The hydrodynamics of particle clusters and sediment entrapment in coarse alluvial channels

ABSTRACT Particles projecting from the bed of an alluvial channel distort the fluid stream to produce a distinctive pressure field. This has considerable significance for both the entrapment and entrainment of other particles and is a primary cause of the widespread occurrence of pebble clusters and boulder shadows. Lift and drag forces are determined on clustered hemispherical particles of varying size. In the wake of an obstructing particle both forces are shown to vary directly with particle separation in a linear fashion. On the stoss side of the cluster, drag is uniform regardless of the separation of the component particles, but lift is shown to increase when particle separation is small, so affecting stability. This mutual interference of neighbouring clustered bed particles is a vital consideration of incipient motion and is shown by field evidence to cause a wide range in transport stage for particles of similar size and shape. On average, 46% of clustered particles are entrained by flood flow compared to 87% of particles in open plane-beds. The influence of clusters is a major determinant of sedimentary sorting.     

Chemical controls of cathodoluminescence of natural dolomites and calcites: new data and review

ABSTRACT Calcites and dolomites from the late Precambrian Bonahaven Formation of Scotland exhibit luminescence when MnCO₃>0.03 wt % (dolomite) or >0.06 wt % (calcite) and FeCO₃<2-2-5 wt % (dolomite). Comparison with other published data shows close similarity in the position of Fe-quenching in dolomite, but more variability in the position of Mn-activation in calcites and dolomites and the onset of Fe-quenching in calcites. These anomalies are thought to relate to chemical controls subsidiary to the dominant factors of Fe and Mn concentration. It is not practical to make a useful comparison between operating conditions of different luminescence instruments because of differences in design and an inability to measure routinely the current actually on the specimen surface.     

The Klokken Gabbro-Syenite Complex, South Greenland: Quantitative Interpretation of Mineral Chemistry

The layered syenite series in the Klokken stock formed by continuousin situ fractionation of a trachytic magma in a chamber linedby gabbro with 3000 m of cover rocks. The following mineralsand reactions are assessed as geothermometers and barometers:two feldspars; hypersolvus ternary feldspars; ferrohedenbergite-ß-wollastonite;clinopyroxene-olivine Fe-Mg exchange; Fe-Ti oxides; sanidine-magnetite-annite;ferroedenite stability. Estimates of silica activity are obtainedfrom the silica-magnetite-fayalite assemblage. The gabbros ended magmatic crystallization at > 1000–1050°C.The less fractionated members of the syenite range probablycrystallized with P_H2O < P_total, at T > 870°C and,P_H2O 800 bars. In the more fractionated syenites P_H2O = P_totalduring intercumulus feldspar growth, and all magmatic phasescrystallized within the interval 940–830°C and P_H2O< 1100 bars. Magmatic f_O2 (bars) was 1 log unit below theQFM buffer. a_SIO2 in gabbros was slightly above the albite-nephelinebuffer, but rose suddenly to just <1 in the syenites, a jumpmirrored by minor elements in pyroxenes and opaque oxides. Biotitegrew subsolidus in most rocks, at f_O2 < QFM, except in intermediaterocks when f_O2 > QFM and was defined by the sanidine-magnetite-biotiteassemblage. In these rocks P_H2O of 450 bars at 760°C isobtained using existing experimental data, but application ofthis data to Fe-rich biotites in the layered series (where biotiteis an intercumulus phase) requires P > 10 kb at magmatictemperatures. High TiO₂ or F: OH probably accounts for increasedT stability of natural annites at low P. The syenitic liquid fractionated down a low temperature zonein a multicomponent system precipitating alk fsp + ol + cpx+ mt and the more fractionated members of the layered serieshad a negligible crystallization interval, a prerequisite forthe development of the unique Klokken-type inversely gradedmineral layering.     

An electromagnetic device for automatic detection of bedload motion and its field applications

A new sensor for the continuous and unmanned detection and recording of bedload motion is described. The sensor acts in the same fashion as a conventional metal detector but is elongate and installed permanently in the bed of an alluvial channel. Artificial clasts are labelled with short lengths of ferrite rod and are seeded upstream from the sensor, replacing bed particles. The entrainment of seeded clasts takes them over the sensor where they distort the magnetic field and produce a change in inductance that is detected and recorded. A field installation demonstrates the value of the sensor by revealing for the first time in coarse-grained alluvium the spasmodic nature of particle motion reminiscent of kinematic waves. It also illustrates the importance of pebble clusters in delaying particle entrainment.     

The oxygen isotopic properties of olivines in the Semarkona ordinary chondrite

Abstract— In order to explore the origin of chondrules and the chondrites, the O isotopic compositions of nine olivine grains in seven chondrules from the primitive Semarkona LL3.0 chondrite have been determined by ion microprobe. The data plot in the same general region of the three-isotope plot as whole-chondrule samples from ordinary chondrites previously measured by other techniques. There are no significant differences between the O isotopic properties of olivine in the various chondrule groups in the present study, but there is a slight indication that the data plot at the ¹⁶O-rich end of the ordinary chondrite field. This might suggest that the mesostasis contains isotopically heavy O. The olivines in the present study have O isotopic compositions unlike the ¹⁶O-rich olivine grains from the Julesburg ordinary chondrite. Even though olivines in group A chondrules have several properties in common with them, the ¹⁶O-rich Julesburg olivines previously reported are not simply olivines from group A chondrules.     

A nitrogen and argon stable isotope study of Allan Hills 84001: Implications for the evolution of the Martian atmosphere

Abstract— The abundances and isotopic compositions of N and Ar have been measured by stepped combustion of the Allan Hills 84001 (ALH 84001) Martian orthopyroxenite. Material described as shocked is N-poor ([N] ～ 0.34 ppm; δ¹⁵N ～ +23%); although during stepped combustion, ¹⁵N-enriched N (δ¹⁵N ～ +143%) is released in a narrow temperature interval between 700 °C and 800 °C (along with ¹³C-enriched C (δ¹³C ～ +19%) and ⁴⁰Ar). Cosmogenic species are found to be negligible at this temperature; thus, the iso-topically heavy component is identified, in part, as Martian atmospheric gas trapped relatively recently in the history of ALH 84001. The N and Ar data show that ALH 84001 contains species from the Martian lithosphere, a component interpreted as ancient trapped atmosphere (in addition to the modern atmospheric species), and excess ⁴⁰Ar from K decay. Deconvolution of radiogenic ⁴⁰Ar from other Ar components, on the basis of end-member ³⁶Ar/¹⁴N and ⁴⁰Ar/³⁶Ar ratios, has enabled calculation of a K-Ar age for ALH 84001 as 3.5–4.6 Ga, depending on assumed K abundance. If the component believed to be Martian palaeoatmos-phere was introduced to ALH 84001 at the time the K-Ar age was set, then the composition of the atmosphere at this time is constrained to: δ¹⁵N ≥ +200%, ⁴⁰Ar/³⁶Ar ≤ 300 and ³⁶Ar/¹⁴N ≥ 17 × 10^?5. In terms of the petrogenetic history of the meteorite, ALH 84001 crystallised soon after differentiation of the planet, may have been shocked and thermally metamorphosed in an early period of bombardment, and then subjected to a second event. This later process did not reset the K-Ar system but perhaps was responsible for introducing (recent) atmospheric gases into ALH 84001; and it might mark the time at which ALH 84001 suffered fluid alteration resulting in the formation of the plagioclase and carbonate mineral assemblages.     

A Strontium Isotopic Investigation of the Bjerkreim--Sokndal Layered Instrusion, Southwest Norway

The 230 km² Proterozoic Bjerkreim—Sokndal layered instrusionhas a monzonoritic bulk composition and comprises a 6 km thick,broadly leuconoritic Layered Series (LS) overlain by unlayeredmangerite and quartz mangerite. In the Bjerkreim lobe the LScomprises six megacyclic units (MCU 0, IA, IB, II, 11 and IV)in a syncline. This lobe is surrounded by migmatitic gneisses;the roof to the instrusion is missing. The mg-number opx, An% and whole-rock initial ⁸⁷Sr/⁸⁶Sr isotope ratios (Sr₀) displaybroadly parallel trends through MCUs II and III, and into MCUIV, with decreasing mg-number (75–58) and An % (50–40)accompanying increasing Sr₀ (0.7050–0.7085). This correlationbreaks down in the upper part of MCU IV and mangerite; the quartzmangerite has Sr₀ values of 0.7085. Abrupt reversals occur acrossthe MCU boundaries. The LS crystallized on the floor of a periodically replenishedmagma chamber that was continually assimilating country rockgneisses. Strong compositional zoning of the magma developedas a result of repeated replenishments with relatively dense,primitive ferrobasaltic magma (Sr₀ = 0.7049) along the floorand the development of extensive buoyant roof melts. Assimilationtook place on a massive scale (up to 50%) in the upper partsof the chamber. KEY WORDS: assimilation; fractional crystallization; hybrid magma; layered intrusion; Sr-isolopes ^*Corresponding author.     

Biotites as Indicators of Fluorine Fugacities in Late-Stage Magmatic Fluids: the Gardar Province of South Greenland

The major element chemistry and fluorine contents of biotitesfrom the Gardar alkaline province of South Greenland providean insight into the F contents of late-stage fluids associatedwith the magmatism. Biotites were taken from composite intrusionsranging from alkali gabbro to syenites, nepheline syenites andalkali granites. In each complex they show a large range ofFe/(Fe+Mg) (from 0•2 to 1) and exhibit strong Fe-F avoidance.There is considerable variation in F for any value of Fe/(Fe+Mg)but for each centre maximum F values plotted against Fe/(Fe+Mg)define a nearly straight line of characteristic slope towardslow or zero F at pure annite. Micas in the SiO₂ undersaturatedcentres have higher F contents than those from oversaturatedcentres. Cl is low (0•69 wt%) except in the Kûngnâtintrusion, where it reaches 1•4 wt%. Phase equilibriumand textural considerations suggest that most or all biotitesgrew subsolidus in a pervasive deuteric fluid. ¹⁸O values suggestthat these fluids were largely magmatic in character and thatextensive reactions with envelope fluids did not occur. Metasomaticresetting of F in biotites in early intrusive units in the aureoleof later units can be demonstrated. Experimental data of Munoz (Mineralogical Society of America,Reviews in Mineralogy 13, 469–494, 1984) were used tocalculate families of curves showing variation in F with Fe/(Fe+Mg)for biotites in equilibrium with fluids of fixed fugacity ratio,f(HF)/f(H₂O), at fixed P and T. The resulting curves cut sharplyacross the maximum fluorine lines observed in the natural examples.As it seems highly unlikely that changes in fluid compositionand T, acting together, could produce the observed linear relationship,we conclude that the partitioning of F between fluid and micain the plutonic environment is not well modelled by the experiments.Possible explanations are short-range order (SRO) of Fe andMg on octahedral sites in biotite (Mason, Canadian Mineralogist30, 343–354, 1992) or effects resulting from differentF speciation in alkaline fluids. If perfect Fe-F avoidance isassumed, Fe-Mg SRO can increase maximum F content. The F levelsseen in the silica-saturated centres are broadly consistentwith a cooling-rate related control. It is possible that thehigher F in biotites in the undersaturated centres reflectsdifferent speciation in the fluid rather than higher F contents,with strongly bound SiF₄° complexes more common. KEY WORDS: biotites; Gardar; fluorine; alkaline rocks; fluids     

Carbon-isotope stratigraphy in shallow-water carbonates: implications for Cretaceous black-shale deposition

A Barremian to Albian succession on Mount Kanala, part of a Tethyan isolated carbonate platform, was investigated for its δ¹³C variations. The limestone sequence is composed of a series of peritidal shallowing-upward cycles with clear petrographic evidence for strong early diagenetic overprinting related to repeated subaerial exposure. Despite significant impact of diagenesis, the observed changes in δ¹³C can be very well correlated with deep-water sections from different ocean basins and shallow water carbonate platforms in the Middle East. This lends further support to the applicability of δ¹³C variations for stratigraphic purposes in shallow-water limestones. Using the δ¹³C signal, time resolution in Lower Cretaceous platform carbonates can be significantly increased, independent of bio-zonations often hampered by ecological variability.
Cyclostratigraphic analysis of the Aptian part of the section shows that strong positive excursions of the cumulative departure from mean cycle thickness of the peritidal shallowing-upward cycles coincide with global positive δ¹³C excursions. This, and the fact that positive shifts in the δ¹³C record are preserved within shallow water limestones, provide evidence that black-shale accumulation in the ocean basins occurred during sea-level rise and flooding of platform tops. Integration of carbon-isotope-, cyclo- and sequence-stratigraphic results from different carbonate platforms indicate that strong positive global δ¹³C shifts and concurrent organic-carbon burial during black-shale deposition are ultimately caused by rapid rises of eustatic sea level. Hence, the rate of change of eustatic sea level is considered to play a crucial role in black-shale accumulation in the global ocean basins during the Cretaceous.     

Fluid Flow During Contact Metamorphism at Mary Kathleen, Queensland, Australia

Corella marbles in the Mary Kathleen Fold Belt were infiltratedby fluids during low-pressure (200-MPa) contact metamorphismassociated with the intrusion of the Burstall granite at 1730–1740Ma. Fluids emanating from the granite [whole-rock (WR) ¹⁸O=8.1–8.6%]produced Fe-rich massive and banded garnet—clinopyroxeneskarns [¹⁸O(WR)=9.1–11.9%]. Outside the skarn zones, marblemineralogies define an increase in temperature (500 to >575C) and XCO₂ (0.05 to >0.12) towards the granite, andmost marbles contain isobarically univariant or invariant assemblagesin the end-member CaO–MgO–Al₂O₃–SiO₂–H₂O–CO₂system. Marbles have calcite (Cc) ¹⁸O and ¹³C values of 12.3–24.6%and –1.0 to –3.9%, respectively. A lack of down-temperaturemineral reactions in the marbles suggests that pervasive fluidinfiltration did not continue after the thermal peak of contactmetamorphism. The timing of fluid flow probably correspondsto a period of high fluid production and high intrinsic permeabilitiesduring prograde contact metamorphism. The petrology and stableisotope geochemistry of the marbles suggest that these rockswere infiltrated by water-rich fluids. If fluid flow occurredup to the peak of contact metamorphism, the mineralogical andisotopic resetting is best explained by fluids flowing up-temperaturetoward the Burstall granite. However, if fluid flow ceased beforthe peak of regional metamorphism, the fluid flow directioncannot be unambiguously determined. At individual outcrops,marble ¹⁸O(Cc) values vary by several permil over a few squaremetres, suggesting that fluid fluxes varied by at least an orderof magnitude on the metre to tens-of-metre scale. Fluids werefocused across lithological layering; however, mesoscopic fracturesare not recognized. The focusing of fluids was possibly viamicrofractures, and the variation in the degree of resettingmay reflect variations in microcrack density and fracture permeability.The marble—skarn contacts represent a sharp discontinuityin both major element geochemistry and ¹⁸O values, suggestingthat, at least locally, little fluid flow occurred across thesecontacts.