Zircon U–Pb geochronology,whole-rock geochemical,and Sr–Nd–Pb isotopic constraints on the timing and origin of Permian and Triassic mafic dykes from eastern North China Craton

Acta Geochimica - This work reports an important episode of extensional, mafic magmatism that impacted the North China Craton (NCC) during the Permo-Triassic and influenced the evolution of this...     

High‐Precision,High‐Accuracy Oxygen Isotope Measurements of Zircon Reference Materials with the SHRIMP‐SI

Analytical protocols for SHRIMP‐SI oxygen isotope analysis (δ¹⁸O) of a suite of zircon reference materials (RMs) are presented. Data reduction involved a robust estimate of uncertainties associated with the individual spot as well as for groups where the spot data are combined. The repeatability of δ¹⁸O measurements is dependent on both the analytical conditions and the choice of the primary reference material. Under optimised conditions, repeatability was often better than 0.4‰ (2s) allowing sample uncertainties to be obtained to better than 0.2‰ (at 95% confidence limit). Single spot uncertainty combined the within‐spot precision with the scatter associated with repeated measurements of the primary zircon reference material during a measurement session. The uncertainty for individual spots measured under optimised conditions was between 0.3 and 0.4‰ (at 95% confidence). The analytical protocols described were used to assess a variety of zircon RMs that have been used for geochronology and for which laser fluorination oxygen isotope data are available (Temora 2, FC1, R33, QGNG and Ple?ovice), as well as zircons that have been used as RMs for trace element or other types of determination (Mud Tank, Monastery, 91500, AS57, AS3, KIM‐5, OG1, SL13, CZ3 and several other Sri Lankan zircons). Repeated analyses over nine sessions and seven different mounts show agreement within analytical uncertainty for Temora 2, FC1, R33, QGNG, Ple?ovice and 91500, when normalised to Mud Tank. For existing ion microprobe mounts with these materials, an appropriate δ¹⁸O can be determined. However, care should be taken when using zircons from the Duluth Complex (i.e., FC1, AS57 and AS3) as reference materials as our data indicated an excess scatter on δ¹⁸O values associated with low‐U zircon grains.     

Tertiary volcanism in the Cape Hillsborough area,North Queensland

Potassium‐argon ages on alkali feldspar phenocrysts from trachyte lavas and on trachyandesites of the Cape Hillsborough Beds, which crop out north of Mackay, Queensland, yield concordant ages of 32.5 ± 0.4 m.y. This age is Early Oligocene and by correlation provides much firmer control than previously available on the age of the sediments deposited in the Hillsborough Basin. Isotopic ages on an altered biotite and on alkali feldspar from the Mount Jukes Syenite Complex, to the southwest of Cape Hillsborough, are indistinguishable from those measured on the Cape Hillsborough Beds, suggesting a possible genetic relationship between the lavas and the syenite intrusions. Present evidence indicates that Cainozoic volcanism in Queensland occurred in two main eruptive episodes. The earlier episode covered much of the Oligocene and its products are widespread in the southern half of the State. After a long hiatus of little or no volcanism there followed a later episode of volcanism in the Pliocene and Quaternary when widespread basaltic eruptions occurred, mainly in North Queensland.     

Holocene cool‐water carbonate and terrigenous sediments from southern Spencer Gulf,South Australia

Holocene sediments from southern Spencer Gulf are cool‐water carbonate‐rich gravels and sands, dominated by molluscs and Bryozoa. Five sedimentary fades are recognized: (i) molluscan gravel; (ii) branching coralline‐algal gravel, associated with shallow partially protected environments; (iii) molluscan‐biyozoan sand; (iv) mixed bioclastic sand, representative of the deeper central region of the lower gulf; and (v) bryozoan gravel, an isolated fades developed in a semi‐protected micro‐environment. The southern gulf is characterized by complex oceanographic conditions together with variations in water depth and substrate. The sediments share the characteristics of both the southern shelf and upper Spencer Gulf. Grain‐size distribution and sedimentary facies are controlled by a combination of all the above processes. Past sea level fluctuations are recognized from sea floor strand‐line deposits. The relic component of the palimpsest sediments has eroded from the Pleistocene aeolianite dunes. The sediments, therefore, reflect both the modern marine and past environments.     

The Younger Dryas in the English Lake District: reconciling geomorphological evidence with numerical model outputs

The geomorphology of the south‐western and central Lake District, England is used to reconstruct the mountain palaeoglaciology pertaining to the Lateglacial and Younger Dryas. Limitations to previous ice‐mass reconstructions and consequent palaeoclimatic inferences include: (i) the use of static (steady‐state) glacier reconstructions, (ii) the assumption of a single‐stage Younger Dryas advance, (iii) greatly varying ice‐volume estimates, (iv) inexplicable spatial variations in ELA (Equilibrium Line Altitude), and (v) a lack of robust extent chronology. Here we present geomorphological mapping based on aerial photography and the NextMap Britain Digital Elevation Model, checked by ground survey. Former glacier extents were inferred and ELAs were calculated using the Balance Ratio method of Osmaston. Independently, a time‐dependant 2‐D ice‐flow model was forced by a regional ELA history that was scaled to the GRIP record. This provided a dynamic reconstruction of a mountain ice field that allowed for non‐steady‐state glacier evolution. Fluctuations in climate during the Younger Dryas resulted in multiple glacial advance positions that show agreement with the location of mapped moraines, and may further explain some of the ELA variations found in previous local and static reconstructions. Modelling based on the GRIP record predicts three phases: an initial maximum extent, a middle minor advance or stillstand, and a pronounced but less extensive final advance. The comparisons find that the reconstructions derived from geomorphological evidence are effective representations of steady‐state glacier geometries, but we do propose different extents for some glaciers and, in particular, a large former glacier in Upper Eskdale.     

A scree slope rockfall model

Certain observed characteristics of scree slopes; namely concavity of profile, straight slope angle less than the angle of repose, and good size sorting of particles, are not consistent with an angle of repose model for accumulation. An alternative model is proposed based upon rockfall and surface stone movement and is tested against experimental data of particle movement in the field. It is found that the mechanical model of stone movement generated adequately explains the motion of particles on scree slopes and that it is in keeping with the characteristics of many screes. The static features of some Isle of Skye screes were also measured and the straight-concave slope form with good downslope sorting of material, characteristic of the rockfall process, was found to be present.     

Determination of the Rare-Earth Elements and Yttrium In 37 International Silicate Reference Materials By Inductively Coupled Plasma-Atomic Emission Spectrometry

The rare-earth elements (REE) and yttrium have been determined in 37 international rock and mineral reference materials. Samples were prepared using conventional rock-dissolution techniques, and the REE and yttrium were separated from the other constituents using cation-exchange chromatography. The REE (La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, Lu) and yttrium were determined simultaneously by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Results for four well-characterised USGS standard rocks agree favourably with "recommended" values and with REE determinations made previously by workers using ICP-AES, isotope dilution mass spectrometry, instrumental neutron activaton analysis and X-ray fluorescence spectrometry.
Les éléments de terres rares (TR) et yttrium ont été dosés dans 37 échantillons internationaux de réféence. Les échantillons ont été décomposés par dissolution acide et les TR séparés par résine échangeuse de cations. Les TR (La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, Lu) et yttrium ont été dosés simultanément par la spectrométrie d'émission-plasma inductif. Les résultats obtenus pour quatre échantillons de référence bien charactérisés de I'USGS se comparent favorablement avec ceux obtenus par la spectrométrie d'émission-plasma, la dilution isotopique, I'activation neutronique et par la spectrométrie d'é fluorescence-X.     

Determination of food web support and trophic position of the mummichog,Fundulus heteroclitus, in New Jersey smooth cordgrass (Spartina alterniflora), common reed (Phragmites australis), and restored salt marshes

The invasion ofPhragmites australis into tidal marshes formerly dominated bySpartina alterniflora has resulted in considerable interest in the consequences of this invasion for the ecological functions of marsh habitat. We examined the provision of trophic support for a resident marsh fish,Fundulus heteroclitus, in marshes dominated byP. australis, byS. alterniflora, and in restored marshes, using multiple stable isotope analysis. We first evaluated our ability to distinguish among potential primary producers using the multiple stable isotope approach. Within a tidal creek system we found significant marsh and elevation effects on microalgal isotope values, and sufficient variability and overlap in primary producer isotope values to create some difficulty in identifying unique end members. The food webs supportingF. heteroclitus production were examined using dual isotope plots. At both sites, the δ¹³C values ofF. heteroclitus were clustered over values for benthic microalgae (BMI) and approximately midway between δ¹³C values ofSpartina andPhragmites. Based on comparisons of fish and primary producer δ¹³C, δ¹⁵N, and δ³⁴S values, and consideration ofF. heteroclitus feeding habits, we conclude that BMI were a significant component of the food web supportingF. heteroclitus in these brackish marshes, especially recently-hatched fish occupying pools on the marsh surface. A 2‰ difference in δ¹³C betweenFundulus occupying nearly adjacentSpartina andPhragmites marshes may be indicative of relatively less reliance on BMI and greater reliance onPhragmites production inPhragmites-dominated marshes, a conclusion consistent with the reduced BMI biomass found inPhragmites marshes. The mean δ¹³C value ofF. heteroclitus from restored marshes was intermediate between values of fish from naturally occurringSpartina marshes and areas invaded byPhragmites. We also examined the isotopic evidence for ontogenetic changes in the trophic position of larval and juvenileF. heteroclitus. We found significant positive relationships betweenF. heteroclitus δ¹⁵N values and total length, reflective of an increase in trophic position as fish grow.F. heteroclitus δ¹⁵N values indicate that these fish are feeding approximately two trophic levels above primary producers.     

A final summary of measurements of cosmogenic AI26 in stone meteorites

A summary is given of the final results of measurements of cosmogenic Al²⁶ activity in a total of 50 stone meteorites, mainly chondrites, carried out using coincidence gamma-spectrometry.