Fine-scale Layering Research Articles

Integrated seismic analysis of the Chalk Group in eastern Denmark—Implications for estimates of maximum palaeo-burial in southwest Scandinavia

The origin of the topography of southwest Scandinavia is subject to discussion. Analysis of borehole seismic velocity has formed the basis for interpretation of several hundred metres of Neogene uplift in parts of Denmark. Here, refraction seismic data constrain a 7.5 km long P-wave velocity model of the Chalk Group below the Stevns peninsula, eastern part of the Danish Basin. The model contains four layers in the ~ 860 m thick Chalk Group with mean velocities of 2.2 km/s, 2.4 km/s, 3.1 km/s, and 3.9–4.3 km/s. Sonic and gamma wireline log data from two cored boreholes represent the upper ~ 450 m of the Chalk Group. The sonic velocities are consistent with the overall seismic layering, although they show additional fine-scale layering. Integration of gamma and sonic log with porosity data shows that seismic velocity is sensitive to clay content. In intervals near boundaries of the refraction model, moderate increases in clay content correlate with reduction of porosity and increase in velocity. Higher clay contents do not further increase velocity. The reduction of porosity and increase in velocity are interpreted as clay causing increased pressure dissolution and cementation. The interpreted velocities are systematically higher than values of a chalk velocity curve determined in previous studies, and it is estimated that a significant part of the velocity anomaly may be explained by the presence of clay. The remaining velocity anomaly can be explained by 450–500 m palaeo-burial of the Chalk Group. The burial anomaly will be over-estimated by ~ 150–200 m if the analysis is based on the average Chalk Group velocity and clay content is disregarded. Burial anomaly values of ~ 450–600 m result if the strongest velocity contrast at ~ 600–650 m depth is interpreted to be a result of diagenetic effects, consistent with the clay-corrected estimates within uncertainty.

Preservation of layered paleodeposits in high-latitude pedestal craters on Mars

An outstanding question in Mars’ climate history is whether or not pedestal craters represent the armored remnants of ice-rich paleodeposits. We address this question using new high-resolution images; in a survey of several hundred high-latitude pedestal craters, we have identified 12 examples in which visible and/or topographically expressed layers are exposed on the marginal scarp of the pedestal. One example, located on the south polar layered deposits, preserves ice-rich layers that have otherwise been completely removed from the polar cap. These observations provide empirical evidence that the pedestal crater formation mechanism is capable of armoring and preserving ice-rich layered paleodeposits. Although layered exposures have not yet been observed in mid-latitude pedestal craters, high-latitude instances of discontinuous, partially covered layers suggest that layers can be readily concealed, likely through mantling and/or mass wasting processes along the marginal scarp. This interpretation is supported by the observation that high-latitude pedestals with exposed layers along their margins are, on average, taller than mid-latitude examples, and have larger, steeper marginal scarps, which may help to maintain layer exposures. These observations favor the interpretation that mid- to high-latitude pedestal craters represent the armored remnants of ice- and dust-rich paleodeposits, which occurred transiently due to changes in the climate regime. Preservation of fine-scale layering of ice and dust at these latitudes implies that the climate change did not involve regional melting conditions.

Role of fine-scale layering and grain alignment in the electrical anisotropy of marine sediments

Electrical resistivities of seafloor sediments determined by controlled source electromagnetic (CSEM) surveys have been found to be significantly greater than those measured by electrical well logging, in some instances by a ratio of as much as 5:1. Because borehole logging techniques invariably measure electrical resistivity using currents circulating in horizontal planes and CSEM surveys are sensitive to the currents circulating in vertical planes, a possible cause of this discrepancy is strong electrical anisotropy of the sediments. We have examined electrical log data from vertical exploration and appraisal wells and deviated production wells in the North Sea. By correlating the same sedimentary units between wells, we are able to compare resistivities measured at different borehole inclination angles. As the inclination angle changes, the amount of vertical and horizontal resistivity contributing to the resistivity changes. Hence we can estimate the electrical anisotropy in the sediments. Results indicate that anisotropy ratios of 1:5 and greater are present within the shale-dominated units. We show that fine-scale horizontal layering can make only a relatively small contribution to this anisotropy, but that a model of horizontal alignment of highly oblate spheroidal grains can account for most or all of it.

Seismic reflectivity of the sediment-covered seafloor: effects of velocity gradients and fine-scale layering

SUMMARY Knowledge of the reflectivity of the sediment-covered seabed is of significant importance to marine seismic data acquisition and interpretation as it governs the generation of reverberations in the water layer. In this context pertinent, but largely unresolved, questions concern the importance of the typically very prominent vertical seismic velocity gradients as well as the potential presence and magnitude of anisotropy in soft surficial seabed sediments. To address these issues, we explore the seismic properties of granulometric end-member-type clastic sedimentary seabed models consisting of sand, silt, and clay as well as scale-invariant stochastic layer sequences of these components characterized by realistic vertical gradients of the P- and S-wave velocities. Using effective media theory, we then assess the nature and magnitude of seismic anisotropy associated with these models. Our results indicate that anisotropy is rather benign for P-waves, and that the S-wave velocities in the axial directions differ only slightly. Because of the very high P -t oS-wave velocity ratios in the vicinity of the seabed our models nevertheless suggest thatS-wave triplications may occur at very small incidence angles. To numerically evaluate the P-wave reflection coefficient of our seabed models, we apply a frequency-slowness technique to the corresponding synthetic seismic wavefields. Comparison with analytical plane-wave reflection coefficients calculated for corresponding isotropic elastic half-space models shows that the differences tend to be most pronounced in the vicinity of the elastic equivalent of the critical angle as well as in the post-critical range. We also find that the presence of intrinsic anisotropy in the clay component of our layered models tends to dramatically reduce the overall magnitude of the P-wave reflection coefficient as well as its variation with incidence angle.

Small-scale composition and haze layering in Titan’s polar vortex

Fine scale layering of haze and composition in Titan’s stratosphere and mesosphere was investigated using visible/UV images from Cassini’s Imaging Science Sub-system (ISS) and IR spectra from Cassini’s Composite Infra-Red Spectrometer (CIRS). Both ISS and CIRS independently show fine layered structures in haze and composition, respectively, in the 150–450 km altitude range with a preferred vertical wavelength of around 50 km. Layers are most pronounced around the north polar winter vortex, although some weaker layers do exist at more southerly latitudes. The amplitude of composition layers in each trace gas profile is proportional to the relative enrichment of that species in the winter polar vortex compared to equatorial latitudes. As enrichment is caused by polar subsidence, this suggests a dynamical origin. We propose that the polar layers are caused by cross-latitude advection across the vortex boundary. This is analogous to processes that lead to ozone laminae formation around Earth’s polar vortices.

Fine-scale layer of hydromedusae is revealed by video plankton recorder (VPR) in a semi-enclosed bay in northern Norway

High resolution images of zooplankton organisms were recorded in situ using a digital autonomous video plankton recorder (VPR) in a semi-enclosed bay in northern Norway. Data from vertical VPR profiles was used to estimate the depth distribution and abundance of zooplankton along a bay-wide grid. Images were recorded simultaneously with hydrographical data, and the spa- tial distribution of different organisms was related to several water column characteristics, such as chlorophyll a, density, and buoyancy frequencies. In June, unusually high abundances of hydrome- dusae (Rathkea octopunctata and Obelia sp.) were located in a fine-scale layer of a few metres thick- ness. This layer could be associated with relatively high buoyancy frequencies, reflecting a density gradient present throughout the entire sampling area. Aggregation of hydromedusae around density discontinuities may have enabled retention of medusae in the bay. Ctenophores and appendiculari- ans were distributed above the medusae layer, while peaks of copepod abundance were found out- side the layer boundaries. We suggest that the bloom of R. octopunctata was initially caused by liber- ation of new medusae from successful benthic hydroid colonies, and that proliferation of this species was sustained by asexual, manubrial budding in the medusae stage.

Petrogenesis of the Yangkou layered garnet-peridotite complex, Sulu UHP terrane, China

The Yangkou garnet-peridotite complex in the Sulu UHP terrane, eastern China, consists of several blocks, tens of meters across, enclosed within gneiss. The blocks consist mainly of garnet Iherzolite and garnet clinopyroxenite with small amounts of eclogite. The complex displays a finely layered structure from outcrop to thin-section scales. Lherzolitic garnet is generally homogenous in composition (Prp 6 4 - 6 7 Alm 2 3 - 2 4 Grs 1 0 - 1 2 Sps 1 ). Garnet (Prp 5 1 - 6 3 ) from clinopyroxenite contains more pyrope component than eclogitic garnets (Prp 2 9 - 3 5 ). Porphyroblastic garnet from clinopyroxenite and eclogite contains abundant exsolved rods of rutile and apatite (′Cpx). Lherzolitic garnet exhibits an unusual sinusoidal REE pattern characterized by a decrease in abundance from La to Pr and an increase from Nd to the HREE, and shows Ba and Pb enrichment and negative Nb and Ni anomalies in primitive mantle-normalized trace-element plots. In contrast, diopside shows a normal REE pattern with decreasing abundances from La to Lu, a peak at Ba and Pb, and depletion in Co and Ni. Garnet from eclogite has a REE pattern with increasing abundances from La to Lu, or a maximum at Eu. Eclogitic clinopyroxene shows humped REE patterns with a peak at Nd. Except for slightly lower δ 1 8 O V S M O W OF +4.40‰ for olivine, O-isotopic compositions (+5.03 to +5.53‰) of analyzed garnets and clinopyroxenes from all three rock types lie in the range of mantle δ 1 8 O values of +5.1 to +6.6‰. Small O-isotope fractionations (AGrt-Cpx: -0.16 to -0.35) in eclogite suggest isotopic equilibrium. On the basis of petrochemical and isotopic data, this garnet-peridotite complex was initially a mantle peridotite cut by mafic veins. These rocks experienced metasomatism and subduction-zone UHP metamorphism at 750 ′ 50 °C and 40 to 45 kbar with a low thermal gradient. The fine-scale layering probably formed as a result of strong shear deformation during exhumation.

Opportunity Runneth Over

### Contents #### Research Articles and Reports [The Opportunity Rover's Athena Science Investigation at Meridiani Planum, Mars][1] S. W. Squyres et al. [Pancam Multispectral Imaging Results from the Opportunity Rover at Meridiani Planum][2] J. F. Bell III et al. [In Situ Evidence for an Ancient Aqueous Environment at Meridiani Planum, Mars][3] S. W. Squyres et al. [Opportunity at Merdiani Planum: Plates][4] [Soils of Eagle Crater and Meridiani Planum at the Opportunity Rover Landing Site][5] L. A. Soderblom et al. [Evidence from Opportunity's Microscopic Imager for Water on Meridiani Planum][6] K. E. Herkenhoff et al. [Localization and Physical Property Experiments Conducted by Opportunity at Meridiani Planum][7] R. E. Arvidson et al. [Minerology at Meridiani Planum from the Mini-TES Experiment on the Opportunity Rover][8] P. R. Christensen et al. [Jarosite and Hematite at Meridiani Planum from Opportunity's Mossbauer Spectrometer][9] G. Klingelhofer et al. [Chemistry of Rocks and Soils at Meridiani Planum from the Alpha Particle X-ray Spectrometer][10] R. Rieder et al. [First Atmospheric Science Results from the Mars Exploration Rovers Mini-TES][11] M. D. Smith et al. [Atmospheric Imaging Results from the Mars Exploration Rovers: Spirit and Opportunity][12] M. T. Lemmon et al. See related [Perspective][13] and the [Report][14] by Formisano et al. A ctually, the Opportunity rover never did run across the martian surface; it was more like a stop-and-go boogie at an average crawl of 3.6 meters per hour. Nor did the mobile rover go very far in its first 56 sols and 56 nights, spending most of its sunlit working hours in the 20-meter-diameter Eagle crater that it serendipitously bounced into. Still, it was the hottest rover on six wheels, on this cold and desolate planet, from the moment that it put up its mast and scanned the broken but distinct outcrop that rimmed part of its landing site. Never before had a slab of martian bedrock been seen or touched by robotic inquisitiveness, including NASA's successful predecessors: Spirit from the Mars Exploration Rovers (MER) mission, Sojourner rover from the Pathfinder mission, and the landers from the Viking 1 and 2 missions. Here within Meridiani Planum, just meters from the landing pod, was a layered rock, bent and fractured by impact events, but still partially intact and providing a history of environmental changes. Opportunity turned a trickle of in situ evidence for liquid water on Mars into a flood. After the overview of the first 90 sols of the mission by Squyres et al. (p. [1698][1]), the next eight papers describe in some detail the fine-scale layering, the hydrated minerals, the enrichment of sulfur, and the salty surfaces that suggest that the layered rock formed by repeated cycles of flooding, evaporation, and desiccation or erosion. Opportunity also found the source of the hematite (Fe2O3) that was remotely sensed over hundreds of kilometers by the Thermal Emission Spectrometer (TES) on the Mars Global Surveyor orbiter. The hematite is mostly concentrated in millimeter-sized spherules, initially called blueberries by the MER team. The spherules are embedded in the layered rocks and distributed as loose particles on the plains. These plains probably represent a lag deposit: a thin layer of larger particles left over after disaggregation of the rocks and removal of smaller particles by the wind. The embedded spherules are randomly oriented relative to the layering, which suggests that they formed later by secondary alteration. The last two papers describe the composition, opacity, and dynamics of the martian atmosphere measured with the instruments on Spirit and Opportunity. In his Perspective (p. [1689][13]), Kargel offers one scenario to explain the bedrock. There are other possible scenarios; however, they all require liquid water and acidic conditions. Both MER rovers are still collecting data almost a year later, and Spirit has recently found some bedrock too. The flood of new in situ evidence for liquid water allows for the possibility of microbial organisms living, perhaps transiently, when the acidic brines were active, like terrestrial acidophiles. Opportunity's run is definitely not over yet. [1]: /lookup/doi/10.1126/science.1106171 [2]: /lookup/doi/10.1126/science.1105245 [3]: /lookup/doi/10.1126/science.1104559 [4]: /lookup/doi/10.1126/science.306.5702.1715 [5]: /lookup/doi/10.1126/science.1105127 [6]: /lookup/doi/10.1126/science.1105286 [7]: /lookup/doi/10.1126/science.1104211 [8]: /lookup/doi/10.1126/science.1104909 [9]: /lookup/doi/10.1126/science.1104653 [10]: /lookup/doi/10.1126/science.1104358 [11]: /lookup/doi/10.1126/science.1104257 [12]: /lookup/doi/10.1126/science.1104474 [13]: /lookup/doi/10.1126/science.1105533 [14]: /lookup/doi/10.1126/science.1101732

The Cretaceous Messum igneous complex, S.W. Etendeka, Namibia: reinterpretation in terms of a downsag-cauldron subsidence model

The Messum igneous complex (MIC) lies within the ENE-trending zone of Lower Cretaceous (∼132 Ma) Damaraland intrusive complexes in Namibia, intruded into Pan-African Damara basement. It is defined by a roughly circular structure ∼18 km in diameter, the bounding ring fault exposed along the eastern sector. Encircling Messum are the volcanic sequences of the Goboboseb Mountains, comprising a lower basalt series (Tafelkop and Tafelberg types) followed, with intervening basalts, by four voluminous quartz latite (QL) eruptions (Goboboseb and Springbok QL units). Inferred stages of development are: (a) an initial very broad basaltic lava shield, comprising the Tafelberg and Tafelkop basalts, and Messum crater basalts (MCB; possibly ponded in near-vent lava lakes). Embedded within the lower basaltic sequence is a localised rhyolite-dominated eruptive centre (ca. 5 km in diameter), interpreted as a funnel caldera located towards the centre of the MIC. (b) Downsagging, extending northwards from Messum, following the Goboboseb QL eruptions (≥2300 km 3). Ponding of overlying basaltic units. (c) Climactic Springbok QL eruption (≥6300 km 3) producing further downsag together with the inward radial dip of all volcanic units towards the MIC. Ring fault initiation. (d) Cauldron subsidence emplacement of a granitoid suite, forming the MIC ‘moat’ (area between the ring fault and the core region). (e) Intrusion of gabbroic cone sheets into incompletely solidified granitic melts within the southeastern moat. Resulting hybridisation and magma mingling produced extensive development of heterogeneous granitoid and hybrid dioritic lithologies. (f) Cone sheet intrusions of the eastern gabbros into more highly solidified granitoids of the southeastern moat. (g) Intrusion of thick (∼1–2 km) western gabbro cone sheets, exhibiting local fine-scale layering, into solidified granitoids, mainly within the western moat. Minor late-stage granitic intrusions. (h) 2–3 Ma quiescent period followed by quartz- and ne-syenite intrusions, and finally basanite dykes, emplaced within the MIC core. Accompanying differential uplift of the core. Uplift/resurgence within the MIC has accompanied intrusion of the moat granitoids and mafic cone sheets, thereby juxtaposing volcanic and intrusive sequences. Phases of both subsidence and uplift have characterised the MIC. The NW Scotland Tertiary central igneous complexes and Messum show evidence of a number of parallel developments, but also important differences. The MIC differs markedly from caldera systems within the western USA and circum-Pacific. Messum is therefore suggested to represent a distinct class of intrusive/extrusive central complex, although probably common in large igneous provinces.

The eclogites of the Marun–Keu complex, Polar Urals (Russia): fluid control on reaction kinetics and metasomatism during high P metamorphism

The Marun–Keu complex (Polar Urals, Russia) is a poorly known member of a group of high P complexes outcropping along the length of the Uralide orogen. The central and southern parts of the complex are metamorphosed at high P, medium T conditions ( T max∼600–650 °C and P∼14–17 kbar) and differ from its northern part and the rest of the Uralian high P complexes which are metamorphosed at blueschist-to-low T eclogite-facies conditions. The Marun–Keu complex consists of Neoproterozoic to Cambrian volcanic-sedimentary sequences with a large variety of intrusive mafic to felsic rocks. Based on the nature of protoliths and the mode of occurrence, it is distinguished: (1) eclogite-facies rocks after intrusive protoliths, which vary from metagabbros to metagranites; (2) eclogitic quartzofeldspathic gneisses; (3) metasomatic eclogites; and (4) amphibolite–eclogite alternations produced by fluid infiltration during uplift. Igneous textures and mineralogy are preserved in non-sheared, intrusive, dry rocks, whereas eclogitization may be complete along centimetre to 10-m-thick shear zones and in domains infiltrated by H 2O-dominated fluids. The most important reaction features are: (1) transformation of igneous diopside (Na∼0.04–0.09 apfu, based on 6 oxygens) into Na-rich diopside and omphacite (Na∼0.24–0.45 apfu) across microfractures and grain boundaries; (2) replacement of plagioclase by tiny aggregates of zoisite/clinozoisite+white mica+garnet (Alm 32–43, Pyr 7–19, Gro 38–60)+kyanite; and (3) double coronas of garnet and orthopyroxene at olivine–clinopyroxene and olivine–plagioclase pseudomorph interfaces and garnet coronas at clinopyroxene–plagioclase pseudomorph interfaces. Eclogitic quartzofeldspathic gneisses display a fine-scale layering of intermediate and felsic rock compositions with omphacite-bearing and omphacite-free assemblages, respectively. Oligoclase is abundant in this type of rocks, coexisting with omphacite in the intermediate rock compositions. The studied area presents a large variety of veins and metasomatic mineral sequences developing in the host-rock adjacent to the veins. High P minerals (garnet (Alm 40–60, Pyr 25–40, Gro 11–23), omphacite, phengite, paragonite) occur in both vein infillings and wall-rock mineral sequences, indicating that metasomatism was caused by the infiltration of out of equilibrium fluids (mostly silica-rich, alkali-rich compositions) during the eclogite-facies metamorphism. In zones of high fracture density, vein networks divide the host-rock into decimetre-scale blocks with omphacite-rich rinds replacing amphibolite cores. In contrast, metasomatic replacement of mafic eclogites by dendritic amphibole with barroisite composition ( M4Na∼0.64 apfu, based on 23 oxygens) also occurred at eclogite-facies conditions. We outline the importance of considering such metasomatic processes for the correct evaluation of P– T– t metamorphic histories. The Marun–Keu complex is another example that highlights the important control exerted by fluids on reaction kinetics during high P metamorphism and further points to an important metasomatic effect of these fluids.

Fine-scale layering on the edge of a stratospheric intrusion

Observations of vertical profiles of ozone, humidity, static stability and VHF radar vertical power at Aberystwyth (52°N, 4°W) on 21 June 1996 revealed a pronounced layered structure at the western edge of a stratospheric intrusion, in a location where one would expect to see a tropopause fold. Despite the 3 km depth of the observed ozone anomaly, it was not represented as a fold in the ECMWF potential vorticity analyses; nor was it evident as a layer of enhanced wind shear. Ozone lidar measurements suggest that the fine-scale layering gave way to a single layer as the day progressed. Weak sporadic turbulence observed by the radar at the edge of the fold showed some mixing between stratospheric and tropospheric air.

Effective Reactive Surface Area: an Anisotropic Property of Physically and Chemically Heterogeneous Porous Media

AbstractAlthough transport calculations are often formulated in terms of mass-based isotropic distribution coefficients, it is the abundance of reactive surface areas of subsurface materials that controls contaminant adsorption. In water-saturated homogeneous systems devoid of advective fluxes (e.g., batch experiments), the available reactive surface area is similar to the total surface area (as measured by conventional methods such as BET gas adsorption). However, in physically and chemically heterogeneous systems with advective fluxes, the effective reactive surface area (i.e., the surface area that a packet of advecting water interacts with) is smaller than the laboratory measured surface area and is a complex function of advective velocity and the correlation structures of the physical and chemical heterogeneities. Theoretical derivations for an important but simple type of heterogeneity (fine-scale horizontal layering) suggest that the effective reactive surface area is an anisotropic property of the medium and is inversely correlated with the anisotropy in hydraulic conductivity. The implications of reactive transport anisotropy include the concept that the retardation factor should be treated as a directional property rather than being treated as a constant. Furthermore, because of the inverse relationship between effective reactive surface area and hydraulic conductivity, batch adsorption experiments tend to overestimate the retention of contaminants relative to intact natural materials.

Simplified anisotropy parameters for transversely isotropic sedimentary rocks

Sedimentary rocks frequently possess an anisotropic structure resulting, for example, from fine scale layering, the presence of oriented microcracks or fractures, or the preferred orientation of nonspherical grains or anisotropic minerals. For many rocks the anisotropy may be described, to a good approximation, as being transversely isotropic. The purpose of this note is to present simplified anisotropy parameters for these rocks that are valid when the P‐wave normal moveout (NMO) and vertical velocities differ by less than 25%. This condition appears reasonable since depths calculated from P‐wave stacking velocities are often within 10% of actual depths (Winterstein, 1986). It is found that when this condition is satisfied the elastic constants [Formula: see text] and [Formula: see text] affect the P‐wave NMO velocity and anellipticity only through the combination [Formula: see text], a combination of elastic constants that can be determined using walkaway VSP data (Miller et al., 1993). The anellipticity quantifies the deviation of the P‐phase slowness from an ellipse and also determines the difference between the vertical and NMO velocities for SV‐waves. Helbig (1983) has shown that a time‐migrated section for which elliptical anisotropy has been taken into account is identical to one that has been determined under the assumption of isotropy. The anellipticity is therefore the important anisotropy parameter for anisotropic time migration. The results given are of interest for anisotropic velocity analysis, time migration, and time‐to‐depth conversion.

Design of multiscalar metallic multilayer composites for high strength, high toughness, and low CTE mismatch

We propose a new class of multilayer composites that consists of alternating tough and strong layers. Both the tough and the strong layers are metallic, effectively reducing the coefficient of thermal expansion (CTE) mismatch problem that often plagues metal-ceramic composites. The high-strength layers are themselves very fine-scale metallic multilayer composites. The high strengths result from Orowan strengthening of these very fine-scale layers. We present detailed analyses of the flow stress, toughness, and thermal stability of these multiscalar metallic multi-layer composites (M3C) as a guide for microstructural optimization. The dominant term in the flow stress is proportional to the volume fraction of the strong layers and scales inversely with thickness of the very fine-scale layers that make up the strong layer. The toughness is dominated by the plastic flow of the tough layers and is proportional to the volume fraction and flow stress of the tough layers, as modified by plastic constraint. The thermal stability of M3Cs is discussed in the context of solubility, length scales, and interdiffusivity of the two metals. Preliminary results suggest that M3Cs do exhibit an unusual combination of high toughness and strength.

Crystal aging and the formation of fine-scale igneous layering

Fine-scale layering in igneous intrusions shows parallels with experimentally produced banding observed in crystallizing salt solutions in which recent advances have demonstrated the importance of crystal aging on the development of banding. These experiments, as well as numerical models of fine-scale layering development during crystal aging, reproduce features observed in fine scale layering that are not readily explained by periodic nucleation and crystal growth models. These include the phenomenon of “lost segments”, in which the development of a layer may be reversed such that the layer disappears over time but does so without affecting the overall spacing of later-developing layers, and the development of “doublet” layers, which results when crystals at the margins of developing layers preferentially grow at the expense of those in the center. While these results suggest that crystal aging is the dominant process in the development of fine-scale layering, crystal aging may enhance the textural and modal features of igneous layering initiated by a variety of other crystal settling, nucleation or growth processes.

Are stringers a manifestation of solute banding?

Fine-scale igneous layering consisting of alternating bands of chromitite and plagioclase often occurs in the Bushveld Complex where it is commonly known as “stringers”. The origin of these features has been unclear but they do have both quantitative and qualitative characteristics of a type of microsegregation (compositional nonhomogeneity) known as “solute banding” which frequently occurs in solidifying industrial melts. The cause of solute banding is due to temperature fluctuations arising from unsteady convection in the melt which generates cotectic shifting at the solidification front, leading to alternating deposition of one phase, then another.

The peridotite plugs of northern Rum

Synopsis Minor intrusions of peridotite occur as plugs and dyke-like bodies within Torridonian country rocks to the north of the Rum Central Complex. These are collectively termed the satellitic plugs to distinguish them from three dyke-like apophyses (tongues) of peridotite which are contiguous with the main layered intrusion (intrusive peridotite plugs also cut the layered sequence locally). The satellitic peridotite plugs are characterized by their lithological uniformity, compared with the cumulates in the central complex. The only exception is the West Sgaorishal plug, which has a less mafic marginal facies, and also displays regular, but subtle, fine-scale matrix banding. This generally dips inwards at moderate angles from the near-vertical margins of the intrusion, and is locally colloform. The layering reflects relatively minor changes in the proportions of interstitial plagioclase and clinopyroxene. Mineral compositions, notably olivine (Fo 88–84 ), are similar to those in the main cumulate succession. The tongues are petrologically similar to the satellitic plugs, and one of them displays the same type of fine-scale layering. The satellitic plugs (and tongues) are thought to represent conduits emanating from the main subvolcanic magma chamber, that were filled with olivine-rich crystal mush displaced from the unconsolidated cumulus pile during major subsidence events, and lubricated by interstitial basaltic magma. The distinctive layering is attributed to a self-regulating oscillatory process affecting the nucleation of plagioclase and clinopyroxene in the intercumulus liquid; perhaps during unusually tranquil conditions.

Acoustic reflection from quasiperiodic sedimentary sequences

A useful theoretical measure of acoustic interaction with the seafloor is the plane wave reflection coefficient (R), which is commonly expressed as bottom loss (−20 log ‖R‖). Predictions indicate that when the seafloor is modeled as a refracting layer over a basement half-space, the bottom loss is proportional to frequency. Bottom loss measurements in the 50–1600-Hz band, however, frequently show a loss that is inversely proportional to frequency. For example, roughly one-third of a set of measured bottom loss data in the North Atlantic exhibit this anomalous frequency dependence. It is concluded that the anomalous frequency dependence is due to sedimentary fine-scale layering arising from turbidity currents. The evidence presented consists of (1) a high correlation between the anomalous bottom loss stations and the bounds of the abyssal plains and (2) favorable predictions between a model accounting for the layering and the measured data. The study area was the western North Atlantic including the Sohm, Hatteras, and Nares abyssal plains. A simple, approximate stochastic model of reflection from a quasiperiodic sedimentary sequence was developed that appears to account for the dominant physical mechanisms important in the reflection process in this type of physiographic province.

Solid-state amorphization and contamination of NiTa powders during ball-milling

High-energy ball-milling is generally used to mechanically alloy metallic powders and develop microstructural refinement as a means to enhance structure-dependent properties. The process involves the repeated fracture, welding and fine-scale layering of the particulate. In some instances it may also induce transformations to highly metastable phases. In particular, when applied to alloy systems with negative heats-of-mixing of the constituent elements and one element that diffuses through the lattice at an anomalously fast rate, the ball-milling almost invariably results in a solid-state amorphization reaction (SSAR). That is, the crystalline material can be driven into a superheated condition which is unstable with respect to the amorphous phase because the free energy of the glass lies below that of the corresponding crystalline solid solution of the same composition. This method of forming the metallic glass state has certain advantages over those involving rapid solidification or vapor deposition. SSAR is convenient to control, usually leads to a much wider glass formation range, and the glasses often have high thermal stability than that of melt-spun ribbons. In this paper the authors chose to ball-mill mixtures of Ni and Ta crystalline powders because they satisfy the requirements mentioned above and SSAR had not been attempted beforemore » with these elements. The authors were successful in producing Ni-Ta amorphous powder alloys of two compositions using a milling container and balls made of WC+Co.« less

Spontaneous development of concentric layering in a solidified siliceous dike, East Greenland

A meter-wide rhyolitic dike near the eastern margin of the Skaergaard Intrusion developed fine concentric layering in a small section where a fault appears to have fractured the dike and enhanced its interaction with meteoric water. Layers two or three millimeters thick consist of quartz alternating with low-temperature albite and potassium feldspar around a core of radiating clots of the same minerals. The layering seems to have resulted from late-stage crystallization and “self-organization” during devitrification in a warm hydrous environment. The development of fine-scale layering can be modeled quantitatively using the model for competitive particle growth (CPG) of Ortoleva and co-workers. Owing to the small differences of surface energy of grains of differing sizes, large crystals tend to grow at the expense of smaller ones, and a repetitive pattern of layering develops from a small initial perturbation of grain size. The spacing of layers predicted by a numerical model using reasonable estimates for the physical parameters, such as diffusivity and the rate of cooling, is in good agreement with the observed geometry and dimensions.