δ13c stratigraphy of the birmania basin rajasthan india implications for the vendian cambrian transition overview the paleogeographic reconstruction of the late neoproterozoic assumes a marked significance due to the fact that the interactions between the biosphere atmosphere hydrosphere and geosphere during this time strongly influenced the evolution of life and initiated a remarkable phase of organic evolution well documented in the rocks of this age time related changes in these carbon reservoirs and processes during the late proterozoic have played an important role in the evolution of the environment and of life des marais 1997 brasier 1992 discussed at length various factors which pushed the shallow marine ecosystem of the late neoproterozoic cambrian towards p limitation and suggested that nitrate fixation density stratification and massive removal of p in the sediments were responsible for the formation of phosphatic deposits within this time interval the east gondwana assembly of continents provides fine examples of stratigraphic successions in which the ideas related to oceanic stratification phosphate deposition enhanced organic production and carbon isotope fluctuations can be verified and correlated from continent to continent banerjee and majumdar 1999

the role of hydrothermal processes in concentrating high field strength elements in the strange lake peralkaline complex northeastern canada abstract the middle proterozoic peralkaline pluton at strange lake quebec labrador comprises hypersolvus to subsolvus phases which are unusually enriched in zr y rees nb be and f as exotic alkali and alkaline earth silicate minerals the highest concentrations of these elements are in subsolvus granite which underwent intense low temperature 200 c hydrothermal alteration involving hematization and the replacement of alkali high field strength element hfse minerals by calcic equivalents this alteration is interpreted to have been caused by meteoric or formational waters high temperature 350 c alteration attributed to orthomagmatic fluids is evident in other parts of the subsolvus granite by the replacement of arfvedsonite by aegirine comparisons of the chemical compositions of fresh and altered rocks indicate that rocks subjected to high temperature alteration were chemically unaffected except for depletion in zr y and hrees these elements were appreciably enriched in rocks that underwent low temperature alteration other elements affected by low temperature alteration include ca and mg which were added and na which was removed available data on hfse speciation in aqueous fluids and the chemistry of the pluton suggest that the hfses were transported as fluoride complexes if this was the case the low temperature fluid could not have been responsible for hfse transport because the high concentration of ca and low solubility of fluorite would have buffered f activity to levels too low to permit significant complexation we propose that hfse mineralization and accompanying alteration were the result of mixing in the apical parts of the pluton of a f rich essentially ca free orthomagmatic fluid containing significant concentrations of hfses with an externally derived meteoric dominated fluid enriched in ca as a result of interaction with calc silicate gneisses and gabbros according to this interpretation the latter fluid was responsible for the exchange of ca for alkalis mainly na in hfse rich minerals and by sharply reducing f activity in the mixed fluid through fluorite precipitation and or increasing ph destabilised the hfse fluoride complexes causing deposition of hfse bearing minerals an important implication of this study is that major hfse enrichment may be restricted to those rare cases where f rich ca free metal leaching environments and ca rich depositional environments are juxtaposed

genesis of the carbonatite hosted fluorite deposit at amba dongar india evidence from fluid inclusions stability isotopes and whole rock mineral geochemistry the amba dongar carbonatite complex is located approximately 400 km northeast of bombay india and consists of a carbonatite ring dike and a number of syenitic intrusions which were emplaced into late cretaceous bagh sandstones and early to late eocene deccan volcanic rocks hydrothermal activity associated with intrusion of the carbonatite was responsible for fenitization of the surrounding sandstones and deposition of economic quantities of fluorite 11 6 mt of 30 caf 2 fluorite mineralization occurs as veins and vug fillings localized along fractures within the calcite carbonatite near its contact with the sandstone fluid inclusions in fluorite indicate a low temperature low salinity 42 and 3 5 respectively the presence of al and s in the fluid the molar equivalence of na and cl and the positive deviation of 18 o and d from the meteoric water line point to a small contribution from orthomagmatic fluids a model is proposed in which the intrusion of a carbonatite magma at high crustal levels caused faulting and fracturing of the surrounding country rocks and was accompanied by the release of orthomagmatic fluids expressed as extensive k and na metasomatism of the surrounding sandstones with the decline of orthomagmatic activity a meteoric water dominated hydrothermal system was initiated by the heat of the intrusion the interaction of ca bearing meteoric fluids with the last vestiges of f bearing orthomagmatic nacl brines caused deposition of large quantities of fluorite at the site of mixing

the nature of orthomagmatic hydrothermal fluids in the oka carbonatite quebec canada evidence from fluid inclusions fluid inclusions in apatite monticellite and calcite from the oka carbonatite complex generally comprise either aqueous liquid vapour or aqueous liquid vapour halite some inclusions also contain a variety of trapped solids including calcite strontianite nahcolite and mirabilite microthermometry and raman spectroscopy indicate that co2 is not an important constituent of these fluids microthermometric data also indicate that the hydrothermal fluids liberated from the carbonatitic magma had a wide range of salinities 4 2 to 49 eq wt nacl and densities 0 77 to 1 26 g cm3 most of the fluids had relatively low concentrations of so42 and co32 6 wt however the nature of the trapped minerals and the phase behaviour of some inclusions indicate that some fluids may have had higher concentrations of so42 co32 and or hco3 textural relationships indicate that the crystallization sequence for the host minerals was monticellite apatite calcite which when combined with fluid inclusion data from the three minerals indicates that fluid salinity decreased and fluid density increased during progressive crystallization of the magma the density increase is thought to have been mainly caused by decreasing temperature isochoric projections indicate that the fluids were exsolved at minimum pressures of 4 to 8 kb these data provide a basis with which to assess the involvement of orthomagmatic aqueous fluids in the formation of hydrothermal rare element mineralization associated with carbonatites

silver base metal epithermal vein and listwanite hosted deposit crnac rogozna mts kosovo part ii a link between magmatic rocks and epithermal mineralization abstract crnac is an intermediate sulfidation pb zn ag epithermal deposit located within the vardar suture zone of the central balkan peninsula the epithermal pb zn ag mineralization consists of i a series of steeply dipping veins hosted within the jurassic amphibolites and ii overlying hydrothermal explosive breccia with angular level iv or rounded fragments of listwanite surface cemented by epithermal mineralization the mineralization is related to the oligocene quartz latite dykes that crosscut the crnac antiform quartz latite rocks predominantly display a shoshonitic character the obtained 40ar 39ar age of fresh quartz latite is 28 9 0 3 ma fine grained sericite from altered quartz latite is dated at 28 6 0 5 ma early alteration related fluid inclusions within quartz latite show coexistence of high density brine and a low density vapor saturated phase that homogenized at 280 405 c phase separation occurs at a paleodepth of 0 6 to 0 9 km epithermal mineralization developed in three stages i early pyrite arsenopyrite pyrrhotite quartz kaolinite ii main sphalerite galena tetrahedrite chalcopyrite and iii late carbonate pyrite arsenopyrite assemblage the onset of mineral deposition within epithermal veins was initiated by boiling of na cl k ca mg fluid at a paleodepth of 0 6 to 0 9 km coexisting vapor and liquid rich inclusions display salinities and trapping temperatures of 4 wt nacl equiv 280 370 c and 2 27 wt nacl equiv 230 375 c respectively boiling continued throughout the deposition of the sphalerite galena tetrahedrite chalcopyrite assemblage late stage carbonate was deposited from diluted non boiling low temperature na ca mg cl co2 fluid 0 2 to 4 8 wt nacl equiv 115 280 c about 100 150 m higher in the system precipitation of listwanite breccia cement began as a result of boiling na cl ca mg k fluid of medium salinities 2 6 to 12 1 wt nacl equiv at temperatures of 245 370 c boiling and dilution of fluids continue throughout the precipitation of the main sphalerite galena tetrahedrite and late mainly carbonate assemblage surface listwanite breccia contain quartz phenocrysts deposited from a homogeneous fluid with a medium salinity 8 10 wt nacl equiv and high temperatures th 295 315 c whereas the early and main stage of a surface listwanite breccia cement precipitated from a boiling fluid of decreasing salinity and temperature aqueous co2 high salinity 16 to 18 wt nacl equiv low temperature 120 c homogeneously trapped fluid that precipitated late stage carbonates is most likely a remnant of boiled off fluid the epithermal assemblage of the surface listwanites precipitated at a paleodepth of 0 4 to 0 6 km the 13c values of the late stage ankerite range from 4 2 to 4 1 whereas 18o range from 9 6 to 17 5 the calculated 18o of fluid that precipitated carbonates within epithermal veins and listwanite breccia cement range from 6 3 to 11 3 indicating a contribution of magmatic water deposition of all mineralization types was initiated by neutralization of primary acidic magmatic fluid by water rock reactions that caused widespread propylitization and sericitization extensive and long lasting boiling combined with dilution by meteoric water increased the ph towards the final stage of hydrothermal activity

volatiles in alkaline magmatism abstract gases extracted during fusion of volcanic glasses substantiate other sources of evidence that alkaline magmatism is distinguished from calc alkaline by higher co2 so2 contents with relatively smaller proportions of h2o in addition to higher c and s activity alkaline magmas are characterised by high cl and f which are released with alkalis during fusion experiments in a graphic reminder that alkalis should be regarded as mobile elements in magmatism the association of c s and halogens in alkaline magmatism is well shown in some high temperature residua characterized by assemblages such as calcite vishnevite calcium is an important cation in many aspects of alkaline magmatism and the links with carbonatite further emphasize the key position of c s cl and f throughout and the subordinate role of h2o

isotopic evolution of the biogeochemical carbon cycle during the proterozoic eon abstract time related changes in the proterozoic sedimentary isotopic record of organic carbon and carbonates are consistent with the following 1 an increase in the oxidation state of the environment 2 a transition towards microbial populations dominated by producers and utilizers of o 2 and so 4 2 and 3 a relatively minor increase in organic 13 c during postdepositional thermal alteration 13 c increased 3 as h c declined from 0 8 to 0 1 the progressive oxidation of the environment was episodic for example between 2 2 and 1 9 ga crustal and atmospheric reservoirs of o 2 fe 3 and so 4 2 increased relatively rapidly and this increase was too large to be attributed solely to the declining rate of oxidant consumption by volcanic and hydrothermal fluxes of reduced species additional fundamental changes seem to be required in the tectonic style of organic sedimentation and preservation episodic changes in both the biogeochemical c cycle and atmospheric o 2 levels during the proterozoic eon likely influenced the evolution of eukarya

thin film dawsonite in jurassic coal measure strata of the yaojie coalfield minhe basin china a natural analogue for mineral carbon storage in wet supercritical co2 abstract a large amount of thin film dawsonite has developed in coal cleats and also in fractures within the roof sandstone in jurassic co 2 rich coal measure strata of the yaojie coalfield minhe basin china the co 2 in these strata has been in a supercritical state since natural emplacement in this study the yaojie coalfield is considered a natural analogue site for co 2 capture utilization and storage ccus the thin film dawsonite in coal measure strata was analyzed to investigate the evolution of carbon in supercritical co 2 after ccus as it provides a possible example of the formation of carbonate minerals from a wet supercritical co 2 fluid the cleat and fracture filling dawsonite takes the form of white or silver white radial thin film crystals generally 2 and co 2 derived from the decomposition of basement marble which resulted from dynamic thermal metamorphism of the f19 fault zone the carbon isotopic compositions of the cleat filling dawsonite from different carbon sources are transitional and the carbon source for all of the sandstone fracture filling dawsonite is the co 2 released from the decomposed basement marble the morphological and mineralogical characteristics of the thin film dawsonite show that it most likely nucleated and grew directly in the liquid like water film formed by wet supercritical co 2 on solid phase surfaces rather than by epitaxial growth on the rocks surrounding the cleats and fractures probably via layer by layer crystallization the sodium and aluminum in the dawsonite were derived from formation water the formation of dawsonite in the coal measure strata of the yaojie coalfield indicates that some of the free state co 2 which increases in content due to desorption of the coal matrix can ultimately be converted into carbonate minerals during ccus

zirconolite allanite and hoegbomite in a marble skarn from the bergell contact aureole implications for mobility of ti zr and ree zirconolite allanite and hoegbomite are present as accessory phases in a metasomatically altered spinel calcite marble from the contact with the bergell intrusives switzerland italy textural relationships indicate a step wise alteration of spinel to 1 hoegbomite or corundum magnetite 2 margarite and 3 chlorite replacement of spinel by hoegbomite can be described by the substitution 1 94 mg2 fe2 zn2 mn2 ca2 ti4 0 12 oh where al3 and fe3 are held constant the average composition of the bergell hoegbomites is given by the formula fe 0 97 2 mg0 69zn0 04ti0 17al3 94fe 0 06 3 o7 98 oh 0 02 and seems to be imposed by the composition of pre existing spinel during the first two steps of spinel alteration calcite was replaced by anorthite phlogopite and the rare earth element ree bearing minerals zirconolite allanite and sphene were formed allanites have characteristic chondrite normalized ree patterns with enrichment in the light ree the zirconolite patterns show a marked increase in concentration from la to ce followed by an almost constant section sphene lacks detectable la and its ree patterns vary from grain to grain contemporaneous formation of phlogopite ree bearing minerals and hoegbomite during replacement of the spinel calcite marble indicates that the metamorphic fluid introduced potassium along with ree and other high valence cations ti4 zr4 u4 th4a3804265 nb5a3804265 y3a3804265 possibly as polynuclear complexes the abundance of fluorine bearing phlogopite and fluor apatite as well as their close association with ree bearing minerals and hoegbomite suggests f and po 4 3 as likely ligands for complexing of the above mentioned elements

a fluid inclusion study of vug minerals in dawsonite altered phonolite sills montreal quebec implications for hfse mobility ordovician limestones in the francon quarr on the island of montreal quebec are host to three sills of cretaceous age composed of phonolite that has been extensively altered to dawsonite an interesting feature of the sills is the presence of abundant vugs containing a wide variety of minerals including several in which one or more high field strength elements zr hf nb ti is a major component the most important of these latter minerals is weloganite a rare strontium zirconium hydrous carbonate first identified in the francon quarry four types of inclusions have been recognized in vug minerals aqueous aqueous carbonic carbonic and solidbearing aqueous inclusions homogenize at temperatures mainly between 70 and 170 c and between 230 and 390 c the homogenization temperatures of primary inclusions cluster around 350 c aqueous inclusions and the aqueous phase in aqueous carbonic inclusions have salinities ranging between 10 and 24 eq wt nacl equivalent primary aqueous carbonic inclusions have low xco2 0 7 carbonic inclusions are all secondary nahcolite dawsonite and weloganite occur as daughter minerals or trapped solids nahcolite and possibly natron or mirabilite appear to form in frozen inclusions analyses of fluid inclusion decrepitates detected high concentrations of na cl al s and c the extraordinarily high concentration of al in the fluid possibly exceeding 1 wt suggests a ph of approximately 10 pressure and temperature conditions estimated from stratigraphic reconstruction and the isochores of primary aqueous fluid inclusions were 450 bar and 360 to 400 c respectively the relatively high temperatures and compositions of primary fluid inclusions suggest that vug filling was the result of mineral precipitation from an orthomagmatic fluid a model is proposed in which a partially crystallized phonolite melt started exsolving a homogeneous low xco2 fluid immediately prior to or after intrusion sodium aluminium chlorine fluorine sulphur and hfse elements such as zr hf nb and ti were partitioned into the hydrous phase in the case of zr possibly to a concentration of 300 ppm the near horizontal orientation of the sills and the chilled margins produced by quenching of the magma created a tight seal that inhibited escape of the fluids as a result the phonolite stewed in its own juices long after crystallization giving rise to widespread replacement of primary igneous minerals by dawsonite and precipitation of this and other minerals in vugs once the sills had colled to temperatures between 200 and 300 c the aqueous fluid exsolved a high co2 fluid which was trapped as the secondary three phase type ii and type iii inclusions decreasing temperature is considered to have been the principal control of mineralization although in the case of the lower temperature minerals decreased bicarbonate or carbonate ion activity and a lower dielectric constant as a result of co2 exsolution may have played a role in the deposition of hfse bearing minerals

nutrient enriched waters and the early skeletal fossil record this paper examines the role of stratified oxygen depleted and nutrient enriched water masses new in transforming the fossil record over the precambrian cambrian boundary interval the following may serve as markers for the encroachment of new over low latitude cratons trace element enriched black shales stone coals and hydrocarbon source rocks phosphorites phosphatized shells and organic matter sponge spicule cherts early appearance of silica and phosphatic skeletons appearance of anabarites and coleoloides biofacies peak diversity of phosphatic species occurrence of black shale lagerstatten and light 13 c values of carbonates nutrient depleted waters ndw are inferred over inner cratonic areas and nutrient starved waters nsw may well have developed over deeper waters at times through the later cambrian related to the effects of sea level rise and salinity stratification carbon isotopic fluctuations are suggested to record the history of these water masses with major changes in carbon burial and palaeoproductivity the latter may well imply episodes of greenhouse climate with globally raised levels of carbon dioxide and relatively high ocean temperatures

diagenetic f rich ferroan calcite and zircon in the offshore scotian basin eastern canada significance for understanding thermal evolution of the basin in the scotian basin offshore eastern canada an unusual combination of high heat flow in the cretaceous and the abundance of halite has resulted in unusual diagenetic minerals such as sphalerite the newburn h 23 well is the most distal well in the basin with good core samples and has two previously unknown diagenetic mineral occurrences fluorine rich ferroan calcite and diagenetic zircon this study uses sem backscattered electron images and eds analyses emp wds mineral analyses and raman spectroscopy to determine mineral chemistry and textures to investigate the diagenetic and thermal significance of these minerals late diagenetic fe calcite contains 1 2 5 wt fluorine mostly from adsorption but rarely as small fluorite crystals fluoride is also adsorbed on the surfaces of some framework minerals and chlorite fluoride was transported in highly saline formation brines derived from the argo salt formation zircon grains 20 40 m in size have crystal outlines that are straight adjacent to pores partially lobate filling porosity and cross cutting other grains these may be diagenetic some zoned detrital zircon grains show 1 3 m wide diagenetic outgrowths neoformation of diagenetic zircon requires temperatures of 250 c transport of zirconium is favored by ligands in low ph solution principally fluoride and phosphate anions with zirconium mobilized during the alteration of metamict detrital zircon under low grade metamorphic conditions the presence of diagenetic sphalerite and the documented mid cretaceous thermal event in the scotian basin indicate conditions that could have been suitable for the formation of diagenetic zircon in this well suitable geological conditions for such diagenetic formation of zircon will be found in a subsiding rift basin with early evaporites that are affected by a subsequent phase of volcanism due to new rifting or subduction

redox dependent distribution of early macro organisms evidence from the terminal ediacaran khatyspyt formation in arctic siberia abstract the ediacaran period witnessed the first appearance of macroscopic animal life in earth s history however the biogeochemical context for the stratigraphic occurrence of early metazoans is largely uncertain in part due to the dearth of integrated paleobiological and chemostratigraphic datasets in this study a comprehensive geochemical analysis was conducted on the fossiliferous khatyspyt formation in arctic siberia in order to gain insights into the ediacaran paleoenvironments this study was designed to specifically address the relationship between paleoredox conditions and ediacaran fossil occurrences in the khatyspyt formation our data reveal a dramatic shift in pyrite sulfur isotope compositions 34 s pyrite from ca 20 to ca 55 and this shift is intriguingly associated with the first occurrence of ediacara type macrofossils at the studied section suggesting a possible link between seawater redox conditions and distribution of early macroscopic organisms based on multiple lines of sedimentological and geochemical evidence we propose that the development of oceanic euxinia which may be widespread in the continental margins due to enhanced oxidative weathering in the terminal ediacaran period may have locally prohibited the colonization of ediacara type organisms and resulted in low 34 s pyrite values in the lower khatyspyt formation in the middle and upper khatyspyt formation progressive secular transition from euxinic to non euxinic and more habitable conditions may have allowed for the colonization of ediacara type and other macro organisms

early cambrian palaeogeography and the probable iberia siberia connection abstract the end of the proterozoic beginning of the cambrian is marked by some of the most dramatic events in the history of earth the fall of the ediacaran biota followed by the cambrian explosion of skeletonised bilaterians a pronounced shift in oceanic and atmospheric chemistry and rapid climatic change from snowball earth to greenhouse conditions all happened within a rather geologically short period of time these events took place against a background of the rearrangement of the prevailing supercontinent some authors view this as a sequence of individual supercontinents such as mesoproterozoic midgardia neoproterozoic rodinia and early cambrian pannotia assembled in the mesoproterozoic this supercontinent appears to have existed through the neoproterozoic into the early cambrian with periodic changes in configuration the final rearrangement took place during the precambrian cambrian transition with the cadomian and related phases of the pan african orogeny the distribution of early cambrian molluscs and other small shelly fossils ssf across all continents indicates a close geographic proximity of all major cratonic basins that is consistent with the continued existence of the supercontinent at that time subsequently rodinia experienced breakup that led to the amalgamation of gondwana separation of laurentia baltica siberia and some small terranes and the emergence of oceanic basins between them spreading oceanic basins caused a gradual geographic isolation of the faunal assemblages that were united during the vendian early cambrian

the genesis of hydrothermal fluorite ree deposits in the gallinas mountains new mexico breccia hosted fluorite bastnaesite deposits associated with epizonal quartz syenite intrusions in the gallinas mountains new mexico provide an important example of hydrothermal rare earth element ree mineralization and an excellent opportunity to investigate the transport and deposition of the ree under hydrothermal conditions fluid inclusion studies show that mineralization commenced with the deposition of quartz at a temperature of approximately 400 c from sulfate rich nacl kcl brines having a salinity of 15 wt percent nacl equiv these fluids are interpreted to have been of orthomagmatic origin fluorite and bastnaesite deposition occurred at slightly lower temperatures and coincided with the introduction of separate co2 bearing and sulfate poor nacl brines into the system these fluids are considered to have been of external origin the latter being a formation water that circulated in surrounding permian sandstones and limestones evaluation of the available thermodynamic data on the aqueous speciation of ree and the preliminary data on the chemistry of the fluids obtained in this study suggest that the ree were dissolved primarily as fluoride complexes a model is proposed in which the ree were transported by the orthomagmatic fluids and deposited as bastnaesite owing to destabilization of ree fluoride complexes the latter occurred in response to the sharp reduction in f activity that accompanied large scale deposition of fluorite as a result of mixing of the orthomagmatic fluids and formation waters this mixing also led to the deposition of anhydrite and barite owing to the interaction of externally introduced calcium and barium ions with orthomagmatic sulfate ions the later stages in the evolution of the gallinas mountains hydrothermal system were marked by increased incursion of external fluids and the alteration of bastnaesite to ca bastnaesite and minor deposition of ca bastnaesite and parisite owing to lowered f batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document co 3 2 mbox end document ratios and or increased ca2 activity the study suggests that ca free ree fluoride bearing fluids are an important ingredient for ree mineralization that mixing of such fluids with ca carbonate bearing fluids or interaction with carbonate rocks is a major control of ree fluorocarbonate mineral deposition and that the occurrence of fluorite could provide an important guide in the exploration for economic ree deposits

sea level changes facies changes and the late precambrian early cambrian evolutionary explosion abstract an explosive increase in the fossil record coincided with a major rise in sea level during latest precambrian early cambrian times the artefactual and real effects of an onshore offshore environmental gradient upon taxonomic diversity are considered with particular reference to 14 fossiliferous sections from around the world within taxon and within habitat evolution through the early cambrian are also discussed the evidence suggests that evolution was a diversity dependent i e kinetic growth process made possible by the colonisation and gradual filling of habitats that were growing both in variety and area archaeocyatha diversified rapidly at first but may have suffered later decline and extinction because facies changes in late early cambrian times reduced their area of habitat

anomalous geochemical signals from phosphatic middle cambrian rocks in the southern georgina basin australia middle cambrian rocks of the georgina basin contain both phosphatic and organic rich sediments the former often occurring as commercially viable phosphate deposits and the latter as minor oil shale occurrences both kinds of rocks occur in the hay river formation penetrated by bureau of mineral resources bmr stratigraphic corehole tobermory 14 detailed sampling and analysis of this core has been undertaken to assess the chemostratigraphy of the hay river formation to examine the possible use as a palaeoceanographic tool of changes in 13c values of carbonates and kerogens and to gain an understanding of phosphate deposition in this part of the basin the upper hay river formation was deposited in a normal marine environment characterized by oxic bottom conditions it contains several episodes of significant trace metal enrichment which appear to indicate periodic replacement of the water body the lower hay river formation however was deposited in an abnormal marine environment low in sulphate and mo and enriched in p its carbonate phase has positive 13c values 1 1 5 0 enriched over the accepted cambrian marine carbonate value of 0 57 0 17 0 the very low sulphate concentration mo depletion and spiking of carbonate 18o values suggests there was an influx of deeper ocean water of sufficient volume to maintain prolonged stable anoxic bottom waters these data support a previous suggestion that phosphogenesis in the georgina basin was related to oceanic anoxic events

rb sr isotope dating of neoproterozoic malani group magmatism from southwest rajasthan india evidence of younger pan african thermal event by 40ar 39ar studies abstract this paper reports rb sr isotope ages of the neoproterozoic volcanics and associated granitoids of the trans aravalli belt of northwestern india all these rocks along with the earlier reported 779 10 ma old felsic volcanics from diri and gurapratap singh of pali district rajasthan constitute the malani group the study indicates that different rock suites belonging to the malani group represent a polyphase igneous activity which spanned for about 100 ma ranging from 780 to 680 ma the granitoids of the malani group i e peraluminous jalore type and peralkaline siwana type were emplaced around 730 and 700 ma ago respectively these plutonic suites represent two different magmatic episodes within a short time interval the initial sr ratios of these granitoids suggest lower crustal derivation of the magma the peralkaline granitoids and the associated peralkaline rhyolites pantellerites are coeval and cogenetic the ultrapotassic rhyolite exposed at manihari of pali district represents the youngest magmatic activity at 681 20 ma having a very high initial sr ratio of 0 7135 0 0033 the high initial sr ratio of these rocks may be due to incorporation of radiogenic 87 sr from the country rock by assimilation or fusion into the residual fraction of the magma in the crust which gave rise to other differentiated rocks of the group 40 ar 39 ar studies of two jalore granite samples indicate presence of post crystallisation thermal disturbance between 500550 ma ago the timing of this thermal overprinting on the malani rocks is related to the widespread pan african thermo tectonic event which is witnessed and magmatically manifested in different part of the indian shield

fossil preservation through phosphatization and silicification in the ediacaran doushantuo formation south china a comparative synthesis phosphatized and silicified microfossils preserved through replication of organic templates by authigenic calcium phosphate and silica respectively form through mechanistically similar mineralization processes but under notably different conditions in order to characterize the environments and taphonomic processes associated with these preservational pathways we compare phosphatized and silicified fossil assemblages in the ediacaran doushantuo formation south china in terms of stratigraphic and paleogeographic distribution preservation and biodiversity several differences are recognized 1 phosphatization and silicification both occurred in sub tidal doushantuo sediments but phosphatized fossils generally occur in shallower sub tidal facies than silicified fossils 2 although both phosphatized and silicified fossils are preserved three dimensionally the latter are typically more compressed and 3 whereas phosphatized assemblages are numerically dominated by multicellular acritarchs putative metazoan eggs and embryos silicified assemblages are dominated by prokaryotes these differences can be explained by environmental and taphonomic distinctions phosphatized fossils occur in intraclastic phosphorites and phosphatic carbonates and were likely fossilized near the suboxic anoxic boundary in sediments under oxic bottom waters with redox and biogeochemical gradients conducive to p remineralization phosphate burial via iron pumping and calcium phosphate precipitation reworking and winnowing of phosphatized fossils preferentially concentrated certain taxa prior to final burial in contrast silicified fossils typically occur in chert nodules which formed deeper in the sediment column in ferruginous pore waters and perhaps under ferruginous bottom waters where bacterial sulfate reduction and pyrite formation created low ph conditions conducive to calcite dissolution and silica precipitation once formed chert nodules mostly remained in situ but some were reworked with turbidites and olistostromes developed in slope facies despite these distinctions phosphatized and silicified fossil assemblages in the doushantuo formation generally overlap in time and space and representatives of all major groups and many common genera of doushantuo organisms are present in both phosphorites and cherts consequently although both preservational pathways have paleoenvironmental and taphonomic biases their relative biases on taxonomic presence absence data are minimal given sufficient sampling efforts and paleobiologic and biostratigraphic information based on presence absence but not abundance data from the doushantuo formation appears to be robust

boron isotopic composition of tourmaline prismatine and grandidierite from granulite facies paragneisses in the larsemann hills prydz bay east antarctica evidence for a non marine evaporite source a unique assemblage of b rich rocks 2 wt b containing the borosilicate minerals tourmaline tur prismatine prs and grandidierite gdd is exposed in the ca 1000 ma brattstrand paragneiss in the larsemann hills east antarctica the b isotope composition of these three minerals in 21 paragneisses and 6 anatectic pegmatites were measured in situ using secondary ion mass spectrometry 11b ranges from 2 8 to 14 4 in tourmaline from 9 6 to 17 8 in prismatine and from 2 8 to 8 7 in grandidierite weighted uncertainties mostly 1 2 per sample in most cases average 11b increases in a given sample prs tur gdd whether the observed b isotope distribution has attained equilibrium were assessed using two criteria microstructural equilibrium and regular distribution of mg and fe in samples for which the two criteria are met we assume the measured distribution of isotopes 11ba b 11ba 11bb represents equilibrium 11btur prs 5 0 1 4 5 of 12 pairs 11btur gdd 3 3 0 8 3 of 5 pairs and 11bgdd prs 7 2 1 3 2 of 2 pairs consistent with the preference of 10b for tetrahedral sites prismatine and 11b for trigonal sites tourmaline grandidierite ab initio computations of b isotope fractionation factors 11btur prs 6 4 1 3 11btur gdd 1 8 1 1 and 11bgdd prs 8 2 1 1 provide confirmation of the measured fractionations the computed relaxed ionic structure of grandidierite gave a shorter 1 368 a b o bond length than in case of dravite 1 385 a which explains the measured enrichment of grandidierite in 11b relative to tourmaline the precursor of the b rich rock least changed by metamorphism tourmaline metaquartzite 11b tur 8 6 to 5 9 is interpreted to be a product of pre metamorphic hydrothermal b metasomatism boron sources consistent with this 11b range include oceanic crust clastic sediments volcanic rocks and non marine evaporite borate but not marine evaporite however dominance of metasediments in the brattstrand paragneiss major and trace element compositions of the b rich rocks and presence of abundant b are consistent with non marine evaporite being the most important source of b for the b rich lithologies of the larsemann hills a possible scenario for precursors of these rocks is a succession of clastic sediments in part tuffaceous intercalated with b rich evaporite deposits in a continental rift basin in which circulating hydrothermal fluids leached b from the evaporite and precipitated it as tourmaline in the associated clastic rocks to form the precursors to the tourmaline metaquartzites

carbon isotope excursions in precambrian cambrian boundary beds morocco the precambrian cambrian boundary is one of the most significant points on the geological timescale because it marks the change in the fossil record from soft bodied organisms to skeletonized faunas the cambrian explosion or radiation event1 saw the evolution of many new life forms especially various molluscs and archaeocyathids from the base of the cambrian tommotian stage others including the trilobites and bivalves evolved a little later in the succeeding atdabanian stage2 3 a massive increase in the biomass of the photic zone through phytoplankton blooms has also been postulated for this time4 particularly in the late riphean early vendian and in the early cambrian5 there has been much discussion concerning where precisely to place the boundary6 7 and here are several contenders in the world for the type section8 i present here a carbon isotope profile from the precambrian cambrian boundary beds in morocco where two distinctive positive 13c excursions are revealed these are interpreted as records of increased organic productivity as part of the precambrian cambrian explosion of life 13c excursions could be useful points for correlation between boundary sections

storm dominated mixed carbonate siliciclastic shelf sequence displaying cycles of hummocky cross stratification late proterozoic wonoka formation south australia abstract the regressive late proterozoic wonoka formation of the adelaide geosyncline in south australia contains a thick mixed carbonate siliciclastic interval of middle to inner shelf origin the shelf was storm dominated and displays abundant hummocky cross stratification hcs of varying scales and other wave formed sedimentary structures some parts of this sequence are notably cyclic two kinds of cycles are described here the first average cycle 8 m thick consists of the alternation of silty and argillaccous limestone with fine grained hummocky cross stratified calcareous sandstone climatic cyclicity e g milankovitch cycles involving alternate wet and dry periods or minor sealevel fluctuations along a low gradient shoreline are considered the most likely cause of the observed sediment cyclicity carbonate deposition predominated during dry periods or high sealevel followed by influxes of sand from deltaic sources during wet periods or times of low sealevel the prevalence of hcs in the sandstones is most likely due to much slower rates of lithification in comparison to the limestones which give evidence of rapid lithification on the seafloor the second type of cycle average cycle 5 10 m thick is characterised by the cyclic appearence of red hummocky cross stratified glauconitic and phosphatic limestones within a sequence dominated by grey green stylonodular limestone tempestites the most likely origin of the red limestone involves contributions of volcanic ash fall material the resulting sediment did not undergo the rapid seafloor lithification of the associated grey green limestones and thus was involved in considerable storm wave reworking before deposition the micritic shelf carbonates of the wonoka formation are considered to represent direct inorganic precipitation into the middle shelf environment rather than reworking of detrital material from shallow carbonate banks

phosphogenesis around the proterozoic phanerozoic transition there was a very significant and widespread phosphogenic event around the proterozoic phanerozoic transition this is reflected in the occurrence of many major and minor latest proterozoic and cambrian phosphate deposits phosphorites in asia australia africa and south america as well as minor occurrences in many other parts of the world the major peak in phosphogenesis was in the tommotian with lesser events in the ediacaran and the lenian bracketed by other minor though significant events recent evidence on the timing of proterozoic phosphogenesis does not now support the suggestion of a phosphogenesis glaciation link these peaks in phosphogenesis are taken as evidence that there was a marked increase in the phosphate content of the shallow seas at about this time possibly as a result of the circulation of deep phosphorus rich ocean waters into the photic zone not only was this responsible for the formation of phosphorites but it also had a profound effect on the biota and may have been a driving mechanism for the cambrian faunal explosion and the initiation of biomineralization

geochemical analysis and paleoecological implications of phosphatic microspherules otoliths from frasnian famennian boundary strata in the great basin usa phosphatic microspherules 1 mm diameter recovered from the upper devonian uppermost frasnian guilmette limestone in eastern nevada are interpreted here to be fish otoliths and thus provide insight into ocean water chemistry just prior to the frasnian famennian mass extinction boundary analysis by scanning electron microscope sem and electron microprobe indicates that they are apatite francolite and consist of radially aligned concentrically banded crystals around a central nucleus this type of microspherule previously interpreted to be conodont pearls has compositions more similar to fish teeth derived from the same unit than to conodonts microspherules and fish teeth have consistently lower concentrations in wt of p2o5 31 88 36 32 f 3 05 5 12 sro 0 15 0 34 and analysis totals indicating higher concentrations of oh and or co3 90 66 96 09 and higher cao 51 67 55 15 so2 0 50 0 90 mgo 0 09 0 15 and fe2o3 0 11 0 21 than the associated conodonts p2o5 37 32 40 01 f 4 99 6 89 sro 0 32 1 79 totals 96 07 100 55 cao 52 32 53 06 so2 0 05 0 21 mgo 0 01 0 05 fe2o3 0 02 0 11 these differences in composition are consistent from core to rim in all microfossils analyzed and reflect primary biogenic compositions rather than diagenetic or metamorphic signatures we interpret the apatite microspherules to be genetically related to the fish teeth rather than to the conodonts based on the geochemical analysis the microspherules are morphologically similar to modern teleost fish otoliths the fish teeth found associated with the microspherules are from acanthodian or actinopterygian fish which possessed otoliths in the devonian and are the distant ancestors to modern teleost fish modern fish otoliths normally have a calcium carbonate composition but their trace element composition is highly sensitive to ambient water temperature and chemistry we speculate that the stratigraphically restricted range and phosphatic composition of the devonian otoliths reflects secretion by fish under conditions of excess dissolved reactive phosphorus in the water column that was most likely associated with upwelling and cooler water conditions on the shelf during maximum transgression

the precambrian cambrian boundary seawater chemistry ocean circulation and nutrient supply in metazoan evolution extinction and biomineralization this paper reviews the evidence for changes in the global environment from the late precambrian into the cambrian against which the evolution of many metazoan groups and the development of biomineralization should be seen with higher carbon dioxide levels precambrian seawater was more supersaturated with respect to caco 3 than phanerozoic seawater and carbonates were precipitated easily from the late precambrian to the early cambrian there was a decrease in the mg ca ratio and an increase in the a co 2 of seawater changes in global climate icehouse to greenhouse and increased plate tectonic activity resulted in major changes in ocean circulation and nutrient levels a rise in global temperature and the formation of extensive shallow seas the vendian cambrian radiation events and onset of biomineralization must have been strongly influenced if not driven by these global environmental changes

phosphorus phosphorites and skeletal evolution at the precambrian cambrian boundary clustering of sedimentary phosphate deposits around the precambrian cambrian boundary is related to a global phosphogenic event this event probably resulted from a complex interplay of factors and had a profound effect on the biota with high concentrations of phosphorus in the photic zone producing an increase in the total shallow marine biomass faunal diversification the precipitation of skeletal structures and explosive evolution

fluid inclusions as metamorphic process indicators in the southern aravalli mountain belt india fluid inclusions from a biotite garnet schist in the southern aravalli mountain belt india give information on both peak metamorphic conditions and post peak metamorphic processes during uplift a combination of careful petrography microthermometry and raman spectroscopy reveals the presence of at least five generations of enclosed fluids lower amphibolite facies pressure temperature conditions of the growth of garnet rims are reproduced by the highest fluid density of the relatively oldest inclusion type of co2 an2 rich compositions a calculated fluid composition in the coh system in equilibrium with the graphite buaer corresponds to a co2 rich fluid at metamorphic condi tions however the results of these calculations are very sensitive to small fluctuations in oxygen fugacity and the accuracy of thermodynamic properties of mineral equilibria re equilibration conceived by specific size density distribution and the absence of an aqueous phase in inclusions that contain nahcolite crystals is monitored in these inclusions as post peak metamorphic processes like partial decrepitation and preferential leakage the other fluid types represent heterogeneous fluid trapping of coexisting aqueous nacl bearing solutions with co2 ch4 rich vapour bubbles in healed cracks and probably the introduction of external fluids containing high salinity aqueous cacl2 rich solutions in nearly pure n2 vapour bubbles at lower p t conditions this study illustrates that fluid inclusions remain a valuable database of peak metamorphic conditions moreover alterations of the entrapped fluids and surrounding crystals are illustrative for specific exhumation evolutions

palaeoenvironment and carbon isotope stratigraphy of upper proterozoic carbonates of the yangtze platform the 13c values for phanerozoic marine carbonates display secular trends mainly in the range 1 to 2 1 these have been interpreted as primary features reflecting changes in the carbon isotope composition of dissolved bicarbonate in seawater brought about largely by variations in the proportions of carbon being sequestered in the oxidized and reduced carbon reservoirs1 2 carbon isotope trends are not as well established for the precambrian but there are indications that upper proterozoic sequences contain carbonates with unusually high 13c values3 9 here we present the results of the first carbon isotopic study of the upper proterozoic carbonate strata from the type locality of the sinian system in the yangtze gorges sections near yichang south china and discuss their implications for accumulation of organic matter we also compare the results for these chinese strata with 13c trends in other upper proterozoic cambrian sequences and assess the extent to which such isotopic data can be used to make stratigraphic correlations between widely spaced sequences

magmatic fluids in the fen carbonatite complex s e norway three different types of carbonatite magma may be recognized in the cambrian fen complex s e norway 1 peralkaline calcite carbonatite magma derived from ijolitic magma 2 alkaline magnesian calcite carbonatite magma which yielded biotite amphibole sovite and dolomite carbonatite and 3 ferrocarbonatite liquids related to 2 and or to alkaline lamprophyre magma damjernite apatite formed during the pre emplacement evolution of 2 contains inclusions of calcite and dolomite devitrified mafic silicate glass and aqueous fluid all of these inclusions have a magmatic origin and were trapped during a mid crustal fractionation event p 4 kbars t 625 c where apatite and carbonates precipitated from a carbonatite magma which coexisted with a mafic silicate melt the fluid inclusions contain water dissolved ionic species mainly nacl with minor polyvalent metal salts and in some cases co2 two main groups of fluid inclusions are recognized type a co2 bearing inclusions of approximate molar composition h2o 88 90 co 27 5 nacl 5 d 0 85 0 87 g cm3 type b co2 free aqueous inclusions with salinities from 1 to 24 wt nacleq and densities betwen 0 7 and 1 0 g cm3 more strongly saline type b inclusions salinity ca 35wt d 1 0 to 1 1 g cm3 contain solid halite at room temperature and occur in overgrowths on apatite type a inclusions probably contain the most primitive fluid from which type b fluids have evolved during fractionation of the magmatic system type b inclusions define a continuous trend from low towards higher salinities and densities and formed as a result of cooling and partitioning of alkali chloride components in the carbonatite system into the fluid phase available petrological data on the carbonatites show that the fluid evolution in the fen complex leads from a regime dominated by juvenile co2 h2o fluids during the magmatic stage to groundwater derived aqueous fluids during post magmatic reequilibration

the marine environments encompassing the neoproterozoic glaciations evidence from c sr and fe isotope ratios in the hecla hoek supergroup in svalbard abstract the neoproterozoic era records several important events in earth history the associations between bif deposition snowball earth events and the redox state of seawater are the key to explain the re appearance of banded iron formations bif during the neoproterozoic unraveling ancient iron cycles provides important information about the linkage between the precipitation of bif and the redox condition in the ocean but current iron isotopic data are limited to sediments deposited only during the sturtian glaciations and ediacaran we conducted a detailed geological survey of the tonian to ediacaran sedimentary succession in nordaustlandet island svalbard our chemostratigraphies of 13 c carb from pre sturtian to post marinoan sedimentary successions in the polarisbreen group are consistent with those in coeval sediments we first analyzed iron isotope ratios 56 54 fe of individual pyrite grains with diverse shapes in carbonates black shales quartz arenites and diamictites in order to constrain the redox condition in seawater we found large variations in iron isotope ratios from ca 2 to ca 4 positive 56 54 fe values over 1 0 are only preserved in euhedral and aggregated pyrite grains in black shales of the backlundtoppen formation and the macdonaldryggen member of the elbobreen formation a partial oxidation of ferrous iron in seawater is necessary to explain such high 56 54 fe values which means that deep seawater was ferruginous ferrous iron rich before and after the sturtian glaciation this stands in opposition to the traditional idea that a glacial cover on an ocean is necessary for the accumulation of ferrous iron in neoproterozoic seawater this study presents the first evidence from iron isotopes that supports a deep ferruginous ocean around the sturtian glaciation if the deep ocean was intrinsically iron rich it is likely that neoproterozoic bifs formed by thermohaline circulation during the glaciation and by massive oxidation due to the expansion of oxic oceans after the glaciation

carbon isotopes and precambrian cambrian boundary geology south australia ocean basin formation seawater chemistry and organic evolution a carbon isotope stratigraphy from the lowest cambrian strata in the adelaide fold belt of south australia is interpreted in terms of the geological context of the sequence major transgression developing circulation patterns and upwelling resulting from an adjacent opening ocean and biomass increases stemming from the cambrian radiation event there is also evidence from the textures of ooids and cements in late precambrian cambrian strata for subtle variations in seawater chemistry across the boundary and these may well have been instrumental in the development of calcareous skeletons in the early metazoans

basal cambrian reworked phosphates from spitsbergen norway and their implications several phosphatic zones are associated with the oldest remains of shelly fossils on the arctic island of spitsbergen the phosphate occurs as reworked nodules and layers associated with a disconformity a gap in the acritarch biostratigraphic record supports a hiatus associated with the phosphatic zone palaeogeographic positions for svalbard both 1 as a single unit and 2 as three isolated parts prior to caledonian tectonism are consistent with conditions favourable to at least minimal amounts of upwelling however upwelling may not have been a prerequisite for development of these phosphate deposits these phosphates were deposited in a nearshore shelf environmnt which contrasts with the outer shelf setting of many modern and recent phosphate deposits geochemistry of the lower cambrian phosphates of spitsbergen varies with the mode of phosphate occurrence concretionary phosphate clasts are chemically zoned such that their centres are enriched in p 2 o 5 and cao and are depleted in a1 2 o 3 sio 2 and k 2 o laminated and thinly bedded phosphate shows no chemical zonation within clasts phosphate cements are the most pure with respect to calcium phosphate this thin phosphatic zone of svalbard is minor when compared with thicker and richer lower cambrian phosphate deposits particularly those in the soviet union southeast asia and australia coupled with the near absence of phosphate in some extensive proterozoic to lower palaeozoic successions e g western north america this emphasizes the fact that widespread lower cambrian phosphate deposition was unevenly distributed althouth the upper proterozoic and lower cambrian are characterized by enhanced phosphogenesis palaeogeographic position was also apparently critical to deposition of phosphatic sediments

on the late neoproterozoic early cambrian transition events in parts of east gondwanaland abstract the east gondwanaland evolved as a result of break up of the rodinia supercontinent the late neoproterozoic early cambrian geochemical events documented in the rocks of the east gondwanaland and siberia suggest variations in the c s and sr isotopic compositions of the contemporary seawater and systematic distribution of phosphorite and evaporite deposits the geochemical records in the peninsular india himalaya south china iran and oman regions have been discussed and used for establishing late neoproterozoic links of these widely separated sedimentary basins

geologic evidence for a mantle superplume event at 1 9 ga 1 both preserved and restored areal distributions of proterozoic marine intracratonic passive margin and platform sediments show a prominent peak at 1 9 ga indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time the chemical index of alteration in shales deposited at this time was high suggesting warm climates possibly due to enhanced co2 levels in the atmosphere high sea level and warms climate may also explain an abundance of black shale banded iron formations and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1 9 ga all of these observations are consistent with a 1 9 ga superplume event the occurrence of only a minor positive carbon isotope shift in marine carbonates at 1 9 ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present slightly low 87sr 86sr isotopic ratios in seawater at 1 9 ga reflect increased mantle input of sr from the proposed superplume event whereas higher ratios at 1 85 1 75 ga may reflect increased input of continental sr from a growing supercontinent the first massive sulfate evaporites in the geologic record at 1 8 1 6 ga follow the possible 1 9 ga superplume event this may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume related volcanism wanes

geology and fluid geochemistry of the na metasomatism u deposits in the longshoushan uranium metallogenic belt nw china constraints on the ore forming process abstract the longshoushan belt in gansu province is a very important uranium metallogenic belt in northwest china and contains a series of different types magmatic na metasomatism supergenetic of uranium deposits and occurrences of all these mineralization types the na metasomatism type is the most predominant with great potential represented by two na metasomatism uranium deposits xinshuijing and jiling different from the fluorite illite quartz dominated alteration minerals in other hydrothermal uranium mineralization system the na metasomatism type uranium deposits are characterized by the unique albite dominated up to 90 vol alteration mineral assemblage which is referred to albitite in many literatures all the uranium orebodies in both the xinshuijing and jiling deposits are hosted within the metasomatic albitites and controlled by the subsidiary faults of the regional malugou fault the mineralization process in both deposits can be divided into four stages namely the na metasomatism stage u mineralization stage post mineralization stage and supergene mineralization stage the na metasomatism stage is predominantly composed of albite with hematite staining with minor chlorite and anatase while the u mineralization stage is characterized by the occurrence of uranium minerals mostly pitchblende intergrown with some tiny albite calcite chlorite pyrite apatite ree minerals and sometimes hydrothermal zircons the mineral assemblages of the na metasomatism stage and u mineralization stage are too intimately related to each other and might be the products of the evolution from the same ore forming fluids the post mineralization stage is mainly the calcite quartz veinlets or occur as aggregate clusters while the supergene mineralization stage includes mineral assemblage of secondary uranium mineralization clay minerals e g kaolinite limonite mineralogical studies and comparison of major and trace elements assay results between fresh biotite granite predominate host rocks albitite and uranium ores suggest that the ore forming fluids resulting in the formation of the na metasomatism alteration are oxidized and rich in na sc v cr co ni u th while the ore forming fluids of the u mineralization stage are relatively reduced and rich in heavy rare earth elements u th po34 fluid inclusion and calcite c o isotope studies indicate that the ore forming fluids are meteoric water dominated which circulated at depth 8 km by fluid inclusion geobarometer and scavenged uranium from the host biotite granites and metamorphic rocks of the longshoushan group the ore precipitation is mainly attributed to physicochemical changes fo2 temperature fluid composition from fluid rock interaction and further temperature decrease of the ore forming fluids while fluid boiling and fluid mixing processes are limited the proposed ore forming process is not only important for the understanding of the ore genesis for the na metasomatism mineralization systems in longshoushan but also highlights the significance of albite alteration which could be used as an important indicator during further uranium exploration