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When basalt magmas are emplaced into continental crust, meltingand generation of silicic magma can be expected. The fluid dynamicaland heat transfer processes at the roof of a basaltic sill inwhich the wall rock melts are investigated theoretically andalso experimentally using waxes and aqueous solutions. At theroof, the low density melt forms a stable melt layer with negligiblemixing with the underlying hot liquid. A quantitative theoryfor the roof melting case has been developed. When applied tobasalt sills in hot crust, the theory predicts that basalt sillsof thicknesses from 10 to 1500 m require only 1 to 270 y tosolidify and would form voluminous overlying layers of convectingsilicic magma. For example, for a 500 m sill with a crustalmelting temperature of 850 ?C, the thickness of the silicicmagma layer generated ranges from 300 to 1000 m for countryrock temperatures from 500 to 850?C. The temperatures of thecrustal melt layers at the time that the basalt solidifies arehigh (900–950?C) so that the process can produce magmasrepresenting large degrees of partial fusion of the crust. Meltingoccurs in the solid roof and the adjacent thermal boundary layer,while at the same time there is crystallization in the convectinginterior. Thus the magmas formed can be highly porphyritic.Our calculations also indicate that such magmas can containsignificant proportions of restite crystals. Much of the refractorycomponents of the crust are dissolved and then re-precipitatedto form genuine igneous phenocrysts. Normally zoned plagioclasefeldspar phenocrysts with discrete calcic cores are commonlyobserved in many granitoids and silicic volcanic rocks. Suchpatterns would be expected in crustal melting, where simultaneouscrystallization is an inevitable consequence of the fluid dynamics. The time-scales for melting and crystallization in basalt-inducedcrustal melting (102–103 y) are very short compared tothe lifetimes of large silicic magma systems (>106 y) orto the time-scale for thermal relaxation of the continentalcrust (> l07 y). Several of the features of silicic igneoussystems can be explained without requiring large, high-level,long-lived magma chambers. Cycles of mafic to increasingly largevolumes of silicic magma with time are commonly observed inmany systems. These can be interpreted as progressive heatingof the crust until the source region is partially molten andbasalt can no longer penetrate. Every input of basalt triggersrapid formation of silicic magma in the source region. Thismagma will freeze again in time-scales of order l02–103y unless it ascends to higher levels. Crystallization can occurin the source region during melting, and eruption of porphyriticmagmas does not require a shallow magma chamber, although suchchambers may develop as magma is intruded into high levels inthe crust. For typical compositions of upper crustal rocks,the model predicts that dacitic volcanic rocks and granodiorite/tonaliteplutons would be the dominant rock types and that these wouldascend-from the source region and form magmas ranging from thosewith high temperature and low crystal content to those withhigh crystal content and a significant proportion of restite. 相似文献
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Komatiites II: Experimental and Theoretical Investigations of Post-emplacement Cooling and Crystallization 总被引：1，自引：2，他引：1
TURNER J. STEWART; HUPPERT HERBERT E.; SPARKS R. STEPHEN J. 《Journal of Petrology》1986,27(2):397-437
When komatiite lavas are emplaced on the sea floor most of theheat transfer occurs through the upper lava-seawater boundary.We have investigated the cooling and crystallization of komatiitesusing a series of analogue laboratory experiments with aqueoussolutions and by theoretical analysis. In komatiites the viscosityis sufficiently low that convection occurs in the interior ofthe flow and these motions, due both to thermal and compositionalvariations, have an important influence on the characteristicfeatures of komatiites such as the strong compositional andtextural layering. The experiments have been conducted with crystallizing aqueoussolutions which display the same overall dynamical processesas solidifying komatiites. The solutions used are simple eutecticsystems having the property that crystallization from a solutionwhich is substantially more concentrated than the eutectic compositionleaves behind residual fluid which is less dense than the originalfluid. This models the decrease in density of komatiite meltson cooling, due to the crystallization of olivine. Such solutionshave been cooled strongly through the metal roof of an otherwiseinsulated container, using a typical fluid depth of 80 mm. Dendriticcrystals grew down vertically from the roof and released lightfluid, depleted in solute, which rose to form a zone of stagnantfluid at the top of the container, while the tips of the crystalsextended just below the bottom of this light layer. A layerof solid eutectic, with a horizontal front, grew more slowlyand filled in the space between the vertically oriented crystals. The growth of the crystals and the eutectic layer were monitoredvisually, and in some experiments the temperatures at the topand in the fluid were recorded, until solidification throughoutthe layer was complete. The solid block was sampled, and themelted products analysed to give vertical concentration profiles.Both the texture and composition are strongly influenced bythe fluid conditions during crystal growth. The top concentrationis that of the original solution, rapidly quenched against theroof, and the mean concentration through the region influencedby the stable fluid layer is also close to the original. Atthe bottom the concentration is high, reflecting the in situgrowth of close-packed crystals, and there is a sharp decreasein concentration at an intermediate level, between the upperand lower crystal layers. The experiments and associated theory shed new light on theconsolidation of komatiites and the development of their characteristictextures and compositions. Since the lava is convecting withinthe interior, the early stages of cooling are characterizedby a rapid decrease in temperature. Initial cooling rates of1 to 100 °C h–1 are calculated. At this stage thecrust remains thin, but as the spinifex zone develops, convectionprogressively decreases in vigour and the cooling rate decreases.Spinifex texture is considered to form by constitutional supercoolingwhich is controlled by compositional convection. As the spinifextexture develops, the olivine dendrites form a layer of depletedfluid. The tips of the crystals extend beyond this differentiatedlayer into a convecting lower region and grow preferentiallyto produce the characteristic vertically oriented spinifex texture.The composition of spinifex zones is shown to be close, butnot identical, to the initial liquid composition. The compositionalprofiles of the solid products of the experiments are similarto those found in komatiites, with the most evolved rock compositionsbeing found just above the cumulate zone. The experiments alsosuggest an alternative explanation to crystal settling for thecumulate zone, in which growth of the spinifex zone by compositionalconvection concentrates crystals suspended within the turbulentlyconvecting lower layer. 相似文献
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Komatiites I: Eruption and Flow 总被引：2，自引：1，他引：2
Because of their high eruption temperatures and ultrabasic composition,komatiite lavas had low viscosities, which typically rangedfrom 0-1 to 10 Pa s. A major consequence of this low viscosityis that most lavas erupted as turbulent flows. An analysis oftheir ascent through the lithosphere suggests ascent velocitiesin the range of 1 to over 10ms–1 and Reynolds numbersmuch greater than the critical value of 2000. The lavas wouldhave remained turbulent for most or all of their subsequentflow and emplacement. Typical horizontal flow rates are estimatedto range from 0?5 to 100 m2 s–1 per unit width of flow.Such turbulent lava flows would have lost their heat by convectionto the surroundings, at rates which are orders of magnitudegreater than the rates for laminar flows, which cool by conduction.A quantitative analysis of the cooling of komatiites indicatescooling rates from over 1000 ?C hr–1 to a few ?C hr–1,while the flows remained turbulent. These rates are in an appropriaterange to cause phenomena such as high nucleation rates, strongsupersaturation of the lava, delayed nucleation of olivine,and skeletal or dendritic crystal morphologies. Komatiites often flowed over ground composed of rocks with lowermelting temperatures. It is proposed that the turbulent lavasmelted the ground to form deep thermal erosion channels. Meltingrates at the lava source are calculated at several metres perday, and deep troughs with depths of several metres to hundredsof metres and lengths of several kilometres probably formed.Laboratory experiments performed to simulate thermal erosionshow qualitative agreement with the theory with channel depthdecreasing downstream. The experiments also revealed that thechannel margins become undercut during thermal erosion to formoverhanging sides of the channel. Some sinuous rilles observedin the mare regions of the Moon are thought to have formed bythermal erosion (Hulme, 1973). They provide analogues of thechannels postulated to form in komatiite eruptions, where conditionswere in fact more favourable for thermal erosion. An assessmentof the role of olivine crystals, precipitated in the cooling,turbulent flows, indicates that they will remain in suspensionuntil the lava has come to rest. Contamination of komatiite lava by underlying rock can be asmuch as 10 per cent. Some illustrative calculations show howthe major element and trace element compositions of residualmelts can be significantly modified by combined assimilationand fractional crystallization in a moving flow. Assimilationof tholeiitic basalt into a komatiite can cause incompatibletrace element ratios, such as Ti/Zr and Y/Zr, and the rare earthpatterns of derivative lavas, to vary substantially. Some ofthe variations in such geochemical parameters, which are oftenascribed to mantle heterogeneity, also could have resulted fromassimilation of the ground. Assimilation could have modifiedthe isotope geochemistry of lava suites and led to apparentages which differ from the true eruption age. The thermal erosionmodel also provides an explanation of the formation of somenickel sulphide ores found at the bottom of thick komatiiteflows. It is proposed that ores can form by assimilation ofsulphur-rich sediment, which combines with Ni from the komatiiteto form an immiscible liquid. 相似文献
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A box model for non-entraining, suspension-driven gravity surges on horizontal surfaces 总被引：4，自引：0，他引：4
The propagation of and the deposition from a turbulent gravity current generated by the release of a finite volume of a dense particle suspension is described by a box model. The approximate model consists of a set of simple equations, a predetermined, depth-dependent leading boundary condition and one experimentally determined parameter describing the trailing boundary condition. It yields predictions that agree well with existing laboratory observations and more complex theoretical models of non-eroding, non-entraining, suspension-driven flows on horizontal surfaces. The essential features of gravity-surge behaviour have been observed and are captured accurately by the box model. These include the increased rate of downstream loss of flow momentum with increased particle setting velocity, the existence of maxima in the thickness of proximal deposits, and the downstream thinning of distal deposits. Our approximation for the final run-out distance, xr, of a surge in deep water is given by xr3(g'oq3o/w2s)1/5, where g'o is the initial reduced gravity of the surge, qo the initial two-dimensional volume, and ws the average settling velocity of the particles in the suspension. A characteristic thickness of the resulting deposit is given by φoqo/xr'where øo is the initial volumetric fraction of sediment suspended in the surge. Our analysis provides additional insight into other features of gravity-surge dynamics and deposits, including the potential for the thickening of currents with time, the maintenance of inertial conditions and the potential for strong feedback in the sorting of particle sizes in the downstream direction at travel distances approaching xr. Box-model approximations for the evolution of gravity surges thus provide a useful starting point for analyses of some naturally occurring turbidity surges and their deposits. 相似文献
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