Viscosity of magmas containing highly deformable bubbles     

M. Manga  M. Loewenberg 《Journal of Volcanology and Geothermal Research》2001,105(1-2)

The shear viscosity of a suspension of deformable bubbles dispersed within a Newtonian fluid is calculated as a function of the shear rate and strain. The relative importance of bubble deformation in the suspension is characterized by the capillary number (Ca), which represents the ratio of viscous and surface tension stresses. For small Ca, bubbles remain nearly spherical, and for sufficiently large strains the viscosity of suspension is greater than that of the suspending fluid, i.e. the relative viscosity is greater than 1. If Ca>O(1) the relative viscosity is less than one. In the limit that Ca→∞ (surface tension is dynamically negligible), numerical calculations for a suspension of spherical bubbles agree well with the experimental measurements of Lejeune et al. (1999, Rheology of bubble-bearing magmas. Earth Planet. Sci. Lett., vol. 166, pp. 71–84). In general, bubbles have a modest effect on the relative viscosity, with viscosity changing by less than a factor of about 3 for volume fractions up to 50%.  相似文献   

Compaction in a mantle with a very small melt concentration: Implications for the generation of carbonatitic and carbonate-bearing high alkaline mafic melt impregnations     

Michel RabinowiczYanick Ricard  Michel Grégoire 《Earth and Planetary Science Letters》2002,203(1):205-220

Xenoliths entrained in alkaline basalts and kimberlites give strong evidence that mantle carbonatitic and carbonated high alkaline mafic silicate melts, which are initially produced at very low degrees of partial melting (?1%), percolate and accumulate to form impregnations with a melt concentration of up to 10%. At present no compaction model has explained such huge local amplification of melt concentration. Recently, Bercovici et al. [1] have shown that the commonly used equations of compaction are not sufficiently general to describe all melt percolation processes in the mantle. In particular, they show that, when the melt concentration in the mantle is very low, the pressure jump ΔP between the solid and liquid fractions of the mantle mush is very important and plays a driving role during compaction. 1-D compaction waves generated with two different systems of equations are computed. Three types of wave-trains are observed, i.e. (1) sinusoidal waves; (2) periodic waves with flat minima and very acute maxima (‘witch hat waves’); (3) periodic solitary waves with flat maxima and extremely narrow minima (‘bowler hat waves’). When the initial melt distribution in the mantle is quite homogeneous, the compaction waves have sinusoidal shapes and can locally amplify the melt concentration by a factor less than two. When there is a drastic obstruction at the top of the wetted domain, the pressure jump ΔP between solid and liquid controls the shape of the waves. If the computation assumes the equality of pressure between the two phases (ΔP=0), the compaction wave has a ‘bowler hat shape’, and locally amplifies the melt concentration by a factor less than 5. Alternatively, simulations taking into account the pressure jump between phases ΔP predict compaction waves with ‘witch hat shape’. These waves collect a large quantity of melt promoting the development of magmons with local melt concentration exceeding 100× the background melt concentration. It is inferred that in a mantle with very low concentrations of carbonatitic or high alkaline mafic silicate melt the magmons are about 1 km thick and reach, in less than 1 Ma, a melt concentration of about 10%. The magmons are likely generated below the lithosphere at some distance away from the center of hot spots. This can explain the development of mantle carbonatitic eruptions in the African rift and the carbonatite and high alkaline mafic silicate volcanic activity in oceanic islands.  相似文献   

Subduction-channel model of prism accretion,melange formation,sediment subduction,and subduction erosion at convergent plate margins: 2. Implications and discussion     

Mark Cloos  Ronald L. Shreve 《Pure and Applied Geophysics》1988,128(3-4):501-545

The subduction-channel model predicts five geologically and geophysically distinctive types of convergent plate margin. They differ primarily in the proportions of incoming sediment and upflowing melange offscraped and underplated near the toe of the overriding block. The model predicts complex patterns of uplift or subsidence of the forearc region due to differential underplating or subduction erosion and to initiation or cessation of melange upflow. It shows how subduction speed, sediment input, and pressure gradient along the channel govern the general distribution and magnitude of subduction-zone earthquakes. It can predict the upward trajectories of exotic blueschist or serpentinized peridotite blocks that become entrained in the upwelling melange; and it suggests mechanisms by which much larger tracts of coherent blueschist can be raised to shallower levels. It predicts characteristic structural and metamorphic histories for geologic units accreted during steady-state subduction, showing for example, that offscraped materials undergo subhorizontally-directed compression during final dewatering and accretion, whereas underplated ones undergo large simple shear. It gives the maximum depths reached by upwelled melange, which bear significantly on the metamorphic changes observed in them. It predicts large-scale melange diapirism in certain rare cases, in which the normally adverse pressure gradient is reversed in a short reach of the channel, as at Barbados Island. Finally, it explains why pelagic sediments are seldom found in accretionary complexes and gives insight into the factors controlling what portion of the incoming sediment, and the¹⁰Be and other tracers it carries, will reach the region of volcanic-arc magmagenesis.  相似文献   

内蒙锡盟新生代玄武岩的岩石学研究   总被引：6，自引：4，他引：6  

杨建军 《岩石学报》1988,4(2)

依照地层顺序和火山机构的完好程度,内蒙锡盟新生代玄武岩可粗略划分为三期。各期玄武岩又可分出若干层(次)。玄武岩由含量不等的橄榄石、单斜辉石和斜长石斑晶及基质组成。部分玄武岩含有二辉橄榄岩等深源包体。斑晶矿物均具有成分环带。矿物化学、岩石化学资料揭示出本区玄武岩从原生岩浆到进化岩浆的主要演化机制是以橄榄石为主的分离结晶作用,同时在部分进化岩浆中还存在岩浆混合作用。稀土元素资料还指出了一个形成本区原生岩浆以及派生岩浆的原生母岩浆的重要机制:上地幔渐进的部分熔融作用。  相似文献   

Causes and consequences of pressurisation in lava dome eruptions   总被引：3，自引：0，他引：3  

R.S.J. Sparks   《Earth and Planetary Science Letters》1997,150(3-4):177-189

High total and fluid pressures develop in the interior of high-viscosity lava domes and in the uppermost parts of the feeding conduit system as a consequence of degassing. Two effects are recognised and are modelled quantitatively. First, large increases in magma viscosity result from degassing during magma ascent. Strong vertical gradients in viscosity result and large excess pressures and pressure gradients develop at the top of the conduit and in the dome. Calculations of conduit flow show that almost all the excess pressure drop from the chamber in an andesitic dome eruption occurs during the last several hundred metres of ascent. Second, microlites grow in the melt phase as a consequence of undercooling caused by gas loss. Rapid microlite growth can cause large excess fluid pressures to develop at shallow levels. Theoretically closed-system microlite crystallization can increase local pressure by a few tens of MPa, although build up of pressure will be countered by gas loss through permeable flow and expansion by viscous flow. Microlite crystallization is most effective in causing excess gas pressures at depths of a few hundred metres in the uppermost parts of the conduit and dome interior. Some of the major phenomena of lava dome eruptions can be attributed to these pressurisation effects, including spurts of growth, cycles of dome growth and subsidence, sudden onset of violent explosive activity and disintegration of lava during formation of pyroclastic flows. The characteristic shallow-level, long-period and hybrid seismicity, characteristic of dome eruptions, is attributed to the excess fluid pressures, which are maintained close to the fracture strength of the dome and wallrock, resulting in fluid movement during formation of tensile and shear fractures within the dome and upper conduit.  相似文献   

Trace element transport rates in subduction zones: evidence from Th, Sr and Pb isotope data for Tonga-Kermadec arc lavas   总被引：2，自引：0，他引：2  

M. Regelous  K.D. Collerson  A. Ewart  J.I. Wendt 《Earth and Planetary Science Letters》1997,150(3-4):291-302

Trace element and Th, Sr and Pb isotope data for young lavas from the Tonga-Kermadec arc in the southwest Pacific suggest that geochemical variations in the lavas along the arc are linked to differences in the material being subducted beneath the arc. Lavas from the southern (Kermadec) segment of the arc have relatively radiogenic Pb isotope compositions, which reflects a contribution from subducted sediment. In contrast, much of the Pb in Tonga lavas is derived from the altered oceanic crust in the subducting Pacific Plate, and lavas from the northernmost Tonga islands of Tafahi and Niuatoputapu contain Pb and Sr derived from the subducted part of the Louisville Seamount Chain. The origin of the Pb in the lavas from these two islands can thus be traced to a point on the subducting slab, and this observation is used to estimate the rate at which trace elements are transported beneath the arc. Our calculations suggest that fluid-soluble elements such as U, Sr and Pb are transported from the subducted slab, across the mantle wedge and back to the surface in lavas over a period of approximately 2–3 Ma, and that magmas are erupted at the surface less than 350 ka after the melts are generated in the mantle wedge.  相似文献   

Phase Relationships of Hydrous Alkalic Magmas at High Pressures: Production of Nepheline Hawaiitic to Mugearitic Liquids by Amphibole-Dominated Fractional Crystallization Within the Lithospheric Mantle     

Irving  Anthony J.; Green  David H. 《Journal of Petrology》2008,49(4):741-756

Experimental melting studies were conducted on a nepheline mugearitecomposition to pressures of 31 kbar in the presence of 0–30%added water. A temperature maximum in the near-liquidus stabilityof amphibole (with olivine) was found for a water content of3·5 wt % at a pressure of 14 kbar. This is interpretedto have petrogenetic significance for the derivation of nephelinemugearite magmas from nepheline hawaiite by amphibole-dominatedfractional crystallization at depth within the lithosphericmantle. Synthetic liquids at progressively lower temperaturesrange to nepheline benmoreite compositions very similar to thoseof natural xenolith-bearing high-pressure lavas elsewhere, andsupport the hypothesis that continued fractional crystallizationcould lead to high-pressure phonolite liquids. Independent experimentaldata for a basanite composition modeled on a lava from the sameigneous province (the Newer Basalts of Victoria) permit theinference that primary asthenospheric basanite magmas undergopolybaric fractional crystallization during ascent, and mayevolve to liquids ranging from nepheline hawaiite to phonoliteupon encountering cooler lithospheric mantle at depths of 42–50km. Such a model is consistent with the presence in some evolvedalkalic lavas of both lithospheric peridotite xenoliths indicativeof similar depths and of megacryst suites that probably representdisrupted pegmatitic segregations precipitated from precursoralkalic magmas in conduit systems within lithospheric mantle. KEY WORDS: experiment; high pressure; alkalic magmas; amphibole; nepheline mugearite; basanite; lithosphere  相似文献   

Experimental effects of pressure and fluorine on apatite saturation in mafic magmas, with reference to layered intrusions and massif anorthosites   总被引：1，自引：0，他引：1  

N. Tollari  D. R. Baker  S.-J. Barnes 《Contributions to Mineralogy and Petrology》2008,156(2):161-175

Apatite is a cumulate phase in the upper parts of some mafic layered intrusions and anorthositic complexes. We investigated the effect of pressure and fluorine on apatite saturation in mafic magmas to better understand under which conditions this mineral crystallizes. Apatite saturation gives information about the formation of silicate rocks, and is of interest in explaining the formation of apatite–oxide-rich rocks (e.g. nelsonites comprising approximately, one-third apatite and two-third Fe–Ti oxide). Two models of formation are proposed for this rock type: crystal fractionation followed by accumulation of apatite and Fe–Ti oxides and liquid immiscibility. New experiments carried out with mafic compositions at 500 MPa confirm that the most important variables on phosphate saturation are SiO₂ and CaO. Fluorine addition leads to apatite saturation at lower SiO₂ and higher CaO concentrations. Comparison of our results with those of previous experimental studies on liquid–liquid immiscibility at upper-to-mid-crustal conditions allows us to investigate the relative importance of apatite saturation versus liquid–liquid immiscibility in the petrogenesis of nelsonites and similar rocks. The liquid line of descent of three natural examples studied (the Sept-Îles intrusive suite, the anorthositic Complex of the Lac-St-Jean and the Skaergaard layered intrusion) do not cross the liquid–liquid immiscibility field before they reach apatite saturation. Thus, the apatite–oxide-rich rock associated with these three intrusive suites are best explained by crystal fractionation followed by accumulation of apatite and Fe–Ti oxides.  相似文献   

小秦岭、熊耳山金矿区中基性岩墙的岩石化学研究   总被引：4，自引：0，他引：4  

王团华  毛景文  谢桂青  叶安旺  李宗彦 《地学前缘》2008,15(1):250-266

通过对小秦岭、熊耳山金矿区岩墙的地质学、岩石学、地球化学的综合研究,查明该岩墙主要由辉绿岩和煌斑岩类组成,属钾质、中等铝质岩石并富含挥发份(>2%)的中基性岩。岩石富集LILE和LREE,与原始地幔相比具有明显的Ti、Nb负异常和Pb正异常。对形成岩墙的岩浆演化过程研究后认为,其岩浆演化大致分为三个关键环节:(1)早中生代时期富钾、富硅熔体交代秦岭造山带岩石圈地幔,导致具富集地幔特征的玄武质岩浆的形成;(2)玄武质岩浆与区域热效应形成的花岗质岩浆的混合(混染)作用;(3)混合岩浆在深部热力学和动力学机制下的结晶分异和连续演化过程。  相似文献   

华南板块北缘东段武功山加里东期花岗岩特征及成因探讨   总被引：17，自引：1，他引：17       下载免费PDF全文

吴富江  张芳荣 《中国地质》2003,30(2):166-172

华南板块北缘东段武功山地区出露大面积的片麻状花岗岩,前人在该区工作时认为属混合成因的“混合岩”,本次工作证实属同熔性花岗岩。采用岩石谱系单位法进行研究,共划分出5个单元,归并为2个超单元。其成岩年龄(U-Pb法)分别为(462.3±3.2)Ma、(409.4±5.1)Ma,属加里东期造山花岗岩。后又经印支—燕山期伸展滑覆构造的叠加、改造,形成片麻状花岗岩。  相似文献