Comb Layering Research Articles

English

Poultry oil is commonly used as a cost-effective energy source in poultry feed. However, when used at higher levels, raw poultry oil has been found to have adverse effects on the liver of birds. Unfortunately, many poultry farmers remain unaware of these harmful effects. The objective of this experiment was to investigate how poultry oil impacts liver health and contributes to fatty liver syndrome in layers. Using a completely randomized design, 120 Lohmann Single Comb (LSL-Lite) layers (25- week-old) were randomly assigned to 4 experimental groups, each of which had 30 birds. Increasing amounts of poultry oil (0, 1.5, 3.0 and 4.5%) were used in experimental diets. Hens were fed these experimental diets for 20 weeks. The results showed maximum liver fat percentage in 4.5% poultry oil in diet, severe liver congestion, degeneration and cell necrosis in caged layers. Plasma lipid profile (cholesterol, HDL, LDL, triglycerides) and liver enzymes (AST, ALT, ALP and LDH) were significantly (p<0.05) affected by poultry oil addition. Higher liver scores indicative of fatty liver syndrome was observed in layers receiving 4.5% poultry oil, while the control group showed the lowest incidence. This study indicated that poultry oil dietary addition at a higher level (4.5%) lead to an increased serum lipid profile and has a negative impact on liver health, ultimately causing fatty liver syndrome in caged layers

Taking the pulse of volcanic eruptions using plagioclase glomerocrysts

Crystallization timescales in subvolcanic systems and the consequences of interaction between ascending magmas and gases remain largely unconstrained, as do links between these processes and monitoring signals at restless volcanoes. We apply diffusion chronometry to radially-oriented plagioclase and associated olivine in a glomerocryst from Tolbachik volcano (Kamchatka, Russia) to elucidate such processes. We show that cm-size glomerocrysts grow in a few days prior to, or during, eruption. Melt inclusions from these glomerocrysts show no compositional evolution during crystallization, implying growth in a melt-rich and dynamic environment. Volatile elements in melt inclusions show significant variability, with increasing CO2 as H2O decreases. This behaviour is inconsistent with normal degassing processes, and more likely reflects CO2-fluxing. On the basis of short residence timescales of glomerocrysts and sharp changes in melt inclusion volatile abundances, we propose that rapid (pre-)eruptive crystallization is controlled by rhythmic fluxing of magmatic H2O and CO2 through the sub-volcanic conduit. This implies that compositional zoning in plagioclase, from resorption textures to oscillatory zoning, record short-term CO2- and H2O-fluxing episodes, consistent with strombolian eruption dynamics. We propose that volcanic glomerocrysts represent the counterpart of vertical igneous layering (or comb layering) in shallow plutons. Magmatic layering and glomerocrysts dominated by radial plagioclase offer novel ways of targeting short-term crystallization and degassing processes in subvolcanic systems.

Ca isotopes record rapid crystal growth in volcanic and subvolcanic systems

Kinetic calcium isotope effects can be used as growth-rate proxies for volcanic and subvolcanic minerals. Here, we analyze Ca isotopic compositions in experimental and natural samples and confirm that large kinetic effects (>2‰) can occur during magmatic plagioclase crystallization. Experiments confirm theoretical predictions that disequilibrium isotope effects depend mainly on the rates for crystal growth relative to liquid phase Ca diffusivity (R/D). Plagioclase phenocrysts from the 1915 Mount Lassen rhyodacite eruption, the ∼650-y-old Deadman Creek Dome eruption, and several mafic subvolcanic orbicules and plagioclase comb layers from Northern California have disequilibrium Ca isotopic compositions that suggest rapid crystal growth rates (>1 cm/y to 15 cm/y). The Ca isotope results, combined with complementary crystal-size distribution analyses, suggest that magmatic rejuvenation (and eruption) events, as reflected in crystal growth times, can be as short as ∼10-3 y. Although mafic systems are predicted to have shorter magmatic rejuvenation periods, we find similarly short timescales in both mafic and silicic systems. These results are consistent with a growing body of evidence suggesting that dominantly crystalline volcanic magma reservoirs can be rapidly reactivated by the injection of fresh magma prior to eruption. By focusing on a common mineral such as plagioclase, this approach can be applied across all major magmatic compositions, suggesting that Ca isotopes can be used as a tool for investigating the dynamics and timing of volcanic eruptions.

Mineral growth in melt conduits as a mechanism for igneous layering in shallow arc plutons: mineral chemistry of Fisher Lake orbicules and comb layers (Sierra Nevada, USA)

Different processes have been proposed to explain the variety of igneous layering in plutonic rocks. To constrain the mechanisms of emplacement and crystallization of ascending magma batches in shallow plutons, we have studied comb layers and orbicules from the Fisher Lake Pluton, Northern Sierra Nevada. Through a detailed study of the mineralogy and bulk chemistry of 70 individual layers, we show that comb layers and orbicule rims show no evidence of forming through a self-organizing, oscillatory crystallization process, but represent crystallization fronts resulting from in situ crystallization and extraction of evolved melt fractions during decompression-driven crystallization, forming a plagioclase-dominated cres-cumulate at the mm- to m-scale. We propose that the crystal content of the melt and the dynamics of the magmatic system control the mechanisms responsible for vertical igneous layering in shallow reservoirs. As comb layers crystallize on wall rocks, the higher thermal gradients will increase the diversity of comb layering, expressed by inefficient melt extraction, thereby forming amphibole comb layers and trapped apatite + quartz saturated evolved melt fractions. High-An plagioclase (An90–An97.5) is a widespread phase in Fisher lake comb layers and orbicule rims. We show that a combination of cooling rate, latent heat of crystallization and pressure variations may account for high-An plagioclase in shallow melt extraction zones.

Varifocal Scanner Using Wafer Bonding

We present a varifocal mirror integrated with a comb-drive scanner (varifocal scanner) fabricated by using a wafer bonding technology. An Au-Au thermocompression bonding technology is utilized for the integration. The bonding technology enables fabricating the multiple single crystalline Si layers on a wafer. The varifocal scanner consists of the three Si layers: 1) the fixed comb layer; 2) the scanner frame layer; and 3) the varifocal mirror layer. The 2-mm-diameter and 10- $\mu \text{m}$ -thick varifocal mirror is fabricated on the $60~\mu \text{m}$ thick comb-drive scanner. The focal length modulation range of the varifocal mirror is from −523 to 139 mm at the frequency of 18.3 kHz. The optical angle is 7° at the resonant frequency of 3.59 kHz. The motion couplings between the varifocal mirror and the scanner are also investigated. [2016-0257]

Comb layering monitors decompressing and fractionating hydrous mafic magmas in subvolcanic plumbing systems (Fisher Lake, Sierra Nevada, USA)

AbstractUnderstanding the dynamics of volcanic eruptions requires constraints on the processes of melt extraction in shallow magma reservoirs. This study focuses on the formation of intraplutonic igneous layering known as comb layers and orbicules within the Fisher Lake Pluton, Sierra Nevada. Comb layers are mineralogically and texturally diverse, ranging from plagioclase‐amphibole comb layers to plagioclase‐pyroxene and olivine comb layers. The majority of individual layers are related to the initiation of plagioclase nucleation and growth on preexisting host rocks and remobilized xenoliths. We use simple phase diagrams to illustrate the formation of comb layers through crystal fractionation along conduit walls and xenoliths, with plagioclase as the liquidus phase. Near‐adiabatic ascent of water‐undersaturated melts through the crust will lead to superheating and crystal resorption, which upon saturation of volatiles at shallow depth can induce heterogeneous nucleation of plagioclase. The rarity of orbicules and comb layers worldwide suggests that adiabatic decompression of crystal‐poor, moderately hydrous melts through the crust is rare. Most melts in intermediate/silicic volcanoes crystallize in multiple reservoirs within the crust prior to emplacement at shallow depth as crystal‐bearing melts. Comb layers and orbicules preserve evidence of multiple melt injections where a sequence of texturally and mineralogically similar comb layers and orbicules reflect the timescale of an individual melt pulse. Short timescales on the order of months to years for the growth of these sequences are estimated using experimental plagioclase growth rates and are interpreted as representing the lifetime of a melt injection within a long‐lived, subvolcanic melt extraction zone.

Sensitivity analysis on the pollutant trapping efficiencies of a novel sewerage overflow screening device

A novel ‘Comb Separator’ was developed and tested with the aim of improving sewer solids capture efficiency and reducing blockages on the screen. Experimental results were compared against the industry standard ‘Hydro-Jet™’ screen. Analysing the parameter sensitivity of a hydraulic screen is a standard practice to get better understanding of the device performance. In order to understand the uncertainties of the Comb Separator's input parameters, it is necessary to undertake sensitivity analysis; this will assist in making informed decisions regarding the use of this device. Such analysis will validate the device's performance in urban sewerage overflow scenarios. The methodology includes multiple linear regression and sampling using the standard Latin hypercube sampling technique to perform sensitivity analysis on different experimental parameters, such as flowrate, effective comb spacing, device runtime, weir opening and comb layers. The input parameters ‘weir opening’ and ‘comb layers’ have an insignificant influence on capture efficiency; hence, they were omitted from further analysis. Among the input parameters, ‘effective spacing’ was the most influential, followed by ‘inflow’ and ‘runtime’. These analyses provide better insights about the sensitivities of the parameters for practical application. This will assist device managers and operators to make informed decisions.

Textural and chemical consequences of interaction between hydrous mafic and felsic magmas: an experimental study

Mantle-derived, hydrous mafic magmas are often invoked as a mechanism to transfer heat, mass and volatiles to felsic plutons in the Earth’s crust. Field observations suggest that mafic, water-rich magmas often intrude viscous felsic crystal-rich mushes. This scenario can advect water from the crystallising mafic magma to the felsic magma, leading to an increase in melt fraction in the felsic mush and subsequent mobilisation, at the same time as the mafic magma becomes quenched through a combination of cooling and water loss. To investigate such a scenario, we conducted experiments on a water-undersaturated (4 wt% H2O in the interstitial melt) dacitic crystal mush (50–80 vol% quartz crystals) subject to volatile supply from a water-saturated (≥6 wt% H2O) andesite magma at 950 °C and 4 kbar. Our experimental run products show unidirectional solidification textures (i.e. comb layering) as crystals nucleate at the mafic–felsic interface and grow into the mafic end-member. This process is driven by isothermal and isobaric undercooling resulting from a change in liquidus temperature as water migrates from the mafic to the felsic magma. We refer to this process as “chemical quenching” and suggest that some textures associated with natural mafic–felsic interactions are not simply cooling-driven in origin, but can be caused by exsolution of volatiles adjacent to an interface, whether a water-undersaturated felsic magma (as in our experiments) or a fracture.

Determination of the molar mass of surface-grafted weak polyelectrolyte brushes using force spectroscopy

The molar mass and dispersity of a polycation, poly[2-(dimethyl amino)ethyl methacrylate] (PDMAEMA) grafted from a poly(methyl methacrylate) (PMMA) backbone, was measured by single-molecule force spectroscopy (SMFS) and shown to be consistent with results from gel permeation chromatography for the same comb polymer in aqueous solution. Comparison was then made between the comb polymer and PDMAEMA brushes that were grown from the substrate, as a function of the pH and ionic strength of the surrounding medium, and the limits of reliable characterization of the polymers are determined. A large discrepancy was observed between the responses of the comb and brush layer at low pH when the PDMAEMA molecules are extended from the supporting substrate. Here it is believed that the atomic force microscope (AFM) tip can penetrate the comb layer and selectively desorb side-chains of the comb. In the case of the well solvated PDMAEMA brushes at high pH, the tip preferentially selects larger chains, resulting in an over-estimate of the brush molar mass. The addition of salt also influenced the molar mass obtained by this technique. It is believed that salted brushes did not adhere well to the AFM tip, with subsequent desorption resulting in an underestimate of the molar mass. However, SMFS was shown to be capable of demonstrating the effect of salt on brush conformation, with greater swelling after the addition of a small amount of NaCl, but a significant decrease when 100 mM is added.

Acoustical impedance characterization of liners using a Bayesian approach

A characterization method of acoustic treatment composed by a perforated plate and a honey comb layer is described. Those treatments are generally used as liners for aircraft nacelle and are characterized by their impedance. Empirical models or Kundt's tube experiments are generally used in order to characterize impedance under specific conditions which are not representative of physics in aircraft liners (plane wave instead complex waves, normal incidence instead grazing incidence...). An inverse method using Bayesian approach was developed to define specific parameters describing impedance of treatments under similar functional conditions. For this, an analytical solution is implemented in order to evaluate the posterior probability density function of parameters and an evolutionary Markov Chain Monte Carlo method (eMCMC) is implemented to explore probability density space. The validation of the inverse method is realized on a simulated data and a real case based on experimental data is presented.

A case for hornblende dominated fractionation of arc magmas: the Chelan Complex (Washington Cascades)

Amphibole fractionation in the deep roots of subduction-related magmatic arcs is a fundamental process for the generation of the continental crust. Field relations and geochemical data of exposed lower crustal igneous rocks can be used to better constrain these processes. The Chelan Complex in the western U.S. forms the lowest level of a 40-km thick exposed crustal section of the North Cascades and is composed of olivine websterite, pyroxenite, hornblendite, and dominantly by hornblende gabbro and tonalite. Magmatic breccias, comb layers and intrusive contacts suggest that the Chelan Complex was build by igneous processes. Phase equilibria, textural observations and mineral chemistry yield emplacement pressures of ∼1.0 GPa followed by isobaric cooling to 700°C. The widespread occurrence of idiomorphic hornblende and interstitial plagioclase together with the lack of Eu anomalies in bulk rock compositions indicate that the differentiation is largely dominated by amphibole. Major and trace element modeling constrained by field observations and bulk chemistry demonstrate that peraluminous tonalite could be derived by removing successively 3% of olivine websterite, 12% of pyroxene hornblendite, 33% of pyroxene hornblendite, 19% of gabbros, 15% of diorite and 2% tonalite. Peraluminous tonalite with high Sr/Y that are worldwide associated with active margin settings can be derived from a parental basaltic melt by crystal fractionation at high pressure provided that amphibole dominates the fractionation process. Crustal assimilation during fractionation is thus not required to generate peraluminous tonalite.

Textural and chemical evolution of a fractionated granitic system: the Podlesí stock, Czech Republic

The Podlesí granite stock (Czech Republic) is a fractionated, peraluminous, F-, Li- and P-rich, and Sn, W, Nb, Ta-bearing rare-metal granite system. Its magmatic evolution involved processes typical of intrusions related to porphyry type deposits (explosive breccia, comb layers), rare-metal granites (stockscheider), and rare metal pegmatites (extreme F–P–Li enrichment, Nb–Ta–Sn minerals, layering). Geological, textural and mineralogical data suggest that the Podlesí granites evolved from fractionated granitic melt progressively enriched in H 2O, F, P, Li, etc. Quartz, K-feldspar, Fe–Li mica and topaz bear evidence of multistage crystallization that alternated with episodes of resorption. Changes in chemical composition between individual crystal zones and/or populations provide evidence of chemical evolution of the melt. Variations in rock textures mirror changes in the pressure and temperature conditions of crystallization. Equilibrium crystallization was interrupted several times by opening of the system and the consequent adiabatic decrease of pressure and temperature resulted in episodes of nonequilibrium crystallization. The Podlesí granites demonstrate that adiabatic fluctuation of pressure (“swinging eutectic”) and boundary-layer crystallization of undercooled melt can explain magmatic layering and unidirectional solidification textures (USTs) in highly fractionated granites.

Exsolved magmatic fluid and its role in the formation of comb-layered quartz at the Cretaceous Logtung W-Mo deposit, Yukon Territory, Canada

ABSTRACT:Comb-layered quartz is a type of unidirectional solidification texture found at the roofs of shallow silicic intrusions that are often associated spatially with Mo and W mineralisation. The texture consists of multiple layers of euhedral, prismatic quartz crystals (Type I) that have grown on subplanar aplite substrates. The layers are separated by porphyritic aplite containing equant phenocrysts of quartz (Type II), which resemble quartz typical of volcanic rocks and porphyry intrusions. At Logtung, Type I quartz within comb layers is zoned with respect to a number of trace elements, including Al and K. Concentrations of these elements as well as Mn, Ti, Ge, Rb and H are anomalous and much higher than found in Type II quartz from Logtung or in igneous quartz reported elsewhere. The two populations appear to have formed under different conditions. The Type II quartz phenocrysts almost certainly grew from a high-silica melt between 600 and 800°C (as β-quartz); in contrast, the morphology of Type I quartz is consistent with precipitation from a hydrothermal solution, possibly as α-quartz grown below 600°C. The bulk compositions of comb-layered rocks, as well as the aplite interlayers, are consistent with the hypothesis that these textures did not precipitate solely from a crystallising silicate melt. Instead, Type I quartz may have grown from pockets of exsolved magmatic fluid located between the magma and its crystallised border. The Type II quartz represents pre-existing phenocrysts in the underlying magma; this magma was quenched to aplite during fracturing/degassing events. Renewed and repeated formation and disruption of the pockets of exsolved aqueous fluid accounts for the rhythmic banding of the rocks.

Oxygen and carbon isotope patterns in the Dicker Willem carbonatite complex, southern Namibia

Carbon and oxygen isotope data are presented for carbonatites and associated alkali silicate rocks from the Tertiary (49 Ma) Dicker Willem complex in southern Namibia. Carbonatites are grouped into: (1) coarse-grained sövites and associated silicate-oxide-phosphate-rich cumulates; (2) finer-grained alvikite intrusions, showing porphyritic and spinifex textures, comb layering and gravity-settled layers; (3) late-stage dykes, pipe breccias, veins and druses. The early sövites carry many inclusions of silicate rocks (ijolites, syenites). The most primitive carbon and oxygen isotope compositions are found in phenocrysts from calcite-phyric microsövite, bulk sövites and interstitial carbonate in the ijolites, with δ 13C ( −5‰ vs. PDB) and δ 18O ( + 7 to +9‰ vs. SMOW). Oxygen isotope fractionation between cumulus pyroxene, magnetite and biotite in the sövites yields near magmatic temperatures of 600–900°C. Carbonates in some cumulates yield magmatic temperatures, but commonly show evidence of secondary alteration. Phenocrysts in dolomite-phyric alvikite are slightly enriched in 13C (average δ 13C= − 3.6‰) and 18O (average δ 18O= + 9.9‰) relative to primitive ratios, but taken together with data for phyric calcite define a linear trend of increasing δ 13C with δ 18O and can be modelled as being the product of combined carbonate-silicate-oxide-phosphate fractionation of a parent sövite. Groundmass carbonate in the porphyritic alvikites, as well as the bulk alvikites, all show variable degrees of 18O enrichment relative to the phenocrysts, and reflect partial recrystallization of carbonate in the presence of low-temperature hydrous fluids.

Oxygen and carbon isotope patterns in the Dicker Willem carbonatite complex, southern Namibia

Carbon and oxygen isotope data are presented for carbonatites and associated alkali silicate rocks from the Tertiary (49 Ma) Dicker Willem complex in southern Namibia. Carbonatites are grouped into: (1) coarse-grained sövites and associated silicate-oxide-phosphate-rich cumulates; (2) finer-grained alvikite intrusions, showing porphyritic and spinifex textures, comb layering and gravity-settled layers; (3) late-stage dykes, pipe breccias, veins and druses. The early sövites carry many inclusions of silicate rocks (ijolites, syenites). The most primitive carbon and oxygen isotope compositions are found in phenocrysts from calcite-phyric microsövite, bulk s6vites and interstitial carbonate in the ijolites, with δ 13C (−5‰ vs. PDB) and δ 18O ( +7 to +9‰ vs. SMOW ). Oxygen isotope fractionation between cumulus pyroxene, magnetite and biotite in the sövites yields near magmatic temperatures of 600–900°C. Carbonates in some cumulates yield magmatic temperatures, but commonly show evidence of secondary alteration. Phenocrysts in dolomite-phyric alvikite are slightly enriched in 13C (average δ 13 C=−3.6‰ ) and 180 (average δ 18 O=+9.9‰ ) relative to primitive ratios, but taken together with data for phyric calcite define a linear trend of increasing δ 13C with δ 18O and can be modelled as being the product of combined carbonate-silicate-oxide-phosphate fractionation of a parent sövite. Groundmass carbonate in the porphyritic alvikites, as well as the bulk alvikites, all show variable degrees of 18O enrichment relative to the phenocrysts, and reflect partial recrystallization of carbonate in the presence of low-temperature hydrous fluids.

Significance of epidote in orbicular diorite from the Grenville Front zone, eastern Labrador

AbstractOrbicules in diorite from the Grenville Front zone of eastern Labrador are defined by shell structures alternately enriched and depleted in biotite, epidote and magnetite. Hornblende occurs locally in orbicule cores and the matrix, but not in the shells. The shells enclose plagioclase-rich (An40–45), leucodioritic cores containing biotite, epidote, magnetite and/or hornblende-bearing mafic clots. The matrix of the orbicules is mineralogically-similar to the orbicule cores, but is mesocratic, and contains relatively sodic plagioclase and accessory quartz and K-feldspar. In places, hornblende contains quartz oikocrysts, implying the resorption of early-formed clinopyroxene, and is rimmed by biotite and epidote. The latter phases also occur as inclusions in quartz-free hornblende interpreted to have crystallized directly from the magma. Epidote has a pistacite content of 21 to 26 and occurs as (1) tiny, idiomorphic crystals (‘epidote I’) enclosed by plagioclase or hornblende, and (2) relatively large (to 1 mm) grains with vermicular textures (‘epidote II’), particularly where in mutual contact with biotite (or hornblende) and plagioclase. These microstructures suggest that epidote is a magmatic phase which formed by direct crystallization from the magma, and by reaction of previously-formed minerals with the magma.The following approximate paragenetic sequence has been inferred for orbicule cores and the matrix: clinopyroxene (clinopyroxene resorbed [→ poikilitic hornblende]), epidote I, Ca-Na plagioclase, biotite, hornblende (biotite and/or hornblende ± plagioclase resorbed [→ epidote II]), quartz + K-feldspar. Biotite compositions became progressively more Fe-rich during crystallization (XMg ⋍ 0.6 → 0.4), and the first-formed plagioclase (inclusions in quartz-free hornblende in orbicule cores) is more calcic (An51) than the last (matrix grains: An35).The appearance of epidote early in the crystallization history of the diorite testifies to elevated PH2O and PTotal (PT). The most aluminous hornblende indicates maximum PT of 5 to 6 kbar. Orbicule shell structures are interpreted to have crystallized from supercooled boundary layers enclosing water-saturated globules within the dioritic magma. Although sufficient to suppress the formation of hornblende in the shell structures, the extent of magma supercooling did not permit the development of comb layering in the orbicules. Supercooling is attributed to an influx into the magma of water from an unidentified source.

Textural evidence for calcite carbonatite magmas, Dicker Willem, southwest Namibia

Carbonatites from a complex in southwestern Namibia retain features such as flow-aligned phenocrysts of calcite or dolomite enclosed in a granular or spinifex-textured calcite groundmass, comb layering, and gravity layering. All textures are compatible with Ca-rich carbonatite magmas carrying liquidus calcite or dolomite quenching on emplacement into a subvolcanic environment. Oxygen isotope fractionation between silicate and oxide minerals in sovite cumulates indicate high temperatures (600-900 °C), compatible with those determined experimentally for crystallization of liquidus calcite or dolomite. We propose that a Ca-rich carbonatite magma (proto-alvikite) separated immiscibly from a carbonate-rich ijolite and evolved by fractional crystallization of magnetite, acmitic pyroxene, and calcite to a dolomite-phyric alvikite.

Reconstruction of the geometry of accretion during formation of the Bay of Islands Ophiolite Complex

Two types of highly allochthonous ophiolitic terrains crop out adjacent to one another in the Bay of Islands region of southwestern Newfoundland, Canada. The first, called the Coastal Complex (CC), is exposed as a long, linear belt along the west coast. It consists of a highly deformed ophiolitic assemblage interpreted as oceanic crust and upper mantle with a previous transform fault tectonic history. The second, called the Bay of Islands Complex (BOIC), crops out to the east and consists of a series of four relatively undeformed ophiolitic massifs interpreted as oceanic crust and upper mantle with a more normal seafloor spreading tectonic history. An autochthonous relationship can be demonstrated between the two types of ophiolitic terrains in the southernmost of these massifs, indicating that each evolved along a single ridge‐transform system. The internal structure of both the BOIC and the CC is consistent with the above interpretations. The NE‐SW structural trends in the CC transform are oriented approximately parallel to the sea‐floor spreading direction inferred from the average orientation of the sheeted diabase dikes within the BOIC. One‐way chilling statistics from the sheeted diabase dike unit, growth directions of dendritic crystals along inclined comb layers in the plutonic section, and the sense of shear along the CC transform all indicate that the BOIC was accreted on the NE side of a NW‐SE trending spreading center (present geographic reference frame). Juxtaposition of the BOIC with the NE‐SW trending CC occurred at a ridge‐transform intersection. The transform exhibited a left‐lateral offset. Magma chamber profiles have been identified in the layered gabbroic sections of the BOIC by using the patterns outlined by the attitudes of fine‐scale igneous layering. These patterns depict arcuate structures which appear to define roughly a cross‐sectional view of a systematically oriented half‐cylinder; the long axis of which is oriented parallel with the plane of the sheeted dikes and major lithologic unit contacts. Thus we believe that a large cylindrical‐shaped crustal level subaxial magma chamber extended in a direction parallel with the ridge axial‐plane at the time of formation of the BOlC. The CC marks the abrupt truncation of this long, linear magma chamber. Petrologic constraints demonstrate that the ultramafic and transition zone cumulates which form the base of the plutonic section, crystallized at a range of high to intermediate pressures possibly along a long narrow conduit extending to 30‐km depth beneath the crustal level magma chamber. After formation, these cumulates were deformed at high temperatures in the zone of mantle upwelling and divergence, then tectonically transported to crustal or near crustal levels to form the basement upon which the remainder of the ophiolite stratigraphy was accreted. A geometric and tectonic model is presented for the accretion process responsible for formation of the Bay of Islands Ophiolite.

A Fluid Dynamic Appraisal of a Model for the Origin of Comb Layering and Orbicular Structure

The destructive power of intruding magmas is sometimes overstated as a consequence of the belief that high drag force is automatically the result of high viscosity, or because a magma contains phenocrysts or xenoliths which cause abrasion. However, consideration of the principles of fluid dynamics indicates that the opposite may generally be the case. Abrasion by particles suspended in a viscous magma is negligible, and the drag forces associated with viscous flow are not necessarily large inasmuch as high viscosity acts to limit flow rate. These contentions are demonstrated analytically for the aqueous suspension model of the formation of orbicules and comb layering proposed by Moore and Lockwood, for which the effects of an aqueous fluid and magma may be compared. As a result of this analysis, a magmatic origin for comb layering and orbicle growth is preferred, a conclusion compatible with other interpretations of comb layering.

Pipes of mafic pegmatite in the stratiform Fiskenaesset anorthosite complex, southwest Greenland

Pipes of hornblende gabbro and ultramafite cut across cumulate layering in the lower part of the Fiskenaesset complex. The pipes form irregular networks in which all vertical portions are massive and pegmatitic, whereas some horizontal portions are layered. Two kinds of layering occur: mineral-graded layers formed by precipitation of plagioclase and hornblende, and comb layers formed by the upward growth of tabular hornblende crystals. Alternations of these layers show that magma with plagioclase phenocrysts was flushed through the pipes in pulses, perhaps associated with erratic propagation of cracks and magmatic drilling.