On the crystallinity, probability of occurrence, and rheology of lava and magma
TL;DR: In this article, a histogram of the total phenocryst content measures the probability of the magma to be erupted as lava, and the eruption probability is defined as the product of the probability for finding the magmas at any state of crystallinity (thermal probability) and the rheological probability (Rpheological probability).
Abstract: Given a set of comagmatic lavas of similar composition but varying crystallinity, a diagram can be constructed using only the modes of the phenocrysts that quantitively shows the sequence of crystallization This is done by plotting the amount of each phenocryst against the total crystallinity or percentage of melt of the lava itself A histogram of the total phenocryst content measures the probability of the magma to be erupted as lava This eruption probability (P
E
) is the product of the probability of finding the magma at any state of crystallinity (thermal probability, P
T
) and the rheological probability (P
R
) of the magma being physically able to erupt (ie P
E
=P
T
P
R
) It is shown that P
E
is given by dX/dT, where X is the crystallinity of the magma as a function of temperature (T) Because crystal production is generally nonlinear—in most rocks it is step-like—P
E
is a bellshaped curve stradling the temperature at which the magma is one half crystallized Near the liquidus it is most favorable rheologically for the magma to erupt But the probability is small of sampling a magma near its liquidus, because it cools quickly there It is maximum when there are high rates of crystal production, because it then cools slowly As the crystallinity increases, it reaches a critical point of maximum packing (ie lowest porosity) around 50–60% crystals where it becomes rheologically impossible to erupt The magma looses its potential to become a lava and it becomes a pluton From a histogram of crystallinity and P
T
,P
R
can be found This technique, as well as the construction of the mode-crystallization (M-C) diagram, is illustrated using a set of Aleutian lavas These lavas also show that the point of critical crystallinity decreases with increasing silica content of the lava Because this critical crystallinity is much lower for granitic magmas, they are much more probable than basaltic magmas to become plutons Beyond this point, granitic magmas can only erupt as ash flows This correlation of critical crystallinity and silica content is used to show a method by which the viscosity of the magma can be estimated as a function of crystallinity This variation is found to compare favorably with Roscoe's equation of the dependence of viscosity on the concentration of suspended solids These results show that differentiation probably can not normally take place beyond this critical crystallinity The extraction of melt beyond this critical point by filter pressing is unlikely because the assemblage dilates upon stressing Only if the phenocrysts deform viscously can additional melt be extracted, and this can probably only occur with large (−30km) bodies
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TL;DR: In this article, a model for the generation of intermediate and silicic igneous rocks is presented, based on experimental data and numerical modeling, which is directed at subduction-related magmatism, but has general applicability to magmas generated in other plate tectonic settings, including continental rift zones.
Abstract: A model for the generation of intermediate and silicic igneous rocks is presented, based on experimental data and numerical modelling. The model is directed at subduction-related magmatism, but has general applicability to magmas generated in other plate tectonic settings, including continental rift zones. In the model mantlederived hydrous basalts emplaced as a succession of sills into the lower crust generate a deep crustal hot zone. Numerical modelling of the hot zone shows that melts are generated from two distinct sources; partial crystallization of basalt sills to produce residual H2O-rich melts; and partial melting of pre-existing crustal rocks. Incubation times between the injection of the first sill and generation of residual melts from basalt crystallization are controlled by the initial geotherm, the magma input rate and the emplacement depth. After this incubation period, the melt fraction and composition of residual melts are controlled by the temperature of the crust into which the basalt is intruded. Heat and H2O transfer from the crystallizing basalt promote partial melting of the surrounding crust, which can include meta-sedimentary and meta-igneous basement rocks and earlier basalt intrusions. Mixing of residual and crustal partial melts leads to diversity in isotope and trace element chemistry. Hot zone melts are H2O-rich. Consequently, they have low viscosity and density, and can readily detach from their source and ascend rapidly. In the case of adiabatic ascent the magma attains a super-liquidus state, because of the relative slopes of the adiabat and the liquidus. This leads to resorption of any entrained crystals or country rock xenoliths. Crystallization begins only when the ascending magma intersects its H2O-saturated liquidus at shallow depths. Decompression and degassing are the driving forces behind crystallization, which takes place at shallow depth on timescales of decades or less. Degassing and crystallization at shallow depth lead to large increases in viscosity and stalling of the magma to form volcano-feeding magma chambers and shallow plutons. It is proposed that chemical diversity in arc magmas is largely acquired in the lower crust, whereas textural diversity is related to shallow-level crystallization.
1,547 citations
Cites methods from "On the crystallinity, probability o..."
...We estimate melt viscosity as a function of temperature, H2O content and composition using experimental data (Dingwell et al., 1998) and take account of the effect of suspended crystals on viscosity using the empirical relationship of Marsh (1981)....
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TL;DR: Herbel et al. as discussed by the authors developed a quantitative theory for the roof melting case and applied it to basalt sills in hot crust, the theory predicts that basalt Sills of thicknesses from 10 to 1500 m require only 1 to 270 y to solidify and would form voluminous overlying layers of convecting silicicic magma.
Abstract: When basalt magmas are emplaced into continental crust, melting and generation of silicic magma can be expected. The fluid dynamical and heat transfer processes at the roof of a basaltic sill in which the wall rock melts are investigated theoretically and also experimentally using waxes and aqueous solutions. At the roof, the low density melt forms a stable melt layer with negligible mixing with the underlying hot liquid. A quantitative theory for the roof melting case has been developed. When applied to basalt sills in hot crust, the theory predicts that basalt sills of thicknesses from 10 to 1500 m require only 1 to 270 y to solidify and would form voluminous overlying layers of convecting silicic magma. For example, for a 500 m sill with a crustal melting temperature of 850 °C, the thickness of the silicic magma layer generated ranges from 300 to 1000 m for country rock temperatures from 500 to 850 °C. The temperatures of the crustal melt layers at the time that the basalt solidifies are high (900-950 °C) so that the process can produce magmas representing large degrees of partial fusion of the crust. Melting occurs in the solid roof and the adjacent thermal boundary layer, while at the same time there is crystallization in the convecting interior. Thus the magmas formed can be highly porphyritic. Our calculations also indicate that such magmas can contain significant proportions of restite crystals. Much of the refractory components of the crust are dissolved and then re-precipitated to form genuine igneous phenocrysts. Normally zoned plagioclase feldspar phenocrysts with discrete calcic cores are commonly observed in many granitoids and silicic volcanic rocks. Such patterns would be expected in crustal melting, where simultaneous crystallization is an inevitable consequence of the fluid dynamics. The time-scales for melting and crystallization in basalt-induced crustal melting (10—10 y) are very short compared to the lifetimes of large silicic magma systems (>10 y) or to the timescale for thermal relaxation of the continental crust (> 10 y). Several of the features of silicic igneous systems can be explained without requiring large, high-level, long-lived magma chambers. Cycles of mafic to increasingly large volumes of silicic magma with time are commonly observed in many systems. These can be interpreted as progressive heating of the crust until the source region is partially molten and basalt can no longer penetrate. Every input of basalt triggers rapid formation of silicic magma in the source region. This magma will freeze again in time-scales of order 10—10 y unless it ascends to higher levels. Crystallization can occur in the source region during melting, and eruption of porphyritic magmas does not require a shallow magma chamber, although such chambers may develop as magma is intruded into high levels in the crust. For typical compositions of upper crustal rocks, the model predicts that dacitic volcanic rocks and granodiorite/tonalite plutons would be the dominant rock types and that these would ascend-from the source region and form magmas ranging from those with high temperature and low crystal content to those with high crystal content and a significant proportion of restite. I N T R O D U C T I O N One of the central questions in igneous petrology concerns the generation of silicic magmas. There is now convincing evidence that most of the large plutonic complexes of granite in the continental crust are the result of crustal anatexis (Pitcher, 1987). There is also [Journal of Petrologf, Vol. 29, Ptn 3, pp 599-«24, 1988] © Oxford Umvcroty Prcu 19S8 600 HERBERT E. HUPPERT AND R. STEPHEN J. SPARKS widespread evidence that basaltic magma from the mantle is often intimately associated with the generation of silicic magmas (Hildreth, 1981). This association of mafic and silicic magmas can occur in orogenic belts above subduction zones, in continental hot-spots, and in regions of crustal extension. In plutonic complexes, mafic and intermediate igneous activity are recorded in contemporaneous dyke swarms, small satellite intrusions, and in mafic enclaves within the granites (Vernon, 1983; Pitcher, 1986, 1987). In silicic volcanic centres, evidence of basaltic magmatism is found in satellite lava fields and cinder cones, early lava shields and stratovolcano complexes prior to the main silicic volcanism (Lipman, 1984), and as mafic inclusions and bands within the silicic volcanic rocks (Smith, 1979; Bacon, 1986). Petrological and geochemical features of many silicic igneous rocks are also convincingly explained by admixture of a mantle-derived (mafic) component with a crustal melt. Regions of high temperature and low pressure metamorphism are commonly associated with granite plutonism and a plausible explanation of this association is that basalt is intruded into the crust, causing melting and high heat flow. Indeed basalt underplating of the crust is a currently popular idea to explain both large scale crustal melting and the strongly layered character of the lower crust. While there may be some silicic magmas that are generated by processes without the aid of basaltic input, such as tectonic thickening of radioactive crust (England & Thompson, 1984; Pitcher, 1987), this paper takes the position that in many cases the additional thermal energy of basalt is essential. The continental crust is strongly layered in terms of its composition, density, and mechanical behaviour. The upper crust is cold and brittle whereas the lower crust is hotter, has a higher density, deforms in a ductile manner, and is commonly characterized by prominent horizontal layering. Basalt magma can be emplaced into the continental crust as dykes and sills and, in some cases where the rate of magma input is high, these intrusions can coalesce to form larger magma chambers. Dyke emplacement does not seem an efficient way of generating large volumes of silicic magma, because dykes are usually small in width and much of the potential heat for melting will not be utilized if the mafic magma erupts. Sills provide a more promising situation in which extensive crustal melting can occur. Horizontal intrusions concentrate their heat at a particular level in the crust and do not dissipate their heat over a large depth range. Sills are intrinsically more efficient than dykes in this respect. Dykes may play an important role in heating up the crust to initiate melting. However, once a region of the crust has become hot, ductile, and partially molten, conditions for dyke propagation become less favourable. A layer or region of partially molten crust provides an effective density barrier and we suggest that basalt magma reaching such a level will spread out as horizontal intrusions. An additional factor which promotes sill formation in the lower parts of the crust is its strongly layered character providing a structural environment in which horizontal intrusions are favoured. For these reasons this paper is concerned principally with the heat transfer and fluid dynamics of sills intruded into hot continental crust. We consider the cooling and crystallization of basaltic sills emplaced into the continental crust. In particular, we emphasize the situation where the roof of the sill is composed of rock which has a fusion temperature that is lower than the magma temperature and the roof rock consequently melts. This is likely to be the normal situation where basalt intrudes into the typical rock types of mature and ancient middle and upper crust which are already at high temperature. However, the concepts developed in this paper are also likely to be applicable to conditions in immature continental crust such as in island arcs, to more refractory lower crust and to lower crust formed by slightly older or even contemporaneous episodes of basalt underplating. In each of these latter cases, lithologies which have relatively low fusion temperatures can form by differentiation processes and can be remelted by further intrusion THE GENERATION OF GRANITIC MAGMAS 601 of basalt. Thus the model is not confined to the origin of granites, but should be relevant to the origins of intermediate rocks such as tonalites and evolved alkaline rocks such as syenite. We present experimental studies on the melting of the roof of a sill. We develop a quantitative model of the melting process at the roof, which describes the rates at which a new layer of roof melt forms and the rates at which the underlying liquid layer solidifies. We discuss possible mechanisms by which the melts can be mixed together and also their implications for magma genesis within the continental crust. A companion paper (Huppert & Sparks, 1988a) describes the melting of the roof of a chamber from a detailed fluid mechanical point of view. Throughout this paper the magma will be considered to be Newtonian. Although magma in reality can be non-Newtonian, especially when it is rich in crystals (McBirney & Murase, 1984) its nonlinear Theological properties and the consequences of its non-Newtonian rheology are poorly understood. Two effects may be evident: there may exist a yield strength, so that for a sufficiently low applied stress the magma will not move; and the nonlinear viscosity may alter the heat flux transferred by a convecting magma. Because of the relatively large values of the Rayleigh number that result in most of our calculations, we anticipate that the yield strength will be exceeded by quite a margin. The alterations in the heat flux are at the moment difficult to anticipate and we suggest that the reader views our quantitative results as an indication of the calculated quantity rather than as a precise value. It may be possible to examine non-Newtonian effects with greater insight in the future, but a Newtonian description illuminates many of the fundamental effects and is a necessary first step in order to form the basis for any comparison. EXPERIMENTAL STUDIES The geological problem i
1,046 citations
TL;DR: In this paper, it was shown that the composition of the melt lens is mainly moderately fractionated ferrobasalt, which is consistent with a model that effectively separates the processes of magma mixing and fractionation into different parts of a composite magma chamber.
Abstract: Geophysical evidence precludes the existence of a large, mainly molten magma chamber beneath portions of the East Pacific Rise (EPR). A reasonable model, consistent with these data, involves a thin (tens to hundreds of meters high), narrow (<1–2 km wide) melt lens overlying a zone of crystal mush that is in turn surrounded by a transition zone of mostly solidified crust with isolated pockets of magma. Evidence from the superfast spreading portion of the EPR suggests that the composition of the melt lens is mainly moderately fractionated ferrobasalt. These results have important implications for magmatic processes occurring beneath mid-ocean ridges and are consistent with a model that effectively separates the processes of magma mixing and fractionation into different parts of a composite magma chamber. Magma mixing, as evidenced by disequilibrium relations between host liquids and included phenocrysts, is especially apparent in samples from low magma supply ridges and probably mainly arises from interactions between crystals of the mush zone and new injections of primitive magma rising out of the mantle. Magmatic differentiation beneath mid-ocean ridges occurs in two parts. Migration of melts through the transition and mush zones can produce chemical trends consistent with in situ fractionation processes. Segregation of melt into molten horizons near the top of a composite magma chamber promotes the more extensive differentiation characteristic of fast spreading ridges. The optimum conditions for the formation of highly differentiated abyssal lavas is where small, discontinuous melt lenses occur, such as at intermediate spreading rates, in the vicinity of propagating rifts, and near ridge offsets at fast spreading ridges. Along-axis homogenization of subaxial magma is inhibited by the thin, high aspect ratio of the melt lens and by the high viscosities expected in the mush and transition zones. Low magma supply ridges are unlikely to be underlain by eruptable magma in a steady state sense, and eruptions at slow spreading ridges are likely to be closely coupled in time to injection events of new magma from the mantle. Extensional events at high magma supply ridges, which are more likely to be underlain by significant volumes of low-viscosity melt, can produce eruptions without requiring associated injection events. The critical magma supply necessary for the development of a melt lens near the top of a composite magma chamber is similar to that of normal ridges spreading at rates of about 50–70 mm/yr, a rate approximately corresponding to that marking an abrupt change in the morphology and gravity signal at the ridge axis. A composite magma chamber model can explain several previous enigmas concerning mid-ocean ridge basalts, including why slow spreading ridges dominantly erupt a narrow range of relatively undifferentiated lavas, why magma mixing is most evident in lavas erupted from slow spreading ridges, why fast spreading ridges erupt a wide range of generally more differentiated compositions, why bimodal lava populations occur in the vicinity of some propagating rifts, and how along-axis geochemical segmentation can occur at a scale shorter than the major tectonic segmentation of ridge axes.
759 citations
TL;DR: In this article, the feasibility of system-wide extraction of high-SiO2 interstitial melt from the mushes was analyzed, which can rationalize a number of observations in both the plutonic and volcanic record, such as abrupt compositional gaps in ignimbrites, the presence of chemically highly evolved bodies at the roof of subvolcanic batholiths, and the observed range of ages recorded by zircons in silicic magmas.
Abstract: The largest accumulations of rhyolitic melt in the upper crust occur in voluminous silicic crystal mushes, which sometimes erupt as unzoned, crystal-rich ignimbrites, but are most frequently preserved as granodioritic batholiths. After approximately 40–50% crystallization, magmas of intermediate composition (andesite–dacite) typically contain high-SiO2 interstitial melt, similar to crystal-poor rhyolites commonly erupted in mature arc and continental settings. This paper analyzes the feasibility of system-wide extraction of this melt from the mush, a mechanism that can rationalize a number of observations in both the plutonic and volcanic record, such as: (1) abrupt compositional gaps in ignimbrites; (2) the presence of chemically highly evolved bodies at the roof of subvolcanic batholiths; (3) the observed range of ages (up to 200–300 ka) recorded by zircons in silicic magmas; (4) extensive zones of low P-wave velocity in the shallow crust under active silicic calderas. We argue that crystal–melt segregation occurs by a combination of several processes (hindered settling, micro-settling, compaction) once convection is hampered as the rheological locking point of the crystal–melt mixture ( 50 vol. % crystals) is attained. We constrain segregation rates by using hindered settling velocities and compaction rates as endmembers. Time scales estimated for the formation of >500 km of crystal-poor rhyolite range from 10 to 10 years, within the estimated residence times of mushes in the upper crust (>10 years, largely based on U/Th and U/Pb dating). This model provides an integrated picture of silicic magmatism, linking the evolution of plutonic and volcanic systems until storage in the upper crust, where granitoids become the leftovers from rhyolitic eruptions.
755 citations
Cites background from "On the crystallinity, probability o..."
...Apart from rhyolites, most intermediate to silicic eruptive products contain >20 vol. % crystals (Marsh, 1981; Ewart, 1982), even though they always tap the least viscous, and, thus, least crystalline magma available in the reservoir....
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...% crystals (Marsh, 1981; Ewart, 1982), even though they always tap the least viscous, and, thus, least crystalline magma available in the reservoir....
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01 Jan 1992
TL;DR: In this article, the authors show that the composition of the melt lens is mainly moderately fractionated ferrobasalt with isolated pockets of magma in the transition zone of the East Pacific Rise (EPR).
Abstract: Geophysical evidence precludes the existence of a large, mainly molten magma chamber beneath portions of the East Pacific Rise (EPR). A reasonable model, consistent with these data, involves a thin (tens to hundreds of meters high), narrow (<1-2 km wide) melt lens overlying a zone of ctystal mush that is in turn surrounded by a transition zone of mostly solidified crust with isolated pockets of magma. Evidence from the superfast spreading portion of the EPR suggests that the composition of the melt lens is mainly moderately fractionated ferrobasalt
681 citations
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46,339 citations
Book•
01 Jan 1965
TL;DR: Low Reynolds number flow theory finds wide application in such diverse fields as sedimentation, fluidization, particle-size classification, dust and mist collection, filtration, centrifugation, polymer and suspension rheology, and a host of other disciplines.
Abstract: Low Reynolds number flow theory finds wide application in such diverse fields as sedimentation, fluidization, particle-size classification, dust and mist collection, filtration, centrifugation, polymer and suspension rheology, flow through porous media, colloid science, aerosol and hydrosal technology, lubrication theory, blood flow, Brownian motion, geophysics, meteorology, and a host of other disciplines. This text provides a comprehensive and detailed account of the physical and mathematical principles underlying such phenomena, heretofore available only in the original literature.
4,648 citations
TL;DR: In this article, a large number of spherical grains of diameter D = 0.13 cm were sheared in Newtonian fluids of varying viscosity (water and a glycerine-water-alcohol mixture) in the annular space between two concentric drums.
Abstract: Dispersions of solid spherical grains of diameter D = 0.13cm were sheared in Newtonian fluids of varying viscosity (water and a glycerine-water-alcohol mixture) in the annular space between two concentric drums. The density σ of the grains was balanced against the density ρ of the fluid, giving a condition of no differential forces due to radial acceleration. The volume concentration C of the grains was varied between 62 and 13 %. A substantial radial dispersive pressure was found to be exerted between the grains. This was measured as an increase of static pressure in the inner stationary drum which had a deformable periphery. The torque on the inner drum was also measured. The dispersive pressure P was found to be proportional to a shear stress λ attributable to the presence of the grains. The linear grain concentration λ is defined as the ratio grain diameter/mean free dispersion distance and is related to C by λ = 1 ( C 0 / C ) 1 2 − 1 where C 0 is the maximum possible static volume concentration. Both the stresses T and P , as dimensionless groups T σ D 2 /λη 2 , and P σ D 2 /λη 2 , were found to bear single-valued empirical relations to a dimensionless shear strain group λ ½ σ D 2 (d U /d y )lη for all the values of λ C = 57% approx.) where d U /d y is the rate of shearing of the grains over one another, and η the fluid viscosity. This relation gives T α σ ( λ D ) 2 ( dU / dy ) 2 and T ∝ λ 1 2 η d U / dy according as d U /d y is large or small, i.e. according to whether grain inertia or fluid viscosity dominate. An alternative semi-empirical relation F = (1+λ)(1+½λ)ηd U /d y was found for the viscous case, when T is the whole shear stress. The ratio T/P was constant at 0·3 approx, in the inertia region, and at 0.75 approx, in the viscous region. The results are applied to a few hitherto unexplained natural phenomena.
2,445 citations
"On the crystallinity, probability o..." refers background in this paper
...Bagnold 1954 ) that upon being sheared a closelypacked assemblage of solids expands or dilates....
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1,797 citations