Covariant Trend Research Articles

Petrological imaging of an active pluton beneath Cerro Uturuncu, Bolivia

Uturuncu is a dormant volcano in the Altiplano of SW Bolivia. A present day ~70 km diameter interferometric synthetic aperture radar (InSAR) anomaly roughly centred on Uturuncu’s edifice is believed to be a result of magma intrusion into an active crustal pluton. Past activity at the volcano, spanning 0.89 to 0.27 Ma, is exclusively effusive and almost all lavas and domes are dacitic with phenocrysts of plagioclase, orthopyroxene, biotite, ilmenite and Ti-magnetite plus or minus quartz, and microlites of plagioclase and orthopyroxene set in rhyolitic groundmass glass. Plagioclase-hosted melt inclusions (MI) are rhyolitic with major element compositions that are similar to groundmass glasses. H2O concentrations plotted versus incompatible elements for individual samples describe a trend typical of near-isobaric, volatile-saturated crystallisation. At 870 °C, the average magma temperature calculated from Fe–Ti oxides, the average H2O of 3.2 ± 0.7 wt% and CO2 typically 0.5 Ma compared to those <0.5 Ma. High microlite contents of domes indicate that effusion rates were probably slowest in dome-forming eruptions. Linear trends in WR major and trace element chemistries, highly variable, bimodal mineral compositions, and the presence of mafic enclaves in lavas demonstrate that intrusion of more mafic magmas into the evolving, shallow plutonic mush also occurred further amplifying local temperature fluctuations. Crystallisation and resorption of accessory phases, particularly ilmenite and apatite, can be detected in MI and groundmass glass trace element covariation trends, which are oblique to WRs. Marked variability of Ba, Sr and La in MI can be attributed to temperature-controlled, localised crystallisation of plagioclase, orthopyroxene and biotite within the evolving mush.

Trends in δ18O and δ13C values in lacustrine tufa mounds: Palaeohydrology of Searles Lake, California

AbstractFour tufa facies have been identified in abundant tufa mounds that accumulated in Searles Lake, California, USA, during the Pleistocene. Covariance of δ18O and δ13C values within the tufa facies has been analysed and correlated with palaeo‐lake history. This study demonstrates that characteristics of δ18O and δ13C values are different in each of the tufa facies and can be related to palaeo‐lake stages. The first facies of tufa formation (P) has relatively low average δ18O and δ13C values (δ18O = −2·7‰ and δ13C = +1·2‰, n = 14). These low values are attributed to rapid increases in the lake volume. This increase was followed by higher average δ18O and δ13C values of both the nodular (N) facies (δ18O = +1·8‰ and δ13C = +4·1‰, n = 24) and the columnar (C) facies (δ18O = +0·6‰ and δ13C = +4·0‰, n = 7). These higher values are interpreted to record a decrease in lake volume due to high evaporation and increased biogenic productivity. Following formation of facies C, the lake was essentially dry during formation of the finely laminated (LC) facies. Facies LC formed subaerially as spring water flowed up through the central conduit of the mounds and cascaded down their periphery. The relatively high δ18O and δ13C values for this facies (δ18O = +1·4‰ and δ13C = +4·4‰, n = 12) are due to evaporation and fast CO2 degassing. Although covariant trends are well‐displayed, variations in lake chemistry alter some of these trends due to periods of hyper‐alkalinity. This study confirms the utility of stable isotopic analyses of lacustrine tufa facies to determine palaeo‐lake history.

Early diagenesis in meteoric versus brackish environments: Example of the Late Oligocene-Early Miocene, littoral, mixed sedimentary succession of Carry-Le-Rouet (southeastern France)

AbstractIn the Carry-Le-Rouet region (southeastern France), the well exposed Late Oligocene-Early Miocene succession allows studying the early diagenesis in meteoric and brackish environments. This area provides unique and continuous outcrops of a mixed sedimentary succession (carbonate-siliciclastic shallow facies) with five particular sedimentary discontinuities resulting from various processes such as subaerial exposure, submarine erosion or bioturbation.Results from the diagenetic characterization (petrography and isotope geochemistry) show that the succession may be divided into three parts. The base of the series is dominated by diagenetic processes associated with brackish water, reflecting a restricted lagoonal environment. Dissolution and mineral replacement occurred directly under the sediment-water interface and a subsequent reprecipitation of the dissolved calcium carbonate took place below this layer. The precipitated calcite is non-ferroan and dull luminescent, showing highly variable stable C- &amp; O-isotopic signatures (−6.31‰ to −0.08‰ for δ13CV-PDB and −5,78‰ to −1,36‰ for δ18OV-PDB), organized in a covariant trend, typical of mixed fluids. The middle part of the series, show a progressive evolution to open marine conditions and records two potential emersion events, associated with the development of meteoric vadose and phreatic conditions. The postulated vadose zones are dominated by mineral replacement rather than leaching and characterized by two cements: a non-ferroan and non-luminescent calcite, changing downward to zoned cement. Isotopic signatures of these calcites are ranging from −6.45‰ to −5.10‰ for δ13CV-PDB and −5,44‰ to −4,17‰ for δ18OV-PDB. Below this zone, a ferroan, non-luminescent calcite (−3.25‰ to −1.18‰ for δ13CV-PDB and −4,93‰ to −3,52‰ for δ18OV-PDB) is observed, suggesting precipitation under reducing conditions associated to a confined phreatic zone, isolated from the overlying zones and the recharge area by a marly interval. The third part of the series (at the top), partly eroded, may have undergone at least one emersion event, with the initial development of a dull cement followed by a non-ferroan, non-luminescent cement. This is linked to a gradual change from downdip slightly reducing to updip oxidizing conditions.The early timing of the diagenetic modifications, the differences of diagenetic patterns observed in each interval and the absence of common trends in the series tend to indicate multiple episodes of early diagenesis, associated to sedimentary discontinuities (more specifically exposure surfaces). These discontinuities act as major controls on the repartition of the diagenetic phases. The excellent preservation of the early diagenetic transformations is linked to the shallow total burial depth, and the lack of mesogenetic and telogenetic imprint. This study also highlighted the influence of the Nerthe massif that acted as a principal meteoric fluid recharge area located less than 1 km away from the sedimentary system.

Aragonite stromatolitic buildups from Santorini (Aegean Sea, Greece): Geochemical and palaeontological constraints of the caldera palaeoenvironment prior to the Minoan eruption (ca 3600 yr bp)

AbstractThe pyroclastic deposits of the Minoan eruption (ca 3600 yr bp) in Santorini contain abundant xenoliths. Most of these deposits are calcareous blocks of laminated‐botryoidal, stromatolite‐like buildups that formed in the shallow waters of the flooded pre‐Minoan caldera; they consist of (i) light laminae, of fibrous aragonite arranged perpendicular to layering, and (ii) dark laminae, with calcified filaments of probable biological origin. These microstructures are absent in the light laminae, suggesting a predominant inorganic precipitation of aragonite on substrates probably colonized by microbes. Internal cavities contain loose skeletal grains (molluscs, ostracods, foraminifera and diatoms) that comprise taxa typical of shallow marine and/or lagoon environments. Most of these forms are typical of warm water environments, although no typical taxa from hydrothermal vents have been observed. Past gasohydrothermal venting is recorded by the occurrence of barite, pyrolusite and pyrite traces. The most striking features of the stable isotopic data set are: (i) an overall wide range in δ13CPDB (0·16 to 12·97‰) with a narrower variation for δ18OPDB (−0·23 to 4·33‰); and (ii) a relatively uniform isotopic composition for the fibrous aragonite (δ13C = 12·40 ± 0·43‰ and δ18O = 2·42 ± 0·77‰, n = 21). The δ13C and δ18O values from molluscs and ostracods display a covariant trend, which reflects a mixing between sea water and a fluid influenced by volcano‐hydrothermal activity. Accordingly, 87Sr/86Sr from the studied carbonates (0·708758 to 0·709011 in fibrous aragonite and 0·708920 to 0·708991 in molluscs) suggests that the aragonite buildups developed in sea water under the influence of a hydrothermal/volcanic source. Significant differences in trace elements have been detected between the fibrous aragonite and modern marine aragonite cements. The caldera water from which the fibrous aragonite crusts formed received an input from a volcano‐hydrothermal system, probably producing diffuse venting of volcanogenic CO2 gas and of a fluid enriched in Ca, Mn and Ba, and depleted in Mg and probably in Sr.

Parallel changes of taxonomic interaction networks in lacustrine bacterial communities induced by a polymetallic perturbation

Heavy metals released by anthropogenic activities such as mining trigger profound changes to bacterial communities. In this study we used 16S SSU rRNA gene high-throughput sequencing to characterize the impact of a polymetallic perturbation and other environmental parameters on taxonomic networks within five lacustrine bacterial communities from sites located near Rouyn-Noranda, Quebec, Canada. The results showed that community equilibrium was disturbed in terms of both diversity and structure. Moreover, heavy metals, especially cadmium combined with water acidity, induced parallel changes among sites via the selection of resistant OTUs (Operational Taxonomic Unit) and taxonomic dominance perturbations favoring the Alphaproteobacteria. Furthermore, under a similar selective pressure, covariation trends between phyla revealed conservation and parallelism within interphylum interactions. Our study sheds light on the importance of analyzing communities not only from a phylogenetic perspective but also including a quantitative approach to provide significant insights into the evolutionary forces that shape the dynamic of the taxonomic interaction networks in bacterial communities.

Depth, degrees and tectonic settings of mantle melting during craton formation: inferences from major and trace element compositions of spinel harzburgite xenoliths from the Udachnaya kimberlite, central Siberia

We examine major and trace element compositions of whole-rocks and minerals of 18 spinel harzburgite xenoliths from the Udachnaya kimberlite in the central Siberian craton. The samples are fresh, modally homogeneous and large enough to provide representative whole-rock samples (>100g); all residual spinel peridotites in our collections that satisfied these criteria were selected for this study. Their Mg# ranges from 0.92 to 0.93. Thirteen xenoliths have 6–22% orthopyroxene (opx) and 0–3% clinopyroxene (cpx); five are opx-rich (31–43%) and contain 3–5% cpx. The low-opx Udachnaya harzburgites define regular co-variation trends on major and minor oxide plots and appear to be pristine melt extraction residues. Their Al–Fe and Al–Mg# trends, in combination with experimental results on melting of fertile peridotites, indicate an origin by ∼40% of polybaric decompression fractional melting between 7−4GPa and ≤1–2GPa. In line with Cr and HREE abundances, garnet was stable in the residues at early melting stages. Thus, the spinel facies cratonic peridotites situated at >40 to 130km (1–4GPa), are residues of melting that took place over a broader depth range; the residues probably underwent density-driven segregation from the denser ambient mantle to form the base of cratonic nuclei. The opx-rich Udachnaya harzbrugites cannot be produced by dry or hydrous melting at 1–7GPa and might be related to fractionation or metamorphic segregation of initial high-P melting residues and their interaction with interstitial liquids retained in the residues during gravity-driven upwelling. Our results, in comparison with published data on arc and fore-arc peridotites, show no evidence that significant enrichments in silica (opx) could be produced during melting or metasomatism in subduction settings, or that initial melting residues were dunites, which were later transformed to harzburgites by melt-rock reactions.

Dolomite cementation and recrystallisation of sedimentary breccias along the Musandam Platform margin (United Arab Emirates)

Two main events of dolomite formation were identified along the southern border of the Late Jurassic Musandam carbonate platform of the UAE. The first dolomitisation phase (type 1) was restricted to specific stratigraphic layers in Jurassic platform limestones that were subsequently brecciated by mass flow and collapse processes on the platform margin. These dolomites are planar-s, have crystal sizes ranging from 5 to 25 µm and exhibit an orange to pink and sometimes zoned red-yellow luminescence. This dolomite phase was formed shortly after deposition by fluids of marine or slightly modified marine composition. The second dolomite phase (type 2) mainly affected dolomite type 1 breccias by recrystallisation, dolomite cementation and replacement. Type 2 dolomites are planar-e and non-planar-c, with crystal sizes between 20 and 70 µm. They have more elongated forms with purple luminescent to non-luminescent cores overgrown by pink to yellow luminescent rims. Stable isotope analyses show a covariant trend between δ 18O and δ 13C from marine (− 4.2 to − 1.8 and + 0.8 to + 2.1‰ VPDB respectively) towards depleted values (− 10.2 and − 8.9‰ VPDB respectively) . This depletion is explained by recrystallisation during type 2 dolomitisation and it is interpreted in terms of high temperatures during precipitation and the incorporation of light carbon as hydrocarbons matured. Dolomite type 2 formation is thought to be the result of tectonically induced fluid flow which supplied hot magnesium-rich fluids. Two possible time scenarios for this fluid flow event are proposed: 1) during thrust emplacement of the tectonic nappes on top of the Musandam Platform in the Late Cretaceous. Volcaniclastic and basaltic rocks of the Hawasina Complex and the Oman-UAE ophiolites are a possible magmatic source for magnesium in this case; or 2) coinciding with the migration of hot, post-evaporative brines along the Hagab thrust, which acted as a fluid conduit during the Cenozoic orogeny.

δ13 C‐δ18 O Covariance: An Effective Indicator of Hydrological Closure for Lakes?

AbstractThe correlation between the δ13 C and δ18 0 in primary carbonates is affected by several factors such as hydrological balance, total CO2 concentrations, climatic condition and lake productivity. The influence of these factors on the δ13 C‐δ18 0 correlation may be different on different time scales. In this paper, two different‐type lakes in southwestern China, Lake Erhai and Lake Chenghai, are selected to investigate the influence of climatic pattern on the δ13 C‐δ18 0 correlation and to evaluate the reliability of the δ13 C‐δ18 0 covariance as an indicator of hydrological closure. The results show that there exists good correlation between the δ13 C and δ18 0 in Lake Erhai (overflowing open lake) and in Lake Chenghai (closed lake). This suggests that the δ13 C‐δ18 0 covariance may be not an effective indicator of hydrological closure for lakes, especially on short time scales. On the one hand, a hydrologically open lake may display covariant δ13 C and δ18 0 as a result of climatic influence. The particular alternate warm‐dry and cold‐wet climatic pattern in southwestern China may be the principal cause of the δ13 C‐δ18 0 covariance in Lake Erhai and Lake Chenghai. On the other hand, a hydrologically closed lake unnecessarily displays covariant trends between δ13 C and δ18 O because of the buffering effect of high CO2 concentration on the δ13 C shift in hyper‐alkaline lakes. We should be prudent when we use the covariance between δ13 C and δ18 O to judge the hydrological closure of lake.

Assessing pedogenic calcite stable-isotope values: Can positive linear covariant trends be used to quantify palaeo-evaporation rates?

Palaeoclimate models suggest enhanced evaporation rates in subtropical regions (15–30° latitude) during greenhouse-world conditions, however, there are no empirical data to support these estimates. Laboratory evaporation experiments have shown that calcites precipitated from variably saturated solutions yield a positive linear covariant trend (PLCT) in δ 18O calcite versus δ 13C calcite values. The goal of the present controlled laboratory experiments is to develop a method to quantify regional palaeo-evaporation rates from palaeosol calcrete PLCTs. Samples of powdered CaCO 3 were dissolved in de-ionized water in pressure sealed containers with a range of elevated atmospheric pCO 2 concentrations for 24 h. The solution was then allowed to evaporate completely from an open container within an incubator with the time of calcite crystallization noted, and aliquots removed for analysis every 24 h. The precipitated calcite produced an array of δ 18O calcite versus δ 13C calcite values that fall upon a PLCT projected from a theoretical meteoric calcite line (MCL). Water analyses yielded δ 18O water enrichments from an initial value of − 4.75‰ VSMOW ranging up to between + 10.0‰ and + 14.8‰. The experimental results show that solutions formed under higher pCO 2 conditions precipitated calcite very early on during evaporation, and thus have δ 18O calcite and δ 13C calcite values that are slightly enriched relative to the MCL. The solutions that formed under low pCO 2 conditions precipitated calcite after much of the fluid had evaporated, and thus yield more enriched δ 18O calcite and δ 13C calcite values. Repeat trials with varying temperature and relative humidity show that the PLCT is steeper under both higher temperature and low relative humidity. The wide range of pCO 2, temperature and relative humidity conditions used simulate meteoric phreatic and meteoric vadose conditions that may affect a calcrete horizon over time during numerous dissolution/precipitation reactions. The results of these experiments show that a dominant factor in the precipitation of vadose calcite is the saturation state of the fluid with respect to CaCO 3, while the primary factors affecting the magnitude and steepness of the PLCT are vadose zone temperature, relative humidity, the starting δ 18O water value and saturation state of the fluid with respect to CaCO 3. Since the pCO 2 of the rooting zone is cross-controlled by the local soil and surface ecology, the magnitude of the PLCT enrichment may not be a direct proxy for palaeo-evaporation rates.

A Yield Model for Integrated Circuits and its Application to Statistical Timing Analysis

A model for process-induced parameter variations is proposed, combining die-to-die, within-die systematic, and within-die random variations. This model is put to use toward finding suitable timing margins and device file settings, to verify whether a circuit meets a desired timing yield. While this parameter model is cognizant of within-die correlations, it does not require specific variation models, layout information, or prior knowledge of intrachip covariance trends. The approach works with a generic critical path, leading to what is referred to as a process-specific statistical-timing-analysis technique that depends only on the process technology, transistor parameters, and circuit style. A key feature is that the variation model can be easily built from process data. The derived results are full-chip, applicable with ease to circuits with millions of components. As such, this provides a way to do a statistical timing analysis without the need for detailed statistical analysis of every path in the design

Sequence stratigraphic controls on synsedimentary cementation and preservation of dinosaur tracks: Example from the lower Cretaceous, (Upper Albian) Dakota Formation, Southeastern Nebraska, U.S.A.

A thin cemented sandstone bed in the Upper Albian Dakota Formation of southeastern Nebraska contains the first dinosaur tracks to be described from the state. Of equal importance to the tracks are stable-isotope (C, O) analyses of cements in the track bed, especially in the context of data derived from generally correlative strata (sandstones and sphaerosiderite-bearing paleosols) in the region. These data provide the framework for interpretations of paleoenvironmental conditions, as well as a novel approach to understanding mechanisms of terrestrial vertebrate track preservation. High minus-cement-porosity (> 47%) and low grain-to-grain contacts (∼2.5) in the track bed indicate early (pre-compaction) lithification. Although phreatic cements dominate, the history of cementation within this stratigraphic interval is complex. Cathodoluminescence petrography reveals two distinct calcite zones in the track-bearing horizon and four cement zones in stratigraphically equivalent strata from a nearby section. The earliest calcite cements from both localities are likely coeval because they exhibit identical positive covariant trends ( δ 18O values of − 9.89 to − 6.32‰ and δ 13C values of − 28.01 to − 19.33‰ VPDB) and record mixing of brackish and meteoric groundwaters. All other calcite cements define meteoric calcite lines with δ 18O values clustering around − 9.42‰ and − 8.21‰ VPDB from the track-bearing horizon, and − 7.74‰, − 5.81‰, and − 3.95‰ VPDB from the neighboring section. Distinct meteoric sphaerosiderite lines from roughly correlative paleosols serve as a proxy for locally recharged groundwaters. Back-calculated paleogroundwater δ 18O estimates from paleosol sphaerosiderites range from − 7.4 to − 4.2‰ SMOW; whereas, meteoric calcite lines from the track horizon are generally more depleted. Differences in cement δ 18O values record changes in paleogroundwater recharge areas over time. Early calcite cements indicate mixing of fresh and brackish groundwaters during the syndepositional lithification of the track horizon. Later calcite cements, however, indicate recharge from a larger catchment basin that extended far inland. Therefore, the cements likely record a rise and subsequent fall in relative sea level. We conclude that scrutiny of the cement isotope geochemistry of genetically significant surfaces, especially track beds, can provide new data for interpreting sea level change.

Lower Eocene carbonate cemented chimneys (Varna, NE Bulgaria): Formation mechanisms and the (a)biological mediation of chimney growth?

In the area of Pobiti Kamani (Varna, northeast Bulgaria), massive carbonate cemented columns (“chimneys”, up to 1.5 m diameter and 8 m high) and horizontal interbeds (≤ 1.5 m thickness) occur in dispersed outcrops over an area of 70 km 2 within loose Lower Eocene sands. Field observations and petrographical and stable isotope geochemical characterisation of four studied locations reveal a relationship between these structures and processes of ancient hydrocarbon seepage. Column and interbed structures both consist of similar well-sorted silt- to sand-sized nummulitic host sediments, predominantly cemented by early diagenetic, low-magnesium calcite. Filamentous textures, about 10 μm in diameter and 80–650 μm long, are only locally detected within interparticle calcite cement of columns. Column samples from two sites reveal a similar, linear and inverse covariant trend of δ 13C– δ 18O values, which was interpreted as a mixing trend between two end member fluid/precipitation conditions, i.e. (1) a methane- and/or higher hydrocarbon-derived carbon member characterised by δ 13C values as low as − 43‰ and marine controlled precipitation conditions with δ 18O of − 1 ± 0.5‰ V-PDB and (2) a member with less contribution of methane which was mixed most likely with less depleted carbon sources explaining δ 13C values ranging up to − 8‰ V-PDB. The corresponding, depleted δ 18O values, with many samples clustering around − 8‰ V-PDB, are interpreted in terms of precipitation at elevated temperatures. This suggests the venting system was not a true “cold” seep, sensu stricto. Furthermore, column cross-transects often document an internal pattern consisting of (concentric) zones with distinct isotopic signatures, which vary between the two end members. The mixing and internal pattern of column isotopic data, together with petrographical observations, are qualitatively interpreted as evidence of alternating precipitation conditions, controlled by varying seepage rates of a single fluid source at depth, during build-up of individual chimney pipes near the sediment surface. Based on several field observations, migration of the hydrocarbon-charged fluids in Lower Eocene times was possibly channelled along NE oriented faults. Isotopic signatures of calcite cemented horizontal interbeds, with depleted δ 18O ratios as low as − 8.88‰ V-PDB and variable δ 13C (− 1‰ to − 16‰, mainly around − 5‰ to − 7‰) suggest that ascending fluids contributed to their cementation or resetted the calcite cement isotopic signature, predominantly during periods of active seepage of warmer fluids. Only few petrographical (and preliminary lipid-biomarker) evidence has been found, pointing to the presence or possibly former activity of microbiota, involved in carbon cycling and calcite precipitation, typical of cold seep settings. This may result from diagenetic alteration of organic components. However, considering the processes of chimney formation, a cementation process, governed by the inorganic oxidation of hydrocarbons in which interstitial oxygen is rapidly consumed without bacterial mediation, is considered.

Platinum-group element systematics in Mid-Oceanic Ridge basaltic glasses from the Pacific, Atlantic, and Indian Oceans

The concentrations of Ir, Ru, Pt and Pd have been determined in 29 Mid-Oceanic Ridge basaltic (MORB) glasses from the Pacific ( N = 7), the Atlantic ( N = 10) and the Indian ( N = 11) oceanic ridges and the Red Sea ( N = 1) spreading centers. The effect of sulfide segregation during magmatic differentiation has been discussed with sample suites deriving from parental melts produced by high (16%) and low (6%) degrees of partial melting, respectively. Both sample suites define positive and distinct covariation trends in platinum-group elements (PGE) vs. Ni binary plots. The high-degree melting suite displays, for a given Ni content, systematically higher PGE contents relative to the low-degree melting suite. The mass fraction of sulfide segregated during crystallization (X sulf ), the achievement of equilibrium between sulfide melt and silicate melts (R eff ), and the respective proportions between fractional and batch crystallization processes (S b ) are key parameters for modeling the PGE partitioning behavior during S-saturated MORB differentiation. Regardless of the model chosen, similar sulfide melt/silicate melt partition coefficients for Ir, Ru, Pt and Pd are needed to model the sulfide segregation process, in agreement with experimental data. When corrected for the effect of magmatic differentiation, the PGE data display coherent variations with partial melting degrees. Iridium, Ru and Pt are found to be compatible in nonsulfide minerals whereas the Pd behaves as a purely chalcophile element. The calculated partition coefficients between mantle sulfides and silicate melts (assuming a PGE concentration in the oceanic mantle at ∼0.007 × CI-chondritic abundances) increase from Pd (∼10 3) to Ir (∼10 5). This contrasting behavior of PGE during S-saturated magmatic differentiation and mantle melting processes can be accounted for by assuming that Monosufide Solid Solution (Mss) controls the PGE budget in MORB melting residues whereas MORB differentiation processes involve Cu-Ni-rich sulfide melt segregation.

Basin-to-platform chemostratigraphy and diagenesis of the Early Cretaceous Vercors Carbonate Platform, SE France

Cathodoluminescence petrography has been combined with bulk-rock and micro-sampled 87Sr/ 86Sr, δ 18O and δ 13C analyses to reconstruct the diagenetic history of Lower Cretaceous (Barremian) limestones exposed at the southern margin of the Vercors carbonate platform in southeastern France. Stratigraphic and geographic trends in these analyses, coupled with mass balance water–rock interaction modeling, have been used to constrain the chemical composition, source, and relative timing of the groundwaters responsible for diagenetic alteration. This study was completed on the Montagnette outcrop, which is composed of two stacked 70-m-thick units of prograding platform-margin limestones that interfinger with marly basinal sediments. The Montagnette exposure is exceptional, in that it is a single complete outcrop of basin-to-platform limestones that are preserved in their original lateral depositional continuity. Cathodoluminescence petrography indicates that the limestones have experienced a complex sequence of carbonate dissolution, precipitation, and fracturing events. Over one hundred bulk-rock and micro-sampled 87Sr/ 86Sr, δ 18O and δ 13C analyses were completed to characterize each major event of deposition and diagenesis. Bulk-rock analyses from the Montagnette basin-to-platform transect exhibited no significant variations in 87Sr/ 86Sr (0.707523 to 0.707555). These values are consistent with estimated Barremian seawater 87Sr/ 86Sr ratios. In contrast, micro-sampled shell material in the platform margin and platform top deposits exhibit large variations in 87Sr/ 86Sr (0.706910 to 0.707450). The least radiogenic Sr (lowest 87Sr/ 86Sr) occurs in shell material located toward the platform, while the more seawater-like ratios occur in sections without shells that are located toward the basin. This pattern implies that selective diagenetic alteration of the shell material took place after basinal brines flowed up into the platform and spread out laterally. The Sr in the diagenetic waters was probably derived from water–rock interaction with Jurassic basinal sediments in the Vocontian Trough. This conclusion is based on: (1) the extremely low 87Sr/ 86Sr composition of shell material in the platform limestone; (2) modeled co-variation trends in 87Sr/ 86Sr, δ 13C, and δ 18O; and (3) the absence of another nearby source of less-radiogenic Sr in the Vercors region.

Mixing-Zone Diagenesis in the Subsurface of Florida and the Bahamas

Abstract Three cores from Great Bahama Bank and southern Florida contain examples of mixing-zone diagenesis where typical shallow-water, aragonite-rich sediments were altered to low-Mg calcite. The 40–50 m thick mixing zones are identified on the basis of downcore covariant trends from depleted meteoric (δ18O = -3.2 ± 0.7‰; δ13C = -1.6 ± 1.8‰) to enriched marine isotopic values (δ18O = +1.0 ± 0.3‰; δ13C = +1.9 ± 0.7‰). The age of the mixing zones varies from late Pliocene to Pleistocene, so these cores provide three independent examples of mixing-zone diagenesis. Mixing-zone diagenesis produced a limestone composed of low-Mg calcite with petrographic fabrics including moldic porosity, blocky to dogtooth calcite cementation, and recrystallization of lime mud to micrite and microspar. The mixing-zone diagenetic model predicts alteration in only part of the mixing zone. Because the alteration recorded in these cores shows continuous alteration through the entire mixing zone, we consider the mixing of waters itself to be largely irrelevant. Rather, diagenesis was probably driven by the difference in solubility between aragonite and calcite with some additional dissolution of aragonite driven by the microbial oxidation of buried organic matter. The mixing zone has been proposed as a likely location for dolomitization. With the exception of some clearly sea-floor dolomite in one core (Unda), the mixing zones in these cores lack dolomite despite the sediment clearly having been altered in the mixing zone. The lack of dolomite in these mixing zones, combined with the extensive literature questioning the chemical reasoning for mixing-zone dolomite, supports rejection of the mixing-zone dolomite model.

Cenozoic calcretes from the Teruel Graben, Spain: microstructure, stable isotope geochemistry and environmental significance

In the Teruel Graben (northeast Spain), laminar and nodular calcretes formed in a variety of Cenozoic deposits, ranging in age from Palaeogene to Pleistocene. These calcretes developed in a relatively small area under the influence of the same highlands. Consequently, differences in their microstructure and isotopic composition (C and O) must be related to differences in host rock, climate, vegetation and duration of development. Nine profiles developed in different sedimentary settings, from proximal areas to lacustrine environments, were studied to determine whether the microstructure and stable isotope composition changed during the Cenozoic. These characteristics might be used as indicators of the prevailing climate and vegetation, and reveal any changes that occurred during this time. Most of the laminar calcretes studied are compound profiles that formed on hard substrates (e.g., Jurassic limestones) or coarse detrital deposits (e.g., Palaeogene, Pliocene and Plio-Pleistocene sandstones and conglomerates). These profiles formed in proximal areas of the basin and also in fluvial terraces. Microstructures include biogenic features such as alveolar septal structures, root tubes, calcified root cells and calcified organic filaments. Underside coatings of micrite and fibrous vadose cements are common around gravel clasts. On the other side, spherulites are only preserved at the very top of the youngest calcretes. Nodular calcretes developed on fine detrital substrates (e.g., on Palaeogene, Miocene and Pliocene red mudstones and sandy mudstones of distal alluvial and floodplain environments), and in some places grade vertically into palustrine limestones. The nodules consist of micrite and were elongated vertically, show desiccation cracks and mottling, micritic coatings, some alveolar septal structures, and root traces. Microcodium of type 1 occurs in both laminar and nodular calcretes, but only in those of Palaeogene age, while calcified root cells ( Microcodium of type 3) are seen in the Pliocene calcretes. The stable isotope composition covers a wide range: −8.23< δ 18O<−4.29‰ PDB and −7.29< δ 13C<−3.42‰ PDB. Nodular, laminar and palustrine facies occupy overlapped isotopic fields, displaying an overall negative covariant trend. Location within the basin, prevailing climate and density of vegetation are responsible for differences in isotope values. The Palaeogene samples show a wide range of values and are difficult to separate from the others; however, there is as significant change to lighter carbon values from the Miocene to the Pliocene profiles. This change may be related to the cooler climatic conditions that prevailed during the Pliocene and is reverse to the expected shift to heavier δ 13C values, which is related to the increase of C 4 vegetation, that occurred in many areas of the world during the Miocene or to the transit to the Pliocene. The formation of these calcretes seems to have occurred in semi-arid conditions with prevailing C 3 vegetation.

Why allanite may swindle about its true age

Allanite from the Tertiary Rieserferner pluton (Austrian Alps) is texturally and chemically heterogeneous. Continuous covariation trends reflect coupled substitution of Ca+Al vs. Fe+REE+Th in allanite, whereas systematic variations in La/Nd demonstrate the increasingly stronger depletion of LREE in the melt during its crystallization. Allanite samples (corrected for 206Pbexcess) from two rocks scatter in the concordia diagram and define discordias from 31.8±0.4 Ma and 32.2±0.4 Ma to ca. 540 Ma. The apparent inheritance does not originate from the inclusion of older allanite or a high-μ phase, such as monazite, xenotime, or zircon, but from the incorporation of radiogenic Pb originating from a precursor. Since allanite requires a high enrichment of Th, (U), and LREE, it may form at the expense of a Th-LREE-rich precursor in metamorphic rocks or where such a phase had dissolved in melts. Likely precursors acquire with time radiogenic Pb isotopic compositions. This Pb, if incorporated in the product mineral, may give the illusion of inheritance. The allanite samples from the Rieserferner pluton show a tendency from high Th/Ucalc (30–50) in samples with an Alpine age to low Th/Ucalc (4–12) in samples with distinct inheritance. This relation between extent of inheritance and apparent Th/Ucalc indicates a lower Th/Ucalc for a possible precursor, falling into the Th/U range commonly encountered for monazite. Precursor monazite would have originated from assimilated Palaeozoic rocks and give rise to localized enrichments of Th and LREE in the melt, thus eventually enabling the growth of allanite.

Sedimentological and palaeohydrological responses to tectonics and climate in a small, closed, lacustrine system: Oligocene As Pontes Basin (Spain)

ABSTRACT A small, closed, lacustrine system developed during the restraining overstep stages of the Oligocene As Pontes strike‐slip basin (Spain). The increase in basin accommodation and the headward spread of the drainage, which increased the water input, triggered a change from shallow, holomictic to deeper, meromictic conditions. The lower, shallow, lacustrine assemblage consists of mudstone–carbonate cycles recording lacustrine–palustrine ramp deposition in a saline lake. High Sr content in some early diagenetic calcites suggests that aragonite and calcite made up the primary carbonate muds. Early dolomitization took place together with widespread pedogenic activity. The upper, deep, freshwater, lacustrine assemblage includes bundles of carbonate–clay rhythmites and fine‐grained turbidite beds. Primary calcite and diagenetic siderite make up the carbonate laminae. The Mg content of the primary carbonates records variations in Mg/Ca ratios in lacustrine waters. δ18O and δ13C covariance trends in calcite reinforce closed drainage conditions. δ18O data indicate that the lake system changed rapidly from short‐lived isotopically light periods (i.e. from seasonal to pluriannual) to longer steady‐state periods of heavier δ18O (i.e. from pluriannual to millennial). The small δ13C changes in the covariant trends were caused by dilute inflow, changing the contributions of dissolved organic carbon in the system and/or internal variations in lacustrine organic productivity and recycling. In both shallow and deep carbonate facies, sulphate reduction and methanogenesis may account, respectively, for the larger negative and positive δ13C shifts recorded in the early diagenetic carbonates (calcite, dolomite and siderite). The lacustrine system was very susceptible to high‐frequency, climatically forced water balance variations. These climatic oscillations interfered with the low‐frequency tectonic and morphological changes in the basin catchment. This resulted in the superposition of high‐order depositional, mineralogical and geochemical cycles and rhythms on the lower order lacustrine infill sequence.

Short-term environmental change in a Jurassic lagoon deduced from geochemical trends in aragonite bivalve shells

The Bathonian age Inverbrora Member of the Brora Coal Formation, northern Scotland, was deposited in a predominantly fresh- water coastal lagoon setting. However, two prominent shell beds within the mostly argillaceous succession have been interpreted as resulting from temporary inundation of marine water and establishment of brackish conditions. The ensuing lagoon hydrology and paleoenvironment were investigated by measuring stable isotopic (δ18O and δ13C) and trace element (Mg, Sr, Mn, Ba) variations within five exceptionally well preserved aragonite shells of the bivalve Isognomon murchisoni . Transects were sampled across the visible growth banding with a resolution of 0.25−0.15 mm for stable isotopes, and 0.05 mm for trace elements. Oxygen and carbon isotope data define a broadly covariant trend (δ18O between +0.4‰ and −4.8‰ Peedee belemnite [PDB]; δ13C between +2.2‰ and −0.7‰ PDB) that supports a variably brackish-water setting with slight seasonal evaporation. Absolute trace element values vary from shell to shell but show consistent patterns of covariation. Profiles of Mg/Ca across shell layers display multiple sharp, episodic minima that are coincident with peaks in Sr/Ca and Ba/Ca. This is interpreted as documenting inundation of the restricted, brackish-water lagoon by relatively cooler seawater, leading to a temporary increase in metabolic activity of the bivalves. Less frequent minima in Mg/Ca, Sr/Ca, δ18O, and δ13C record fresh-water influx and dramatic salinity reduction in the lagoon. Bivalve metabolic efficiency was decreased, and in some cases breaks in shell growth occurred. This study shows that in hydrodynamically complex settings, trace element variation in bivalve shells can provide a detailed record of short-term environmental change.

Correlating paleomagnetic, geochemical and petrographic evidence to date diagenetic and fluid flow events in the Mississippian Turner Valley Formation, Moose Field, Alberta, Canada

Petrographic, geochemical and paleomagnetic analyses of the Mississippian Turner Valley Formation provide constraints on diagenesis and fluid flow events in the Western Canada Sedimentary Basin. Paleomagnetic plugs and companion geochemical samples were taken from two drillcores, with Fullbore MicroImage log orientations. Dolomite from both wells yielded two magnetization directions. The low-temperature, low-coercivity direction is a drilling-induced remanence rather than a viscous remanent magnetization. The high-temperature, high-coercivity remanence direction is Cretaceous, and there is no sign of a primary Mississippian direction. Geochemical analyses of matrix dolomite yield δ 18 O values ranging from 0.65 to −3.34‰ (VPDB standard) and δ 13 C values ranging from 1.77 to 3.05‰ VPDB. The least depleted samples have stable isotope values consistent with, or only slightly depleted from, postulated Mississippian dolomite values. The remaining sample values exhibit a negative covariant trend consistent with either mixing with another diagenetic fluid or recrystallization during burial. Petrographic analysis reveals the presence of a recrystallization event that caused zoning and a gradual increase size of the dolomite crystals. This event is thought to have caused both the Cretaceous paleomagnetic remanence and the altered geochemical values. The minor enrichment in Sr radiogenic isotopes, relative to coeval seawater values, suggests that both an extrabasinal source for any fluid and large-scale fluid flow are unlikely. The results also indicate that magnetic remanences are very sensitive to visually minor changes in carbonate recrystallization from heat or pressure, so that great care must be taken in correlating paleomagnetic and geochemical data.