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1.
A model for radial dike emplacement in composite cones based on observations from Summer Coon volcano,Colorado, USA 总被引：1，自引：0，他引：1
We mapped the geometry of 13 silicic dikes at Summer Coon, an eroded Oligocene stratovolcano in southern Colorado, to investigate
various characteristics of radial dike emplacement in composite volcanoes. Exposed dikes are up to about 7 km in length and
have numerous offset segments along their upper peripheries. Surprisingly, most dikes at Summer Coon increase in thickness
with distance from the center of the volcano. Magma pressure in a dike is expected to lessen away from the pressurized source
region, which would encourage a blade-like dike to decrease in thickness with distance from the center of the volcano. We
attribute the observed thickness pattern as evidence of a driving pressure gradient, which is caused by decreasing host rock
shear modulus and horizontal stress, both due to decreasing emplacement depths beneath the sloping flanks of the volcano.
Based on data from Summer Coon, we propose that radial dikes originate at depth below the summit of a host volcano and follow
steeply inclined paths towards the surface. Near the interface between volcanic cone and basement, which may represent a neutral
buoyancy surface or stress barrier, magma is transported subhorizontally and radially away from the center of the volcano
in blade-like dikes. The dikes thicken with increasing radial distance, and offset segments and fingers form along the upper
peripheries of the intrusions. Eruptions may occur anywhere along the length of the dikes, but the erupted volume will generally
be greater for dike-fed eruptions far from the center of the host volcano owing to the increase in driving pressure with distance
from the source. Observed eruptive volumes, vent locations, and vent-area intrusions from inferred post-glacial dike-fed eruptions
at Mount Adams, Washington, USA, support the proposed model. Hazards associated with radial dike emplacement are therefore
greater for longer dikes that propagate to the outer flanks of a volcano. 相似文献
2.
The origin of dike-like bodies along the Hilina fault scarp on the south flank of Kilauea Volcano. Hawaii has been the subject of recent controversy. Some geologists favour an origin by intrusion of magma from below, others favour « intrusion » of lava derived from above — lava derived from fluid surface flows which poured down open cracks. In order to distinguish between deep versus surface sources for the bodies, a suite of dike and other samples were analyzed for S, H2O, and Cl. All surface flows are degassed, whereas known dikes are volatile-rich. Samples of the Hilina dikes, and dikes from the Ninole Formation, Mauna Loa are degassed, indicating that these dikes were surface-fed — formed by magma which had been de-volatized by surface transport. A model is presented whereby the Hilina dikes form in talus and lava cones that drape the Hilina fault scarp. Seismic activity during eruption may have played an important role in the formation of the Hilina dikes. Similar dikes in the Ninole Formation probably formed in a similar environment. 相似文献
3.
Philip E. Wannamaker Jeffrey B. Hulen Matthew T. Heizler 《Journal of Volcanology and Geothermal Research》2000,96(3-4)
Newly discovered olivine phlogopite lamproite dikes intrude Jurassic siliciclastic strata in the Green River Desert subregion of the western Colorado Plateau tectonic province in southeastern Utah. The dikes yield an age of 22 Ma both from 40Ar/39Ar step-heating of phlogopite and from isochron modeling of laser-fused sanidine. This age is similar to those of mica-rich minettes and melanephelinites of the Wasatch Plateau about 125 km northwest and within the age range of the Navajo potassic volcanic field about 150 km to the southeast. The dikes intruded a pre-existing, northwest-oriented fracture system containing previously introduced bitumen, indicating that some regional lineaments of this trend are Early Miocene or older. The dikes are highly LREE-enriched, and display lamproite-specific REE ratios and phlogopite and sanidine compositions. Incompatible element and radiogenic isotope (Nd–Sr–Pb) ratios suggest that lithospheric source material modified by ancient subduction processes, together with younger asthenospheric source components, produced the melt. Timing of the intrusion coincides with the transition from Early–Middle Cenozoic, calc-alkaline plutonism to the dominantly mafic, Basin and Range type volcanism of the Late Cenozoic. While the lamproite occurrence indicates thermal input from the mantle, model non-uniqueness for both magma source depths and geophysical structure prevents quantitative comparison of Early Miocene with present-day lithospheric thickness. 相似文献
4.
Akira Takada 《Bulletin of Volcanology》1988,50(2):106-118
The Subvolcanic structure of the central dike swarm associated with the Miocene Otoge ring complex and the Shitara igneous complex, central Japan, has been reconstructed. The central dike swarm was supplied from several aligned magma reservoirs. Flow lineations observed at the margin of the dikes converge towards a region that is regarded as a magma reservoir about 1–2 km below present sea level. The minimum diameter of the magma reservoir corresponds to the width of the central dike swarm, estimated to be about 3–4 km. The inferred magma reservoir of the Otoge ring complex, may have a zoned structure, as suggested by the flow lineations of dikes and the arrangement of cone sheets. Felsic magma occupied the upper part, about 1–2 km below present sea level, and basic magma the lower part, deeper than 2 km. The centre of the Shitara igneous complex is interpreted to be composed of several other shallow magma reservoirs. The distribution pattern in plan view of the central dike swarm is summarized from the frequency of dikes (defined by the number of dikes per kilometre in the direction normal to the trend of the dike swarm) and the variations of the different properties of individual dikes along the dike swarm. It has a plane of symmetry normal to the dike swarm above the magma reservoir. The patterns critical to a general understanding of the dike formation are:
相似文献
| 1. | A region of low dike frequency is present above the magma reservoir and a radial dike pattern occurs around the magma reservoir. |
| 2. | From both sides of the magma reservoir, the axes of high dike frequency extend symmetrically along the central zone of the dike swarm. |
| 3. | The number as well as the individual and total thickness of felsic dikes increases towards the magma reservoir. |
| 4. | The number of basic dikes increases towards both sides of the magma reservoir, while the individual thicknesses of basic dikes increase with distance from the magma reservoir. |
5.
Two contacts between Sudbury norite and northwest-trending diabase dikes and two contacts between the overlying micropegmatite and northwest dikes were investigated in order to estimate the depth of burial of the present erosion surface at the time of dike emplacement. A zone of hybrid paleomagnetic direction representing the vectorial sum of an older host component and an intrusion component of decreasing highest blocking temperature and intensity with distance from the intrusion was sought. Subtracting the calculated thermal effect of the intrusion from this highest blocking temperature yields the temperature of the host at the time of magma emplacement. Dividing this host temperature by an estimated paleogeothermal gradient yields the burial depth of the present erosion (or sampling) surface at the time of magma emplacement. Remanence direction in one of the dikes and norite contact zones is not typical for the Sudbury dike swarm of 1250 Ma age, and this contact is not further considered. An earlier published result for a norite-dike contact was reconsidered because of complicated dike geometry and included in this study. In one of the four usable contacts the hybrid zone is represented by three samples, in another by one sample, and in the remaining two only the contact zone width could be used. The final host temperature results are based on 4 individual calculations and show fair consistency with mean values of 287°C (s.d. 13°) and 267°C (s.d. 11°) calculated without and with a correction for viscosity of the host remanence respectively. Using a gradient of 26°C/km for 1250 Ma ago indicates a burial depth of9.5 ± 2km at that time. The fair consistency encourages the use of the method to deduce quantitatively the history of vertical motions of Precambrian terranes, the detail obtained being dependent on the presence of hybrid zones and of intrusions of various ages. 相似文献
6.
Most flank eruptions within a central stratovolcano are triggered by lateral draining of magma from its central conduit, and only few eruptions appear to be independent of the central conduit. In order to better highlight the dynamics of flank eruptions in a central stratovolcano, we review the eruptive history of Etna over the last 100 years. In particular, we take into consideration the Mount Etna eruption in 2001, which showed both summit activity and a flank eruption interpreted to be independent from the summit system. The eruption started with the emplacement of a ~N-S trending peripheral dike, responsible for the extrusion of 75% of the total volume of the erupted products. The rest of the magma was extruded through the summit conduit system (SE crater), feeding two radial dikes. The distribution of the seismicity and structures related to the propagation of the peripheral dike and volumetric considerations on the erupted magmas exclude a shallow connection between the summit and the peripheral magmatic systems during the eruption. Even though the summit and the peripheral magmatic systems were independent at shallow depths (<3 km b.s.l.), petro-chemical data suggest that a common magma rising from depth fed the two systems. This deep connection resulted in the extrusion of residual magma from the summit system and of new magma from the peripheral system. Gravitational stresses predominate at the surface, controlling the emplacement of the dikes radiating from the summit; conversely, regional tectonics, possibly related to N-S trending structures, remains the most likely factor to have controlled at depth the rise of magma feeding the peripheral eruption. 相似文献
7.
Gabbroic xenoliths that represent cumulate environments within Mauna Kea Volcano are, in rare examples, penetrated by small-scale
(<7 cm) dikes. We examined four dike/host composite xenoliths to establish how this evidence for magma seemingly injected
into cumulate gabbro fits into the evolution of igneous processes in shield volcano magma reservoirs. Olivine, clinopyroxene,
and plagioclase compositions in both host gabbros and dikes are characteristically tholeiitic and evolved (Fo71–66, cpx-Mg # 79–77, An72–51) with respect to Hawaiian magmatism. Dikes, however, when compared with their host gabbros, have slightly greater abundances
of some incompatible elements and slightly more evolved olivine compositions (e.g., Fo68 vs Fo71). Compared with Mauna Kea lava compositions, both host gabbros and dikes have lower incompatible-element abundances, positive
Eu anomalies, and, notable for dikes, major-element compositions unlike those of lavas (e.g., SiO2<46 wt.%). The small-scale dikes, therefore, also have cumulate characteristics. We interpret them as representing late-stage
liquids (e.g., <5 wt.% MgO, based on <Fo70) "squeezed" from solidifying cumulus piles of evolved (e.g., ∼Fo70) gabbroic assemblages. The compositions of the dikes, however, do not match those of the most evolved liquids expected in
reservoirs because they appear to have lost interstitial liquids (e.g., positive Eu anomalies, low abundances of some trace
elements). Because minerals in the dikes were in equilibrium with highly evolved liquids, conditions for small-scale dike
formation in cumulate environments apparently occur only at the last stages of reservoir magma differentiation and solidification.
Received: 25 February 1997 / Accepted: 14 June 1997 相似文献
8.
The emplacement conditions for 39 igneous dikes cutting basalts in northwestern Ethiopia are evaluated by analyzing their displacement–length scaling relations. Maximum opening displacements and lengths of the dikes demonstrate displacement–length scaling of the form Dmax = 0.088L0.48, consistent with other populations of dikes and veins and different than the power-law scaling relation typically found for faults. The dikes propagated through the thin Trap basalt sequence under conditions of constant fracture toughness, with values corrected for three-dimensional dike geometry of ~ 77–273 MPa m1/2. The large values of fracture toughness are likely associated with (1) the toughening effects of near-tip damage, (2) mixed-mode dike propagation, as shown by magma flow fabric analysis through anisotropy of magnetic susceptibility (AMS) and image analysis of thin sections, and (3) elevated temperature within the blocky and ductile basaltic host rock, evidence of which has been found in the field. 相似文献
9.
Dikes within stratovolcanoes are commonly expected to have radial patterns. However, other patterns may also be found, due
to regional stresses, magmatic reservoirs and topographic variations. Here, we investigate dike patterns within volcanic edifices
by studying dike and fissure complexes at Somma-Vesuvius and Etna (Italy) using analogue models. At the surface, the dikes
and fissures show a radial configuration. At depths of tens to several hundreds of metres, in areas exposed by erosion, tangential
and oblique dikes are also present. Analogue models indicate that dikes approaching the flanks of cones, regardless of their
initial orientation, reorient to become radial (parallel to the maximum gravitational stress). This re-orientation is a significant
process in shallow magma migration and may also control the emplacement of dike-fed fissures reaching the lower slopes of
the volcano. 相似文献
10.
A 45-km-long regional dike was emplaced over a period of 2 weeks in August 2014 at the boundary between the East and North Volcanic Zones in Iceland. This is the first regional dike emplacement in Iceland monitored with modern geophysical networks, the importance of which relates to regional dikes feeding most of the large fissure (e.g., Eldgja 934 and Laki 1783) and lava shield (e.g. early Holocene Skjaldbreidur and Trölladyngja) eruptions. During this time, the dike generated some 17,000 earthquakes, more than produced in Iceland as a whole over a normal year. The dike initiated close to the Bardarbunga Volcano but gradually extended to the northeast until it crossed the boundary between the East Volcanic Zone (EVZ) and the North Volcanic Zone (NVZ). We infer that the strike of the dike changes abruptly at a point, from about N45°E (coinciding with the trend of the EVZ) to N15°E (coinciding with the trend of the NVZ). This change in strike occurs at latitude 64.7°, exactly the same latitude at which about 10 Ma dikes in East Iceland change strike in a similar way. This suggests that the change in the regional stress field from the southern to the northern part of Iceland has been maintained at this latitude for 10 million years. Analytical and numerical models indicate that the dike-induced stress field results in stress concentration around faults and particularly shallow magma chambers and calderas in its vicinity, such as Tungnafellsjökull, Kverkfjöll, and Askja. In particular, the dike has induced high compressive, shear, and tensile stresses at the location of the Bardarbunga shallow chamber and (caldera) ring-fault where numerous earthquakes occurred during the dike emplacement, many of which have exceeded M5 (the largest M5.7). The first segment of the dike induced high tensile stresses in the nearby part of the Bardarbunga magma chamber/ring-fault resulting in radially outward injection of a dike from the chamber at a high angle to the strike of the regional dike. The location of maximum stress at Bardarbunga fluctuates along the chamber/ring-fault boundary in harmony with dike size and/or pressure changes and encourages ring-dike formation and associated magma flow within the chamber. Caldera collapse and/or eruption in some of these volcanoes is possible, most likely in Bardarbunga, but depends largely on the future development of the regional dike. 相似文献
11.
Elisabeth A. Parfitt Tracy K. P. Gregg Deborah K. Smith 《Journal of Volcanology and Geothermal Research》2002,113(1-2)
Eruption styles on the subaerial East Rift Zone (ERZ) of Kilauea volcano are reviewed and a classification scheme for the different types of eruption is proposed. The various eruption types are produced by differing thermal and driving pressure behaviour in the feeder dikes. Existing evidence is reviewed and new evidence presented of the types and volumes of eruptions on the Puna Ridge, which is the submarine extension of the ERZ. Eruptions on the Puna Ridge fall into the same five classes as, and are of comparable volume to, those on the subaerial ERZ. Evidence is presented which suggests that feeder dikes for Puna Ridge eruptions are more thermally viable than those feeding subaerial eruptions, and this difference causes long-lived, large-volume eruptions to be more common on the Puna Ridge than on the subaerial ERZ. This systematic variation in thermal viability may be due to increased dike width for Puna Ridge dikes or increased pressure gradients driving magma flow. Lateral dike emplacement is common to many basaltic systems including on other Hawaiian volcanoes, in Iceland and at mid-ocean ridges. The systematic trend inferred for the ERZ of Kilauea implies that in the other systems large-volume eruptions may also be more common at great distances than they are close to the magma centre. 相似文献
12.
Numerical models show that maximum dike width at oceanic spreading centers should scale with axial lithospheric thickness if the pre-diking horizontal stress is close to the Andersonian normal faulting stress and the stress is fully released in one dike intrusion. Dikes at slow-spreading ridges could be over 5 m wide and maximum dike width should decrease with increasing plate spreading rate. However, data from ophiolites and tectonic windows into recently active spreading ridges show that mean dike width ranges from 0.5 m to 1.5 m, and does not clearly correlate with plate spreading rate. Dike width is reduced if either the pre-diking horizontal stress difference is lower than the faulting stress or the stress is not fully released by a dike. Partial stress release during a dike intrusion is the more plausible explanation, and is also consistent with the fact that dikes intrude in episodes at Iceland and Afar. Partial stress release can result from limited magma supply when a crustal magma chamber acts as a closed source during dike intrusions. Limited magma supply sets the upper limit on the width of dikes, and multiple dike intrusions in an episode may be required to fully release the axial lithospheric tectonic stress. The observation of dikes that are wider than a few meters (such as the recent event in Afar) indicates that large tectonic stress and large magma supply sometimes exist. 相似文献
13.
The 14 ka Puketarata eruption of Maroa caldera in Taupo Volcanic Zone was a dome-related event in which the bulk of the 0.25 km3 of eruption products were emplaced as phreatomagmatic fall and surge deposits. A rhyolitic dike encountered shallow groundwater during emplacement along a NE-trending normal fault, leading to shallow-seated explosions characterised by low to moderate water/magma ratios. The eruption products consist of two lava domes, a proximal tuff ring, three phreatic collapse craters, and a widespread fall deposit. The pyroclastic deposits contain dominantly dense juvenile clasts and few foreign lithics, and relate to very shallow-level disruption of the growing dome and its feeder dike with relatively little involvement of country rock. The distal fall deposit, representing 88% of the eruption products is, despite its uniform appearance and apparently subplinian dispersal, a composite feature equivalent to numerous discrete proximal phreatomagmatic lapilli fall layers, each deposited from a short-lived eruption column. The Puketarata products are subdivided into four units related to successive phases of:(A) shallow lava intrusion and initial dome growth; (B) rapid growth and destruction of dome lobes; (C) slower, sustained dome growth and restriction of explosive disruption to the dome margins; and (D) post-dome withdrawal of magma and crater-collapse. Phase D was phreatic, phases A and C had moderate water: magma ratios, and phase B a low water: magma ratio. Dome extrusion was most rapid during phase B, but so was destruction, and hence dome growth was largely accomplished during phase C. The Puketarata eruption illustrates how vent geometry and the presence of groundwater may control the style of silicic volcanism. Early activity was dominated by these external influences and sustained dome growth only followed after effective exclusion of external water from newly emplaced magma. 相似文献
14.
Dating of altered mafic intrusions by applying a zircon fission track thermochronometer to baked country rock,and implications for the timing of volcanic activity during the opening of the Japan Sea 
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下载免费PDF全文 Once a mafic intrusive rock has become altered, it is generally difficult to obtain a reliable intrusion age using conventional isotopic dating methods. To overcome this problem, this study used zircon fission track (ZFT) thermochronometry to determine the timing of crystallization of altered mafic intrusions. ZFT dating was carried out on samples of baked granite country rock adjacent to dolerite dikes (5–10 m thick) in the Takato area of central Japan. Three granite samples collected within 8 mm of a dike contact yielded consistent ZFT ages of 17–16 Ma, with confined track lengths indicative of the complete annealing of pre‐existing tracks by reheating due to dike intrusion. An older ZFT age was obtained for one granite sample collected within 20 mm of the contact, but confined track length measurements indicate that this is an incompletely reset age that lies between the ZFT age of the unbaked granitic country rocks (ca. 55 Ma) and the emplacement age of the dike. Petrographic examinations suggest that post‐intrusion hydrothermal activity did not influence the ZFT ages. We conclude that the 17–16 Ma ZFT age represents the emplacement age of the dikes. Our results show that ZFT dating of baked country rock is an effective tool for dating altered mafic intrusions, for which other dating techniques are not applicable. In the eastern part of Southwest Japan, dispersed volcanic activity occurred in the late Early to early Middle Miocene (18–15 Ma), and the volcanic belt extended into the forearc. This pulse of activity was possibly related to the injection of asthenospheric material into the trench‐side mantle wedge beneath the Japan arc. We also present young apatite fission track ages (ca. 4 Ma) that may reflect a Middle Miocene or later thermal event associated with local magmatic activity near the Takato area. 相似文献
15.
Allan M Rubin 《Bulletin of Volcanology》1990,52(4):302-319
Observations of eroded volcanic rift zones indicate that dikes in Iceland are typically several times thicker than those in Hawaii. Geodetic and seismic observations of active rifts, however, suggest that dike heights in the two regions are similar. Provided the elastic properties of the rift zones are the same, this implies that dikes are intruded with higher driving pressures (magma pressure minus compressive stress perpendicular to the dike plane) in Iceland than Hawaii. A second major difference between the two regions is the greater prevalence of large normal fault scarps in rift zones in Iceland. From this it can be infered that a lower percentage of dikes breach the surface in Iceland than in Hawaii. Thus, although dikes in Iceland are intruded with higher driving pressures, they possess lower absolute magma pressures than in Hawaii. These differences can be interpreted in terms of the tectonic settings in the two regions. In Iceland, a steady remote extension reduces the horizontal stress perpendicular to the rift zone, allowing dikes to be intruded with low absolute pressures but high driving pressures when magma becomes available. In Hawaii, a more continuous magma supply on the timescale over which the dike-induced stresses are relaxed, and perhaps a greater role for intrusions in driving long-term rift extension, ensure that the rift-compressive stress is not relaxed significantly before the next dike is intruded. Thus the magma pressure must be nearly sufficient for eruption in order for intrusion to occur. If the mechanism for relaxing the rift-compressive stress were less efficient still, then an even higher percentage of dikes would erupt, and at times the rift zone trend could become an unfavorable orientation for dike intrusion. Such might be the case at Mauna Loa, which lacks large rift-zone faults and fissures and possesses numerous radial vents outside its two main rift zones. 相似文献
16.
The McClure Mountain — Iron Mountain alkalic complex, the largest of three Precambrian alkalic complexes in south-central Colorado, is an irregularly ovoid body about 8×6.5 kilometers in plan. It contains major petrologic units (in age sequence): 1) peridotite and ilmenite-magnetite lenses, 2) gabbro, 3) hornblende syenite, 4) ijolite and 5) nepheline syenite, whose internal arrangement is irregular. Dikes are numerous and varied both within the complex and in the surrounding Precambrian metasediments and meta-volcanics (Idaho Springs formation), granitoids and migmatites to a distance of 27 kilometers from the complex. Fenitization has only locally affected these wall rocks at their contacts with the complex. Dikes within the complex are chiefly light colored and of syenitic or nepheline syenitic composition. Texturally they range from aphanitic, through porphyritic with an aphanitic to fine-grained matrix, to uniformly medium-grained. Most of the intracomplex dikes (save the few carbonatites) are represented by compositionally equivalent major units within the complex. In contrast, the extracomplex dikes are chiefly lamprophyres and carbonatites, none of which has a compositionally similar unit within the complex. The carbonatite dikes are younger than the lamprophyres; where the two types co-occupy the same fracture, the lamprophyre has been carbonatized. The lamprophyres are represented by an extraordinary number of textural varieties, involving phenocrysts and phenocryst combinations of augite, barkevikite, olivine and plagioclase in varying matrix combinations of olivine, augite, barkevikite, biotite, magnetite, apatite, plagioclase and orthoclase. Many of the dikes can be classed as camptonites or olivine camptonites. The Goldie lamprophyres contain local marginal phases spectacularly marked by concentrically zoned ovoids. The Cabin dike is studded by megaphenocrysts of augite. The carbonatites are of three main types:
- Relatively homogeneous calcitic or dolomitic carbonatites with few accessories.
- Composite carbonate — hematitic potash feldspar bodies which, with diminution of carbonate, grade into feldspathic Th-RE veins.
- Carbonatites with abundant barite and/or fluorite, and rarer species. The Goldie carbonatite also contains a complex suite of aluminofluorides.
17.
Variations in thickness of fault‐controlled dikes and its implications: A case study of Gosung area,South‐East Korea 下载免费PDF全文
下载免费PDF全文 Dikes are natural records that can be used to understand the way magma flows in the crust. Coastal platform outcrops in Gosung, South Korea, show clear evidences that their intrusion took place along pre‐existing fractures. We analyzed outcropping dikes, measuring variations in dike thickness as well as fracture density (cumulative number of fractures along strike) and geometry around the dikes. The geometry and thickness variations of dikes intruded along pre‐existing fractures can be interpreted to understand the effect of pre‐existing fractures to evolution on magma flow, especially related with fault damage zones. This helps us to gain a better understanding of magma and fluid flow along pre‐existing fractures. Magma flow is greater along planes that strike perpendicular to the direction of least compressive horizontal stress, and along well connected fractures that show a high degree of connectivity. At the fault tip and linkage damage zone, there is a concentration of extensional fractures; in these areas injected dikelets can form. As faults become linked, the fracture density increases, until they become fully linked and act as one through‐going fault plane. As faults evolve, the boundary conditions of the faults vary and this has an impact on dike characteristics. Fracture geometry around dikes that intruded pre‐existing faults can be used as a record of fault evolution and this can give insights into how the maturity of a fault system can affect to the related magma or fluid flow characteristics. 相似文献
18.
Laurence A. Coogan Simone A. Kasemann Sumit Chakraborty 《Earth and Planetary Science Letters》2005,240(2):415-424
Episodic emplacement and cooling of lavas and dikes at mid-ocean ridges leads to large fluctuations in hydrothermal fluxes and biological activity. However, the processes operating beneath the seafloor during these transient events such as permeability creation and dike cooling are poorly understood. We have developed a new approach to determine the cooling rate of the sheeted dike complex based on the extent of diffusion of lithium from plagioclase into clinopyroxene during cooling. We have calibrated this Li-geospeedometer using new high-temperature experiments to determine both the temperature dependence of the partitioning of Li between plagioclase and clinopyroxene and the diffusion coefficient for Li in clinopyroxene. Application of this method to lavas and dikes from ODP Hole 504B shows that cooling rates vary dramatically with depth in the upper oceanic crust. Extremely rapid cooling rates (> 450 °C hr− 1) in the upper part of the sheeted dike complex are sufficient to power hydrothermal megaplume formation within the overlying water column. 相似文献
19.
Fujii and Uyeda (1974) postulated that viscous dissipation may lead to thermal instability and explosive eruptions in the case of volcanic conduits or dikes. Although their conclusions were based on a viscosity function which was valid over a very narrow temperature range, calculations presented here lead to the same result for critical dike width. A simple forced intrusion model, without viscous dissipation effects, is also developed and found to be sufficient to explain the observed width of volcanic conduits and dikes. The mechanism of thermal runaway may present problems for magma energy extraction. 相似文献
20.
Geometry and mode of emplacement of dike swarms around the Birnudalstindur igneous centre,SE Iceland
《Journal of Volcanology and Geothermal Research》2006,151(4):340-356
Dike geometries around the well-exposed periphery of the Birnudalstindur igneous centre (SE Iceland) are constrained by moving averages of strike, dip, thickness and dilation by 775 mafic dikes, mapped along three strategically placed transects. On the basis of spatial analysis of dike strikes, a rift-parallel swarm is distinguished from a cross-cutting tri-axial swarm pattern of ‘brown dolerites’ that clearly post-dates the volcano's cone sheet swarm. Dikes are on average orientated at right angles to the lava pile and consequently used to constrain the ‘flexured’ geometry of the host lava pile, subsequently back-rotated to horizontal. This produces two end-member scenarios, which can be tentatively used to evaluate the dynamic formation of Icelandic crust. Dike dilations above a prominent stratigraphical transition into hyaloclastite breccias are markedly lower than in the underlying plateau lava pile, suggesting that vertical dike propagations were inhibited along this density/stress boundary. Lined up with the Birnudalstindur igneous centre, average dike thicknesses decrease towards the axes of both the rift-parallel dike swarm and the rift-perpendicular branch of the tri-axial swarm. This arguably links all dike swarms to the Birnudalstindur igneous centre, even if it remains inconclusive whether rift-parallel dikes fed and/or were injected laterally away from its sub-volcanic magma chamber. It seems more likely that the slightly offset tri-axial swarm of brown dolerites was preferentially emplaced along a peripheral bulge that was created around the ‘down-sagging’ volcano. 相似文献