Diagenetic Apatite Research Articles

Metamorphism obscures primary taphonomic pathways in the early Cambrian Sirius Passet Lagerstätte, North Greenland

Abstract Correct interpretation of soft-bodied fossils relies on a thorough understanding of their taphonomy. While the focus has often been on the primary roles of decay and early diagenesis, the impacts of deeper burial and metamorphism on fossil preservation are less well understood. We document a sequence of late-stage mineral replacements in panarthropod fossils from the Sirius Passet Lagerstätte (North Greenland), an important early Cambrian Burgess Shale–type (BST) biota. Muscle and gut diverticula were initially stabilized by early diagenetic apatite, prior to being pervasively replaced by quartz and then subordinate chlorite, muscovite, and chloritoid during very low- to low-grade metamorphism. Each new mineral replicates the soft tissues with different precision and occurs in particular anatomical regions, imposing strong biases on the biological information retained. Muscovite and chloritoid largely obliterate the tissues’ original detail, suggesting that aluminum-rich protoliths may have least potential for conserving mineralized soft tissues in metamorphism. Overall, the fossils exhibit a marked shift toward mineralogical equilibration with the matrix, obscuring primary taphonomic modes. Sequential replacement of the phosphatized soft tissues released phosphorus to form new accessory monazite (and apatite and xenotime), whose presence in other BST biotas might signal the prior, more widespread, occurrence of this primary mode of preservation. Our results provide critical context for interpreting the Sirius Passet biota and for identifying late-stage overprints in other biotas.

Alteration mineralogy and pathfinder element inventory in the footprint of the McArthur River unconformity-related uranium deposit, Canada

ABSTRACTPathfinder elements associated with the exploration footprint of the McArthur River unconformity-related U deposit include U, radiogenic Pb, V, Ni, Co, Cu, Mo, As, Zn, and rare earth elements. In this study, the mineralogical and paragenetic context for their occurrence was established by integrating in situ mineral chemistry and laser ablation mass spectrometry chemical mapping of interstitial assemblages, detrital grains, and cements with whole-rock analyses of drill core samples from the diagenetically altered background and the hydrothermally altered sandstone host rocks. Diagenetically altered background sandstones contain a matrix assemblage of illite and dickite, with trace to minor aluminum-phosphate-sulfate (APS) minerals, apatite, and Fe-Ti oxide minerals. Aluminum-phosphate-sulfate minerals account for the majority of the Sr and light rare earth element concentrations, whereas early diagenetic apatite, monazite, and apatite inclusions in detrital quartz and detrital zircon contribute significant U and heavy rare earth elements to samples analyzed with an aggressive leach (partial digestion) such as aqua regia. Hydrothermally altered sandstone host rocks also contain variable assemblages of Al-Mg chlorite (sudoite), alkali-deficient tourmaline, APS minerals, kaolinite, illite, Fe-oxide, and sulfide minerals. Late pre-mineralization chlorite accounts for a significant portion of the observed Ni concentrations, whereas Co, Cu, Mo, and Zn occur predominantly in cryptic sub-micron sulfide and sulfarsenide inclusions within clay mineral aggregates and in association with Fe-Ti oxides. Elevated concentrations of U were observed in cryptic micro-inclusions associated with sulfides in quartz overgrowths, with Fe-Ti oxide micro-inclusions in kaolinite, and in post-mineralization Fe-oxide veins. The distribution of pathfinder elements throughout the deposit footprint appears to be less related to the primary dispersion of alteration minerals from the hydrothermal system than to the secondary dispersion of elements post-mineralization. Their occurrence throughout pre-, syn-, and post-mineralization assemblages further demonstrates the limitations to defining geochemical footprints from pathfinder element concentrations expressed in lithogeochemical data sets without structural, lithological, and mineralogical context.

Decay and mineralization of soft tissue in Early Devonian boring ctenostome bryozoans

The Early Devonian of Podolia, Ukraine, has yielded phosphatized colonies of the boring ctenostome bryozoan Podoliapora doroshivi with 3-D preservation of soft tissues. However, the feeding zooids are not anatomically complete, their preserved soft tissues comprising decay-resistant structures such as the protective cuticular polypide sacs with presumed parietal muscles inside the wall of the sacs, the setigerous collars, the membranous orificial walls and remains of the muscle tissues. Early diagenetic apatite mineralization occured in numerous feeding zooids of Podoliapora at different stages of decay and may be important for the interpretation of decay processes in these colonial soft-bodied fossil organisms. A setigerous collar, which is a characteristic of extant ctenostomes, occurs in P. doroshivi in several stages of decay showing progressive collapse and eventual complete loss. This study indicates that the morphological changes of collars induced by decay often resulted in connection with the membranous orificial wall, producing false anatomical structures, unrelated to structures observed in the earlier stages of decay or to the anatomical structures of extant ctenostomes. The most decay-resistant cuticular polypide sacs mineralized as cryptocrystalline apatite in early stage of decay became degraded in later stages of decay. These data provide evidence that the anatomical interpretation of soft-bodied fossils preserved only in the later stages of decay may have led to imprecise morphological interpretations.

The REE-composition and petrography of apatite in 2 Ga Zaonega Formation, Russia: The environmental setting for phosphogenesis

The first significant P-rich deposits appear in the global rock record during the Paleoproterozoic around 2Ga, however the specific triggers that led to apatite precipitation are still under debate. The ca. 2Ga Zaonega Formation, Karelia, Russia contains P-rich intervals in its upper part with abundantly occurring apatite. These apatites have been studied for their Rare Earth Element (REE) composition using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with an emphasis on the environmental condition during phosphogenesis. Petrographic observations by scanning electron microscopy (SEM) integrated with LA-ICP-MS results allow recognition of variously recrystallized apatite, and distinction of best-preserved diagenetic apatite which presumably records syn-depositional REE characteristics. Diagenetic apatite exhibits moderately negative Ce anomalies that indicate an at least partially oxygenated water column. A variable but typically positive Eu anomaly is consistent with geologic evidence suggesting intensive magmatic activity and hydrothermal influence during apatite precipitation. We conclude that phosphatic sediments in the Paleoproterozoic Zaonega Formation record phosphogenesis in a vent/seep influenced setting that experienced fluctuating redox conditions at the sediment–water interface.

REE redistribution during hydrothermal alteration of ores of the Kalahari Manganese Deposit

The Kalahari Manganese Deposit (KMD) is the largest land-based manganese deposit, hosting approximately 80% of the world's known, mineable manganese resources. The deposit, located near Kuruman in the Northern Cape Province of South Africa, is one of five erosional relics of the Paleoproterozoic (ca. 2.2Ga) Hotazel Formation, with sedimentary manganese ores occurring as up to 50m thick beds interbedded with banded iron-formation (BIF) and hematite lutite.The study focuses on the manganese ores of the Nchwaning–Gloria mining area of the northern KMD. In this area, pronounced mineralogical and major element alteration was imparted on the sedimentary manganese ores by a structurally-controlled hydrothermal fluid flow event. Most notable effects of hydrothermal alteration are the decomposition and leaching of Ca- and Mg-carbonate, and marked residual enrichment of manganese. On the basis of mineral assemblage, grade, texture and geochemical characteristics, three ore types were distinguished in the studied sample set, classified into least altered (LA), partially altered (PA) and advanced altered (AA) types. Advanced altered ores may be further classified into five different types, based on mineral assemblages that contain hausmannite and/or braunite as significant minerals. The rare earth element (REE) geochemistry of these fundamental ore types was studied in detail, to document REE mobility during hydrothermal alteration.Total REE concentrations in LA ores were found to be very low (14–22ppm) and remarkably uniform, within the range typically observed for BIF. Hydrothermal alteration results in residual enrichment and a much larger scatter in REE contents. A small Ce anomaly observed in the protolith remains similar in magnitude when observed in PAAS-normalised REE plots. The data define, however, a power trend in the (Ce/Ce*) vs (Pr/Pr*) diagram. Such behaviour is interpreted in terms of a conservative system that was predominantly protolith-buffered. Local remobilisation of REE during hydrothermal alteration is attributed to the dissolution of diagenetic apatite and redistribution of hydrothermal trace minerals, including neoformed apatite, monazite and cerianite.

Metamorphic alteration, mineral paragenesis and geochemical re-equilibration of early Archean quartz–amphibole–pyroxene gneiss from Akilia, Southwest Greenland

Quartz–amphibole–pyroxene gneiss from the island of Akilia, Southwest Greenland has been claimed to contain the earliest traces of life on Earth in the form of biogenic carbonaceous matter encapsulated as inclusions in apatite crystals. Various lines of evidence, including petrography, geochronology, field relations, and geochemistry, have, however, been presented that challenge this interpretation. Textural relationships and geochemical signatures in this controversial gneiss presented here manifest a complex, spatially variable metamorphic history that includes granulite- and amphibolite-facies overprints and metasomatism. A peak metamorphic, granulite-facies, quartz–orthopyroxene–clinopyroxene–amphibole–magnetite assemblage is preserved in only a few centimeter-scale layers within the 5-m-thick, quartz–amphibole–pyroxene gneiss unit. Calcite veinlets that appear to postdate the peak metamorphism occur in pyroxene. The quartz–amphibole–pyroxene gneiss unit has subsequently experienced isochemical (except hydration) amphibolite-facies alteration during which pyroxenes were retrogressed to amphiboles and magnetite, and calcite was consumed. Parts of the quartz–amphibole–pyroxene gneiss that contain texturally late hornblende have experienced metasomatic alteration by Al-carrying fluids. These fluids controlled the alteration of pyroxenes and amphiboles to hornblende, and modified the trace-element composition by remobilizing LREE and Eu. Apatite has variable REE composition and 87Sr/86Sr in the quartz–amphibole–pyroxene gneiss, but on the local scale (cm) is in equilibrium with co-existing silicates. Effective recrystallization of apatite crystals as well as co-existing silicates during several stages of the metamorphic history makes the intact preservation of diagenetic apatite with encapsulated primary carbonaceous matter implausible. Hence, it is highly unlikely that Akilia apatite could serve as repository of the earliest traces of life on Earth.

Orogenesis vs. diagenesis: Can we use organic-rich shales to interpret the tectonic evolution of a depositional basin?

Black shales from the southern Appalachian Basin and the southwest Welsh Basin have anomalous U–Pb and Nd model ages suggesting syn- and post-depositional resetting of the Sm–Nd and U–Pb isotopic systems. This alteration to the primary detrital signature of these two shale sequences is indicative of black shale diagenetic/depositional processes that obscure paleo-environmental and provenance information recorded prior to and during deposition. The trace element and isotopic signatures of these two shale sequences reveal a syn-/post-depositional history that is de-coupled from the coeval orogenic history of the region making it difficult to reconstruct the tectonic and oceanographic conditions present at the time of deposition. Both the Ordovivian Welsh Basin and the Devonian Appalachian Basin sequences host REE- and U-bearing diagenetic phosphate minerals that play a critical role in the whole rock REE and U budgets. In the Welsh Basin shales, early diagenetic apatite and a later monazite phase dominate the REE budget and cause the redistribution of REE early in the basin's history (ca. 460 Ma). This redistribution is recorded by the Sm–Nd system (450 ± 90 Ma) and the Nd model ages that are anomalously old by as much as 20% (T DM > 2.0 Ga). This early history is complicated by a Permo-Triassic fluid event affecting the whole rock U-budget and resetting the U–Pb isotopic system at 193 ± 45 Ma. The Appalachian Basin sequence appears to have a much less complicated history yet still records a significant disturbance in both the Sm–Nd isotopic system (392 ± 76 Ma) and the Pb isotopic system (340 ± 50 Ma) at about the time of deposition (ca. 365 Ma). These two sequences suggest a pattern of diagenetic disturbance common to black shales. These processes are unique to black shales and must be considered when interpreting provenance and paleo-environmental information from the black shale sequences. Although these rocks are susceptible to alteration, the alteration may provide extensive information on the post-depositional history of the basin while still retaining some primary depositional information. If black shale processes are considered during the interpretation of isotopic and trace element signatures from organic-rich shales, it may be possible to recover an extensive basin history.

Postdepositional Sm/Nd Fractionation in Sandstones: Implications for Neodymium-Isotope Stratigraphy

ABSTRACT Sandstone reservoirs from the Norwegian continental shelf commonly show varying degrees of postdepositional fractionation of Sm/Nd ratio, apparently due to preferential incorporation of Sm in the crystal lattice of diagenetic apatite. Based on data from leaching experiments, an apatite mineral separate, and petrography, it is proposed that fractionation occurred mainly during closed-system burial diagenesis, as phosphate deposited on the sea floor recrystallized to apatite and incorporated rare earth elements (REE) from the surrounding rock matrix. As a result, the Sm-Nd model age parameter in general contains two components of information: A) the true crustal age of the sediment and B) a secondary component of Sm/Nd fractionation, which can be either negligible or dominant. It is difficult to separate these components and thus correct for component B. Even in data showing no obvious signs of postdepositional fractionation, model ages may be variably disturbed by subordinate amounts of diagenetic alteration. However, the isotope ratio 143Nd/144Nd is expected to be relatively resistant to alteration by secondary Sm/Nd fractionation and should therefore be used rather than model age as the principal correlation parameter in stratigraphic studies. In this way, Nd-isotope stratigraphy can still be used as a valuable and unique tool for studies of provenance and correlation, even in strata showing strong Sm/Nd fractionation effects. Strongly Sm/Nd-fractionated horizons (recognized by bulk-rock Sm/Nd ratios > 0.23) may also be useful for stratigraphic correlation, but it is clear that their occurrence reflects special depositional conditions rather than shifts in sediment provenance.

Early diagenetic stabilization of trace elements in reptile bone remains as an indicator of Maastrichtian–Late Paleocene climatic changes: evidence from the Naran Bulak locality, the Gobi Desert (South Mongolia)

Abstract Maastrichtian dinosaur bone remains from the Naran Bulak locality (the Gobi Desert) with well-preserved bone textural features are enriched in some trace elements, primarily in REE. These features of vertebrate fossils were formed during diagenesis following rapid burial in mudflow sediments, and prior to postfossilization epigenetic changes. Trace elements are mainly concentrated in diagenetic apatite. Their contents in the bones correlate with that in their enclosing sediments for both maxima and minima. Fossil and sediment compositions were established under the influence of paleoclimate. They are correlated with long-term climatic changes with the aridity maximum at the K/T boundary. Climatic changes were recorded via the change of salinity of waters interacting with the buried vertebrate remains.

Mineralogic controls on REE mobility during black-shale diagenesis

ABSTRACT The Llandeilo-Caradoc black shales of southwest Wales, U.K. record a paragenetic sequence that requires redistribution of rare earth elements (REE) on a mineralogic scale during diagenesis. The main evidence for this redistribution is formation and replacement of REE-rich apatite and monazite and variations in whole-rock REE patterns. The processes that produce these mineralogic changes create a diagenetic overprint on the Llandeilo-Caradoc black shales that obscures geochemical information related to both provenance and paleoenvironmental conditions of deposition. Periods of open-system conditions existed during the diagenetic alteration of the Llandeilo-Caradoc black shales, which allow an evaluation of the extent and timing of the REE redistribution inferred from petrography. The observed variations in La/Sm ratios in the whole-rock REE patterns are strong evidence that REE were redistributed on a scale greater than an individual hand sample. This change in the whole-rock REE pattern altered the Cerium anomaly by as much as 20% and produced a range in the Sm/Nd ratio from 0.14 to 0.20. This change in the whole-rock Sm/Nd ratio indicates a disturbance in the Sm-Nd isotopic system at about 460 Ma that can be directly linked to the formation and dissolution of diagenetic apatite and monazite during diagenesis. One effect of this disturbance is to alter the range in Nd model age for the Llandeilo-Caradoc black shales from 1.6 to 1.8 Ga to about 1.4 to 2.25 Ga, thus complicating any interpretation of provenance. The ability to link petrographic observations from a black shale to measurable whole-rock geochemical changes provides us with one tool to evaluate the presence of a diagenetic overprint on trace elements commonly used in paleoenvironmental reconstruction.

Origin of chlorite enrichment and neodymium isotopic anomalies in Haltenbanken sandstones

Thisis really two papers rolled into one. We have chosen to present them together because both topics derive from the same series of Nd isotopic analyses of some chlorite-coated, Jurassic-age sandstone reservoirs from the Norwegian continental shelf. The first topic is a comparison of the Nd isotopic composition of chlorite-rich vs chlorite-poor sandstones to determine whether systematic differences in sediment provenance have controlled the occurrence of grain-coating Fe-chlorite cement. The results show that both chlorite-rich and chlorite-poor sandstones have a wide range of 143Nd/ 144Nd, consistent with mixing of at least two main provenance terranes of differing mean crustal age in both groups. This does not support the model of chlorite enrichment due to localized influx of an unique Fe-rich provenance component, such as mafic igneous material, but is consistent with an alternative model of chlorite enrichment by Fe-clay authigenesis on the sea-floor, localized by fluvial discharge into a nearshore marine setting. The implication is that studies of depositional setting rather than sand provenance should be the key to predicting occurrence and geometry of high- porosity zones in deeply buried sandstones with chlorite-related porosity preservation. The second topic deriving from this set of Nd isotopic data concerns the discovery of an association between anomalously high SmNd model ages and elevated contents of bulk-rock phosphorous. This sub-set of the samples is anomalous in that the SmNd model ages are too high to be interpreted as detrital provenance ages and also because SmNd model age correlates with Sm/Nd ratio. Furthermore, the anomalous SmNd signature can be removed by acid leaching, leaving a residue with apparently non- anomalous SmNd characteristics. Analysis of an apatite mineral separate from one sample confirms that the anomalous SmNd signature resides within diagenetic apatite. The anomalies are suggested to result from REE fractionation due to the crystal chemistry of apatite, which favors preferential incorporation of the intermediate-weight REE, thus increasing Sm:Nd and SmNd model age. The apatite probably formed by recrystallization of phosphate that was present at the time of sand deposition, either as organic detritus or as the product of precipitation on the sea floor. The isotopic composition of the apatite indicates that Sm/Nd fractionation occurred within the past 100 million years and thus was not syndepositional. Formation of the Nd isotopic anomalies is therefore interpreted to be a burial diagenetic process involving local REE enrichment of apatite and complementary REE depletion of the surrounding rock mass. Experimental apatite:melt partition coe.cients determined at magmatic temperatures predict much lower Sm/Nd fractionation than is observed in the present samples, but we speculate that the degree of fractionation may increase greatly toward the lower, diagenetic temperatures relevant for the present samples. The discovery of these anomalies does not invalidate the use of SmNd model age profiling as a stratigraphic correlation tool and provenance indicator, but indicates that precautions should be taken when dealing with marine sandstones. Specifically, data sets should be examined for correlation between SmNd model age and Sm:Nd ratio, which may indicate the presence of anomalous fractionation effects. Even in such cases, the isotopic ratio 143Nd/ 144Nd can provide a reliable provenance signature. 1998 Elsevier Science Ltd. All rights reserved.

The migration of uranium into peat-rich soils at Broubster, Caithness, Scotland, U.K.

Uranium is being actively transported from uraniferous sedimentary rocks into a peat bog at the Broubster natural analogue site in Caithness, Scotland. Massive calcareous sandstone within the Caithness Flags sequence is the main source of uranium which resides primarily within diagenetic apatite and dispersed USiTi phases. Supergene weathering processes have decalcified the sandstone and are effective in mobilising uranium by groundwater leaching, primarily along a fault zone. Uranium transport in solution by means of groundwater and surface flows is effectively terminated by retardation within 4-kyr-old peat deposits laid down on boulder clay. This process of secondary fixation has resulted in a young uranium anomaly in the peat which comprises in excess of 0.1 wt% U. The site has been investigated comprehensively to define the geometry of the anomaly together with the hydrogeology, hydrochemistry, petrology, mineralogy and the nature of the peat sink-term. The main physical and geochemical properties of the system, including the uranium decay series radionuclide distributions in water and solid samples, are documented in this paper. From these data, the processes governing the distribution of uranium have been quantified using a three-dimensional groundwater flow package and an equilibrium speciation model incorporating a recently developed electrostatic surface complexation model to account for cation-organic interactions. The results described form part of a coordinated project on natural radionuclide migration undertaken to improve confidence in predictive methods used for radiological assessment.

Paleomagnetism and the evolution of fluids in the Proterozoic Athabasca Basin, northern Saskatchewan, Canada

In the Athabasca Basin, diagenetic hematite of variable paragenesis occurs throughout the sandstones and underlying paleoregolith. This hematite carries three distinct, single-component magnetizations: A (D = 158°, I = 62°, α95 = 5°, n = 21); B (D = 11°, I = −36°, α95 = 7°, n = 6); and C (D = 18°, I = 79°, α95 = 3°, n = 27). In some areas of the sandstones, such as near reactivated fault zones, the diagenetic hematite has been altered to goethite which yields a very low-intensity, incoherent D magnetization. Ages for the A, B, and C magnetizations, inferred from comparisons with paleomagnetic directions in Precambrian rocks whose ages are known approximately, are 1750–1600, 1600–1450, and about 900 Ma, respectively. The A magnetization is carried by the earliest formed hematite, and its estimated age compares well with U–Pb ages of 1650–1700 Ma for early diagenetic apatite. U–Pb and Rb–Sr ages of approximately 1500 and 900 Ma for uraninite and illite coeval with hematite that carries the B and C magnetizations compare well with their ages estimated from paleomagnetism. The development of B magnetization appears to be coeval with high-grade, unconformity-type uranium mineralization.Petrographic and field relationships indicate that the A magnetization is carried by hematite formed during initial diagenesis of the Athabasca sandstones, the B magnetization is carried by hematite formed during peak diagenesis, and the C magnetization is carried by hematite formed during subsequent high-temperature hydrothermal alteration. The incoherent D magnetizations have resulted from degradation of hematite to goethite as a result of incursion of low-temperature meteoric waters along fault zones that have been continuously reactivated since the late Precambrian. δ18O values of clay minerals and of the coeval hematite which carries the B and C magnetization indicate that they were formed from a fluid having temperatures of 150–200 °C and δ18O values near 1.0‰. Fluids that deposited the early formed hematite carrying the A magnetism are relatively 18O depleted, with values of approximately 0.8‰ and somewhat lower temperatures of 120–160 °C. Intermingling of A, B, and C magnetizations indicates either that hematite may be deposited by one fluid and reprecipitated by a subsequent fluid, or that fluid flow was controlled by local variations in permeability. Evidently, fluid flow has been episodic and basin wide and has occurred over a time span on the order of 108 years. It is suggested that the stratigraphy of the sandstones controlled the basin-wide lateral migration of the basinal fluids and that faults facilitated interformational fluid flow.

The origin and sedimentary, diagenetic and metamorphic evolution of chlorite–mica stacks in Llandovery sediments of central Wales, U.K.

AbstractChlorite–mica stacks in the Llandovery sedimentary rocks of central Wales have been studied, both within cleaved turbidite mudrocks, and within early diagenetic apatite, carbonate and monazite concretions. Stacks show a graded distribution within turbidite units, indicating an origin as detrital particles. Concretions provided an arrested diagenetic environment which restricted subsequent deformation and so revealed the nature of the stack precursor minerals. These included detrital mica, and also probable pyroxenes, amphiboles and volcanic rock particles, relic textures of which can be seen preserved within the concretions. Diagenetic alteration to chlorite, probably with some white mica intergrowth, occurred, most probably via smectite and then mixed-layer or swelling chlorite intermediates. Subsequent modification, during the Caledonian cleavage-forming episode, determined the present shape and mineralogy of the stacks. Outside concretions, chlorite pseudomorphs were rotated by tectonic compression, crushed at their margins and pulled apart along (001) cleavage and slip planes. Growth of at least two generations of secondary mica infillings has been recognized, and this was probably accompanied by the growth of secondary chlorite. This resulted in barrel-shaped stacks, elongated parallel with cleavage, and with the (001) plane rotated from the bedding towards the perpendicular to the cleavage. Within early diagenetic concretions it is evident that some stacks had formed prior to lithification. However, these have a relatively simple morphology and the original bedding-parallel grain shapes are largely preserved.

Authigenic aluminium phosphate‐sulphates in sandstones of the Mitterberg Formation, Northern Calcareous Alps, Austria

ABSTRACTComplex aluminium phosphate‐sulphate minerals of the hinsdalite group have been identified as early diagenetic precipitates in Late Permian sandstones of the Northern Calcareous Alps, western Austria. According to their chemical composition they can be regarded as solid solutions between woodhouseite, svanbergite, crandallite and goyazite. The model proposed for their origin involves the dissolution of detrital apatite in a low‐pH environment and subsequent precipitation of aluminium phosphate‐sulphate minerals, which clearly pre‐date syntaxial quartz cementation of the sediment. Their occurrence probably has been overlooked in other sandstones showing diagenetic apatite dissolution. The recognition of such minerals could provide a significant insight into early phosphate diagenesis.

Age of the Richmond Gulf Group and implications for rifting in the Trans-Hudson Orogen, Canada

Abstract The Aphebian Richmond Gulf Group of the east coast of Hudson Bay is a fluvial sandstone sequence containing basalt flows. It was shed eastward from a rifting-related thermal dome situated in the present central part of Hudson Bay. Sandstone from the base of the group contains a uraniferous diagenetic apatite cement. U-Pb and Pb-Pb isochron analysis from this material show the age of the apatite-haematite cement to be 2025 ± 25 Ma old. These data provide an older age bracket for the deposition of the Richmond Gulf, Nastapoka and Belcher Groups. They also date the formation of the Richmond Gulf Graben, an aulacogen related to the initial rifting of the Trans-Hudson Orogen.

Biogenic and diagenetic Sr/Ca in Plio-Pleistocene fossils of the Omo Shungura Formation

Under conditions of normal calcium metabolism, strontium/calcium ratios (Sr/Ca) have been shown to reflect the trophic level of contemporary and recent terrestrial fauna. These ratios therefore offer a potential means of studying fossil ecosystems and the diet of prehistoric humans. In cases in which suitable controls have demonstrated the preservation of biogenic Sr/Ca, it has been possible to investigate the proportionate importance of meat vs. vegetable foods in the diets of prehistoric humans. However, diagenetic change after interment has made it impossible to discern biogenic Sr/Ca in faunal and human skeletons over 15,000 y b.p. A procedure is investigated for the analysis of biogenic and diagenetic apatite in vertebrate fossils, on the basis of solubility differences among carbonate, hydroxy-, and fluorapatites. When applied to the 2 ma b.p. fauna of the Omo Basin (Ethiopia), distinct characterization of the herbivore, omnivore, and carnivore fauna in conformity with trophism was discerned, in spite of anomalous Sr/Ca of one highly specialized carnivore,Homotherium.Possible metabolic and/or taphonomic explanations of this anomaly are discussed, and future basic research into the solubility profile procedure is outlined.

Geochemistry of the Hepworth Carboniferous sediment sequence and origin of the diagenetic iron minerals and concretions

The major element geochemistry of a mudstone sequence of Westphalian age has been studied in detail and mineralogical distributions calculated. Quartz and zircon trends indicate the energy of the depositional environment and this is closely reflected in the clay mineralogical composition. Of the detrital clays kaolinite is concentrated in the finer-grained sediments relative to illite. Chlorite and interstratified illite-smectite probably underwent diagenetic modification and small amounts of authigenic clays may have formed. Siderite, which occurs in both concretionary and disseminated form, and pyrite developed during diagenesis under reducing conditions and account for most of the iron present. Pyrite grew first deriving its iron partly from detrital clays. Carbon isotopic evidence suggests that early siderite precipitated in pore space as nodules and later in dispersed form. A second concretionary phase, pistomesite, grew during late dewatering. Siderite iron was derived mainly from Mn-rich oxidate phases. Titanium occurs as anatase uniformly distributed through the sequence. Phosphorus is mainly in diagenetic apatite and the sodium potassium ratio is controlled by the micas. Adsorbed water is closely related to grain size and clay content.