Conduit Width Research Articles

Sedimentological and GPR Studies of Subglacial Deposits in the Joux Valley (Vaud, Switzerland): Backset Accretion in an Esker Followed by an Erosive Jökulhlaup

During the Würmian glaciation, the Jura ice sheet covered the Joux Valley (Vaud, Switzerland). A geomorphological study reveals many drumlins in this valley. Some are composed of gravels and sand, others of till. Outcrops show that the surface of the sandy-gravel drumlins is a major and sharp erosion surface. Given the lack of shearing structures in sediments below this erosion level, its origin cannot be linked to ice action of the glacier. Very high-energy subglacial meltwater floods (jökulhlaups), probably due to the drainage of subglacial or supra-glacial lakes, are the more likely cause of the erosion. Results of a ground penetrating radar (GPR) survey show the internal structure of one of these sandy-gravel drumlins to depth of 15 m. These GPR data, together with sedimentological observations, indicate that prior to erosion, subglacial sedimentation occurred in closed conduits (eskers) with strong and rapid flow variations. The sediments contain large chute-and-pool structures (high flow energy backset accretion) with dimensions comparable to the conduit width. Therefore, we interpret these sandy-gravel drumlins as portions of eskers, their present drumlin shape being the result of erosion by one or many jökulhlaups.The good preservation of the subglacial meltwater deposits is the result of the closed-basin geometry of the Joux Valley, which limited movement at the base of the glacier. This new contribution to the interpretation of the Joux Valley glacial features underlines the importance of meltwater in sedimentological processes under the Jura ice sheet.

Relations between vulnerability to xylem embolism and xylem conduit dimensions in young trees of Quercus corris

ABSTRACTMeasurements of xylem conduit length and width and the distribution of xylem conduit ends were made in inter‐nodes (I), nodes (N) and twig junctions (J) of 1‐, 2‐ and 3‐year‐old twigs of plants of Quercus cerris L. Parallel measurements were also made of the loss of hydraulic conductivity of twigs subjected to pressure differentials across conduit pit membranes, equalling the leaf water potential at the turgor loss point. The loss of theoretical hydraulic conductivity was calculated as the ratio of i esivr4 (where r is the conduit radius) of the non‐conducting conduits to that of all the conduits in the outermost wood ring of I, N and J. Stem zones such as 1‐year‐old nodes and junctions were localized with narrower and shorter xylem conduits and with higher percentages of conduit ends than internodes. Such ‘constricted zonesrsquo; were less vulnerable to embolism than internodes. Latewood conduits were consistently narrower, shorter and less vulnerable to embolism than earlywood ones. A positive relation therefore existed between conduit diameter and length and vulnerability to embolism. The overall vulnerability to embolism of Q. cerris plants is discussed in terms of xylem conduit width and length and of the distribution of conduit ends.

Xylem conduction and cavitation inHevea brasiliensis

Clones of Hevea were studied in an attempt to discover the reasons for differences in the hydraulic performance of xylem. Differences between clones were determined, including hydraulic conductivity and conduit width and length distributions. However, it has proved difficult to reconcile anatomical differences with physiological performance for use in future plant breeding programmes. When leaf relative water content (RWC) had been reduced from about 95% to 85%, the hydraulic conductivity of petioles decreased sharply to about 40% of the initial value. This value corresponded with xylem sap tensions of 1.8-2.0 MPa. Acoustic detection experiments revealed that this reduction in hydraulic conductivity coincided with the greatest occurrence of cavitation. It seems inescapable that the reduction in hydraulic conductivity was caused by embolization ; thereafter gas bubbles blocked the flow of water inside many of the conduits. There was some indication that eventually such bubbles might be dissolved, because the hydraulic conductivity increased again if specimens were fully rehydrated. Apparently, the incidence of cavitation coincides with the entry of gas bubbles via ultramicroscopic pores into the conduits through the walls according to the air-seeding hypothesis. When a petiolate leaf is tested in a pressure chamber it is impossible to make satisfactory measurements of a balancing pressure beyond c. 1.8-2.0 MPa, because air bubbles, mixed with sap and escaping from the conduits, form a persistent froth. Xylem transport in Hevea seems to be disrupted relatively easily under water stress which is a feature of other tropical species adapted to rainforest-type environments.

Three‐dimensional flow beneath a slow spreading ridge axis: A dynamic contribution to the deepening of the Median Valley toward fracture zones

The axial valley along a slow spreading mid‐ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis. Along a ridge segment, the axial valley floor deepens by as much as 2–3 km over distances of several tens of kilometers approaching a ridge‐transform intersection. This deepening may be explained, in part, by a pressure gradient associated with horizontal flow in the ridge axis conduit. Vanishing vertical velocity on a conduit end wall at a ridge‐transform intersection results in reduced vertical flow within about one conduit width of the intersection. Horizontal flow along the conduit, toward the intersection, must occur to form lithosphere at a uniform rate along the ridge axis. A simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge‐transform intersections is considered. The model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls. This model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection. For reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection.

Carbon dioxide in petrogenesis III: role of volatiles in the ascent of alkaline magma with special reference to xenolith-bearing mafic lavas

Kinetic and fluid dynamic constraints on deep-seated magma migration rates suggest ascent velocities in the range 10 to 30 m/s, 10−1 to 10 m/s and 10−2 to 5 m/s for kimberlitic, garnet peridotite-bearing and spinel peridotite-bearing alkalic magmas. These rates virtually demand translithospheric magma transport by a fracture as opposed to diapiric mechanism. The hypothesis that volatile exsolution accelerates magma through the deep lithosphere is tested by solution of the appropriate set of conservation, mass balance and volatile component solubility equations governing the steady ascent (decompression) of compressible, two-phase magma (melt+H2O+CO2) in which irreversible phenomena (friction, heat transfer) are accounted for. The results of the numerical experiments were designed to test the importance of melt bulk composition (kimberlite, nephelinite, alkali basalt), initial conditions (mass flux (M), heat transfer coefficient (B), lumped friction factor (C f )), conduit width (D), initial magma volatile content and geothermal gradients. The fractional increase in ascent rate (Δu/u i ) is rarely greater than approximately 2 during translithospheric migration. The propellant hypothesis is rejected as a first-order mechanism driving magma acceleration during ascent. The most influential parameters governing ascent dynamics are M, C f , D, B and the geotherm. Because of the relatively incompressible nature of the magmatic volatile phase at P>100 MPa, the initial magma volatile content plays a secondary (although demonstrable) role. The main role of volatiles is in controlling the initial magma flux (M) and the magma pressure during ascent. In adiabatic (B=0) simulations, magma ascends nearly isothermally. Generally, however, the assumption of adiabaticity is a poor one especially for flow through narrow (0.5 to 2 m) conduits in old (cold) lithosphere at rates ∼10−1 m/s. The proposed fluid dynamic model is consistent with and complementary to the magma-driven crack propagation models. The generation of mantle metasomatic fluid is a corollary of the non-adiabatic ascent of volatile-bearing magma through the lithosphere. Magma heat death is an important process for the creation of mantle heterogeneity.

Dynamics of a passive spreading center

At a passive spreading center the motion of the fluid rising in the conduit between diverging lithosphere plates is generated by the motion and weight of the plates. Combined gravity and viscous forces exerted by the rising material on the overhanging conduit walls deform the adjacent lithosphere to produce sea floor topography. If viscous forces in the conduit and long-term strength of the young lithosphere were negligible, topography could be in approximate isostatic balance, consistent with the observation that the gravity field is relatively undisturbed. However, deep axial valleys for slow spreading suggest that viscous forces are not negligible; they evidently suppress the rise in the conduit and support the gravitationally unstable lithosphere. For a general class of conduits that could occur in nature (those with a bottleneck near the top) the upward viscous force equals the weight of the mass deficiency in the conduit. Dynamically supported topography above such conduits could account for the gross mass balance without limiting the role of viscous forces. A static pressure deficiency at the base of the conduit drives the upflow against viscous resistance; for given physical properties this driving pressure is determined by the elevation of the sea floor at the axis. When V changes from one steady value to another, the rate of upflow in the conduit must change to balance the new rate of lithosphere production. The upflow may be adjusted by varying the static driving pressure through changes in axial elevation (‘damping’) or by changing the resistance to upflow by varying the conduit width (‘throttling‘). However, the mechanism by which conduit fluid changes to lithosphere solid determines conduit width as a function of velocity. Hence as velocity changes, the axial elevation must adjust to assure compatibility among the dynamics of conduit flow, the dynamics of accretion, and the condition of mass continuity. If conduit width is proportional to velocity (an approximation for the case of isothermal conduit walls), axial elevation must decrease sharply as small velocities decrease. This is consistent with observed deep axial valleys in slow spreading and small axial ridges in fast spreading; it is illustrated with analytical results for Newtonian flow in a wedge-shaped conduit. At small velocities, slower spreading plates will be harder to separate. If body forces and tractions elsewhere on the plates cannot equilibrate the horizontal component of suction caused by deep valleys required at very small velocities, steady spreading at these velocities is impossible, and the plates will coalesce. This could be why the earth's lithosphere is so well organized and not fragmented into many small plates. Resistance to plate separation associated with dissipation of mechanical energy in the axial region can be expressed in terms of the dynamic model, and hence so can the stability conditions for oblique spreading and transform faults. Oblique spreading and absence of the characteristic axial valley on the Reykjanes Ridge can be attributed to anomalously low asthenosphere viscosity associated with the Iceland volcanic center. In the Reykjanes conduit the reduced viscous suppression could eliminate the axial valley, and reduced dissipation of mechanical energy could lead to stable spreading in the oblique mode. The passive dynamic model is easily generalized to simple active cases.

Segregation of Magma from a Mostly Crystalline Mush

A model based on the fluid dynamics of intermixed fluids indicated that favorable conditions for segregation of melt from a mostly crystalline mush upwelling from the asthenosphere included a high concentration of melt, a narrow conduit containing the mush, a large grain size, and a large ratio of the viscosity of grains to the viscosity of melt. The greater depth, inferred geochemically, of the source regions of seamount lavas, compared with mid-oceanic ridge lavas, is attributable to the dependence of segregation on conduit width. For thermal reasons, conduit width is smaller at greater depths for small sources of material such as seamounts. Observed systematic enrichment of seamount and island-arc lavas in radiogenic isotopics (which, for chemical reasons, would be most abundant in domains containing high fractions of water) relative to mid-oceanic ridge lavas may be due to disproportionate representation of ubiquitous, small-scale, water-rich domains in the source region of seamount lavas. The fraction of melt is strongly dependent on water content in the deep source regions of seamount lavas but only weakly dependent in the shallow source regions of mid-oceanic ridge lavas. This also complicates identification of subducted sediments in island-arc lavas. The viscosity of partial melt in the asthenosphere, inferred from seismic attenuation studies, is too high for that melt to segregate efficiently.