphides in the generation of hydrothermal gold and volcanogenic massive sulphide deposits abstract immiscible magmatic sulphides exert a major control over both the absolute precious and base metal contents of magmas and on the availability of these metals from rocks during ore formation it is concluded that only those mafic magmas that attained s saturation at a late stage are not depleted in the chalcophile elements and are hence suitable source rocks for metals in hydrothermal ore deposits immiscible magmatic sulphides are not only strongly enriched in the ore forming metals but they are also much more reactive than silicates the ore forming metals may be liberated from the sulphides at the sea floor alteration stage and deposited in interflow chemical sediments au deposits or volcanogenic massive sulphide deposits subsequent mobilization remobilization of au from the interflow sediments may concentrate au within the interflow sediments to form economic au deposits the mobilized remobilized au may also be transported to form epigenetic style gold deposits in archaean greenstone belts and in palaeozoic slate belts

ground truthing late ordovician climate models using the paleobiogeography of graptolites 1 the 18o records obtained from conodonts suggest that during the mid ordovician equatorial temperatures stabilized at close to the present day a hypothesis broadly supported by published climate models however the degree to which equatorial temperatures represent global climate state and varied between different climatic modes greenhouse icehouse interglacial glacial and to what extent ordovician 18o and climate models truly reflect the global climate remain to be tested here we present paleobiogeographical data for the planktonic graptolites from the gracilis time slice i e early sandbian stage that just postdates the postulated onset of climate stabilization twinspan analysis and constrained seriation provide robust ecological groupings in paleobiogeographical presence absence data the highest level groups reflect tropical subtropical assemblages and a high southerly latitude assemblage constrained seriation defines latitude distinct biotopes that are considered equivalent to modern zooplanktonic provinces the distribution pattern of graptolite biotopes in the gracilis time slice show 1 that models explaining local graptolite ecological assemblages using lateral differentiation e g upwelling are to be favored above those using depth stratification and 2 a steep faunal gradient from equator to pole which is typical of a cool nongreenhouse world and comparable to the modern situation we therefore broadly support the climate stabilization hypothesis paradoxically the climate of the early late ordovician appears similar to that of the present day despite the higher atmospheric pco2 levels graptolite species were indicative of temperature controlled ocean water masses in much the same way as the modern zooplankton

an evaluation of the solubility of sulfide sulfur in silicate melts from experimental data and natural samples abstract an evaluation of the solubility of sulfur in silicate melts from experimental data obtained under a wide variety of experimental conditions has been undertaken under reducing conditions fo2 below qfm sulfur is present as sulfide ii oxidation state and precipitates as pyrrhotite or an fe s o sulfide phase when the sulfur concentration in the silicate melt is sufficiently high the basic solubility reaction is 1 2 s 2 g o 2 silicate melt 1 2 o 2 g s 2 silicate melt for silicate melts with high feo contents 10 wt feo sulfur dissolves as an fe3so2 or 2feo fes species when saturated with pyrrhotite fes sulfide melt 2feo silicate melt fe 3 so 2 silicate melt log k log x s 21ogx feo 0 8 at 1200 c 1 bar under saturated and unsaturated conditions 1 2 s 2 g 3feo silicate melt fe 3 so 2 silicate melt 1 2 o 2 g log k log x s 1 2 1og o 2 3 log x feo 1 2 log s 2 4 1 at 1200 c 1 bar log k also shows a positive pressure dependence 0 1 per kbar for silicate melts with low feo contents fes sulfide melt o 2 silicate melt feo silicate melt s 2 silicate melt log k log x s log x feo 5 8 9007ndash 1000 c 1 6 kbar for silicate melts with intermediate feo contents 1 10 wt feo the behavior of sulfur is more complex with sulfur probably present as an fes species fes sulfide melt fes silicate melt log k log x s 1 05 3000 t k 850 1200 c 1 bar 2 2 kbar the effects of temperature pressure melt composition and other experimental conditions have been assessed the ability to predict the solubility of sulfur in silicate melts has implications for the formation of ore deposits associated with igneous systems application of the results to natural systems shows that some samples have a concentration of sulfur higher than that which could have been dissolved in the silicate melt indicating that the magma carried excess sulfur and or sulfur has been added after crystallization of the magma

the difficulties of dating mafic dykes an antarctic example archaen gneisses of the vestfold hills of east antarctica are transected by several compositionally discrete suites of tholeiitic dykes a representative of one of those suites which has been dated in the present study shows that not only rb sr whole rock isochrons but also u pb zircon techniques if not properly applied can produce erroneous crystallisation ages two zircon populations were recovered from the mafic dyke itself one of which is 2 483 9 ma in age and clearly of xenocrystic origin the other yields an age of 1 025 56 ma which is not ascribed to the magmatic crystallisation of the dyke but rather to the time that it underwent metamorphic metasomatic alteration as a response to high grade tectonism in the adjacent mobile belt it is presumed that the zircon in question formed by the breakdown of another mineral or minerals possibly magmatic baddeleyite due either to ingress of a siliceous fluid or more probably by the release of silica from the breakdown of pyroxene to amphibole a cogenetic 1 2 cm wide felsic vein of late magmatic early hydrothermal origin also contains two zircon populations again most of the grains therein which are interpreted as of xenocrystic origin grew at 2 483 9 ma however a few euhedral zircons with very high u and th contents grew at 1 248 4 ma which is taken to be the formation age of both the felsic vein and the enclosing mafic dyke

normalograptus kufraensis a new species of graptolite from the western margin of the kufra basin libya normalograptus kufraensis sp nov occurs as monospecific assemblages in the tanezzuft formation at the western margin of the kufra basin jabal eghei southern libya these graptolites have parallel sided rhabdosomes with long straight virgellae climacograptid thecae and a full straight median septum n kufraensis is intermediate between ordovician graptolites from the n angustus perner lineage and the younger sister species n ajjeri legrand and n arrikini legrand n kufraensis differs from these taxa as follows it is broader than n angustus it has greater thecal spacing than n ajjeri or n arrikini a table comparing measurements of n kufraensis with 44 other normalograptus taxa differentiates it from other members of this morphologically conservative group even though n angustus and n ajjeri are very long ranging graptolites a stratophenetic approach suggests that the specimens from jabal eghei may be of late hirnantian or younger age the faunal composition and preservation suggests these graptolites occupied the cratonic invader biotope the stratigraphic succession records deglacial flooding and fluctuating of redox in the tanezzuft formation with the graptolites indicating a short lived interval of anoxia

helium isotopes in north sea gas fields and the rhine rift the popular assumption that all natural hydrocarbon accumulations are biogenic in origin has recently been questioned1 2 with the suggestion that deep source mantle derived hydrocarbons may contribute to accumulated biogenically produced hydrocarbons and also give rise to hydrocarbon deposits in unexpected parts of the crust the possibility that deep source volatiles are present in significant quantities in the hydrocarbon bearing parts of the crust of western europe is assessed here using analyses of 3he abundances in groundwaters and natural gas accumulations within the uk mainland and continental shelf for comparative purposes analyses are also presented for groundwaters from the southern part of the rhine graben a region that has been relatively active tectonically and the site of tertiary and quaternary volcanic activity in contrast the north sea sedimentary basins and central graben have no recorded tertiary or younger volcanic activity helium associated with north sea gas fields and groundwaters on the uk mainland is dominantly radiogenic produced by decay of u and th in the continental crust even where heat flow exceeds 100 mw m 2 a minor component 15 in groundwater samples fault structures seem to be important for the transport of mantle helium through the brittle upper crust the results are entirely consistent with a biogenic origin for north sea hydrocarbons

the platinum group element and gold contents of the marginal rocks and sills of the bushveld complex previous work has defined three suites of sills and marginal rocks adjacent to and below the rustenburg layered suite of the bushveld complex namely an ultramafic mgo approximately 30 a magnesian basaltic mgo approximately 12 and a tholeiitic suite mgo approximately 7 these three suites are thought to represent magmas which have intruded and been incorporated into the rustenburg layered suite at various times during its formation the elements ir ru rh pd pt and au have been analyzed in 17 samples representative of the three suites the ultramafic sills have the highest total noble metal content 46 ppb followed by the magnesian basaltic suite 37 ppb and the tholeiitic suite 21 ppb the ultramafic sills are characterized by a relatively fiat chondrite normalized platinum group element pattern with an average pt ir ratio of 3 52 and a pronounced rh anomaly the magnesian basaltic and tholeiitic suites both have steep positive chondrite normalized patterns with average pt ir ratios of 16 14 and 40 5 respectively and no rh anomaly modeling done using incompatible trace elements and petrographic mixing suggests the ultramafic sills represent crystal mushes of cumulate olivine and chromite that were intruded into the floor during the crystallization of the lower portion of the rustenburg layered suite the enrichment of ir ru and rh in the ultramafic sills appears to be associated with the cumulate enrichment of olivine or chromite in these sills the chondrite normalized patterns found for the merensky reef and the platreef are similar to those of the magnesian basaltic magma the pattern found for the ug 2 is much flatter and in some respects very similar to that found for the ultramafic sills on a worldwide basis the platinum group element contents of the magnesian basaltic and tholeiitic magmas generally fall in the range of basalts and chill zones of other layered suites the magnesian basaltic magma does have one of the highest pt contents measured for a magma

platinum group element segregation and mineralization in a noritic ring complex formed from proterozoic siliceous high magnesium basalt magmas in the vestfold hills antarctica

a major helium 3 source at 15 s on the east pacific rise an extensive plume of water enriched with helium 3 has been discovered in the deep pacific ocean at latitude 15 s on the east pacific rise in the core of the plume at a depth of 2500 meters over the ridge crest the helium 3 helium 4 ratio is 50 percent higher than the ratio in atmospheric helium indicating a strong injection of mantle or primordial helium at the spreading center axis through local hydrothermal systems the helium 3 plume is completely absent east of the rise but it can be traced over 2000 kilometers to the west above a newly observed physical feature a density discontinuity here caled the ridge crest front the injected plume provides a unique deep sea tracer with an asymmetric distribution which shows that the deep circulation across the rise is from east to west the striking intensity and lateral extent of this helium 3 anomaly compared to observations at known oceanic hydrohrmal sites suggest that the largest hydrothermal fields in the ocean are yet to be discovered and that they will be found near 15 s on the east pacific rise

helium isotopes and tectonics in southern italy geodynamic evolution of southern italy can be understood within the framework of the mediterranean alpine system subduction of a plate along the sicily calabrian forearc under the tyrrhenian sea has been suggested by many geophysicists although it is not yet confirmed and remains somewhat controversial helium isotope ratios provide useful information on the geotectonic structure of the region the authors report here the sup 3 h sup 4 he ratios of terrestrial gas samples from southern italy the observed sup 3 he sup 4 he ratios are relatively high in the eolian volcanic arc region and low in the other areas dichotomous explanations are presented firstly volcanic arc forearc hypothesis suggests the subduction along the sicily calabrian forearc secondly horizontal transport hypothesis is described based on the relationship between the ratios and radial distance from the recent spreading basin in southern tyrrhenian sea