Magma Density Research Articles

Convection and mixing in magma chambers

Convection and mixing in magma chambers

Replenishment of magma chambers by light inputs

Magma chambers, particularly those of basaltic composition, are often replenished by an influx of magma whose density is less than that of the resident magma. This paper describes the fundamental fluid mechanics involved in the replenishment by light inputs. If ρ denotes the uniform density of the resident magma and ρ — Δρ that of the input, the situation is described by the reduced gravity g’ = gΔρ/ρ, the volume flux Q, and the viscosities of the resident and input magmas νe and νi, respectively. The (nondimensional) Reynolds numbers, Ree = (g’Q3)1/5/νe and Rei = (g’Q3)1/5/νi and chamber geometry then completely specify the system. For sufficiently low values of the two Reynolds numbers (each less than approximately 10), the input rises as a laminar conduit. For larger values of the Reynolds numbers, the conduit may break down and exhibit either a varicose or a meander instability and entrain some resident magma. At still larger Reynolds numbers, the flow will become quite unsteady and finally turbulent. The values of the Reynolds numbers at which these transitions occur have been documented by a series of experiments with water, glycerine, and corn syrup. If the input rises as a turbulent plume, significant entrainment of the resident magma can take place. The final spatial distribution of the mixed magma depends on the geometry of the chamber. If the chamber is much wider than it is high, the mixed magma forms a compositionally stratified region between the roof and a sharp front above uncontaminated magma. In the other geometrical extreme, the input magma is mixed with almost all of the resident magma. If the density of the resident magma is already stratified, the input plume may penetrate only part way into the chamber, even though its initial density is less than that of the lowest density resident magma. The plume will then intrude horizontally and form a hybrid layer at an intermediate depth. This provides a mechanism for preventing even primitive basaltic magmas of minimum density from erupting at the surface. By conducting an experiment using aqueous solutions, we show that entrainment can lead to crystallization of the magma in the input plume by making it locally supersaturated. All these effects are discussed and illustrated by photographs of laboratory experiments.

Gravity fields and the interpretation of volcanic structures: Geological discrimination and temporal evolution

Abstract A review of gravity data reflecting shallow concealed structures on volcanoes demonstrates that valuable information may be derived on the geological development of such structures and on the dynamic evolution of active volcanoes. To a first approximation, all recorded examples of anomalous gravity fields associated with volcanoes are concentric with and have sources that lie within the volcanic edifice. Positive anomalies with wavelengths less than 20 km and amplitudes up to ca. 30 mGal, characterise mainly basaltic volcanoes from various tectonic settings, and are caused by a relatively dense intrusive complex/magma body which contrasts with its surroundings either because the body is more mafic than average or, more likely, because near surface, previously erupted materials are uncompacted. Negative anomalies with wavelengths up to 100 km and amplitudes up to ca. 60 mGal, occur over much larger volcanic calderas, many of which have erupted highly silicic pyroclastic ash and pumice; uncompacted silicic caldera infill, with a possible contribution from low-density magma bodies, is responsible for the observed anomalies. There is some evidence for a continuum of gravity anomaly types that corresponds to the geological evolution of volcanic systems generally from primitive rift or subduction-related basaltic andesite to mature, high-level silicic calderas. Repeated microgravity observations over active volcanoes enable magma movements, variations in magma input and changes in magma density to be monitored closely. Dynamic modelling of volcanic systems is providing new evidence on the behaviour of concealed magma bodies and could have considerable potential for predicting eruptive events.

Eruptive history and structural development of the Toquima caldera complex, central Nevada

The Toquima caldera complex, located in the Toquima Range of central Nevada, consists of three overlapping to nested calderas. The Moores Creek caldera is the largest (∼30 by 20 km); it formed ∼27.2 Ma in response to eruption of the high-silica rhyolite tuff of Moores Creek. Because of recurrent volcanic activity and subsequent basin-range faulting, only the northern segment of the Moores Creek caldera is preserved; its eastern and western margins are downfaulted below valley fill, and its southern part was obscured by collapse of the Mount Jefferson caldera. Eruption of the tuff of Mount Jefferson resulted in collapse of the 18- by 20-km Mount Jefferson caldera ∼26.5 Ma. The ash-flow tuff exposed at Round Mountain is a silicic outflow-facies equivalent of the compositionally zoned (76–67 wt % SiO 2 ) intracaldera tuff of Mount Jefferson. Pyroclastic eruptive activity in the complex concluded ∼23.6 Ma with formation of the comparatively small 8- by 10-km Trail Canyon caldera. With time, caldera size diminished, and the focus of volcanic activity shifted progressively southeastward. The southeastward migration of volcanism in the complex approximately parallels regional northwest-striking faults, suggesting fundamental structural control in the rise and eruption of magma. Arcuate faults and small, aphyric to porphyritic plugs outline the structural margins of the calderas. Some plugs, on the basis of gradational textural changes from vitroclastic in the tuff to flow-layered and porphyro-aphanitic in the plug, appear to represent lava-choked, ash-flow-tuff, feeder vents. Caldera collapse breccias are locally well exposed in all three calderas. Caldera resurgence is not strongly developed in the Toquima caldera complex. The Mount Jefferson massif, which dominates the complex at an elevation of 3,640 m, is an artifact of young basin-range block faulting. The limited development of resurgence in the Mount Jefferson caldera is attributed to ponding of the bulk of erupted material within the caldera. Intermediate to mafic volcanic rocks, premonitory or coeval to silicic ash-flow tuff, are volumetrically minor over central Nevada and are lacking in the Toquima caldera complex. The less silicic magma apparently became lodged in the middle to lower crust below an extensive zone of more silicic, less dense magma.

Mid-ocean ridge basalts from the Galápagos spreading center: Direct probes of magma chambers

Volcanic material erupting on the earth surface provides the one direct means to study the physical conditions, chemical composition, and mineralogy of magmas in magma chambers. Most magmas from the Galapagos spreading center have not cooled or crystallized perceptibly on the way to the surface, and these magmas can be used to determine the composition, temperature, density, viscosity, and crystal content of magmas in magma chambers below the Galapagos spreading center. The constraints placed on these ridge crest magmas indicate that they will not erupt if they cool to less than 1150 °C, if their viscosity is more than 820 P, or if their density is greater than 2.8 g/cm 3 . The magma chambers are zoned: high-temperature (∼1200 °C), low-density, crystal-free magma is at the top, and low-temperature (∼1170 °C), high-density, crystal-rich magma is at the bottom. These conclusions can be extended to all ocean ridges that erupt basalts with low crystal content and unzoned olivine phenocrysts.

Postcumulus processes in layered intrusions

AbstractDuring the postcumulus stage of solidification in layered intrusions, fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks. One mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir. Convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma. Another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir. This process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile. Calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions, but could not form pure monomineralic rocks. Intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment. Dense magma, emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers. Reactions between melt and matrix may lead to changes in mineral compositions, mineral textures and whole rock isotope compositions. Another important mechanism for forming adcumulate rocks is compaction, in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled. For cumulate layers from 10 to 1000 metres in thickness, compaction can reduce porosities to very low values (< 1%) and form monomineralic rocks. The characteristic time-scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion. During compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix. Both compaction and compositional convection can be interrupted by solidification in the pore spaces. Compositional convection will only occur if the Rayleigh number is larger than 40, if the residual melt becomes lower in density, and the convective velocity exceeds the solidification velocity (measured by the rate of crystal accumulation in the chamber). Orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection.

35. Density of magmas at high pressures(Abstracts of Papers Presented at the Fall Meeting of the Society)

35. Density of magmas at high pressures(Abstracts of Papers Presented at the Fall Meeting of the Society)

Implications of volcano and swell heights for thinning of the lithosphere by hotspots

The heights of hotspot volcanoes are modeled by assuming isostatic equilibrium between the magma column and the adjacent lithosphere. The depth to the level of compensation is assumed to be related to the thickness of the lithosphere after it has been reheated and thinned by a hotspot. There is a square‐root relationship between volcano height and lithospheric age. Relationships between volcano height and lithospheric thickness and between reheated thickness and age of the lithosphere can be constrained within the limits imposed by uncertainties in lithospheric and magma densities. If the reheated thickness determined from the isostatic model can be compared with lithospheric thickness determined from thermal models of the lithosphere, then a relationship between the thickness of the lithosphere before and after reheating can be derived. Combining the upper limit on the reheated thickness/age relationship with a theoretical expression, derived by S. T. Crough, which relates swell height to lithospheric age, reset thickness, and physical constants, yields a swell‐height/age relationship that is in good agreement with empirical swell‐height data. This agreement supports the assumption that the lithospheric thickness defined isostatically by volcano heights is closely related to the thermal thickness of the lithosphere, a result that, in turn, supports the thermal origin of hotspot swells.

Geochemical evolution of the Menengai Caldera Volcano, Kenya

The Menengai volcano is composed almost entirely of strongly peralkaline, Si‐oversaturated trachytes. No mafic or intermediate products have yet been identified. The volcano has had a complex geochemical evolution, resulting from the interplay of magma mixing, crystal fractionation and liquid state differentiation. The controlling mechanism at any time was related to the growth stage of the complex, the presence of volatile gradients in the chamber, and the distribution of magma densities in the chamber. Prior to a major ash flow eruption, the magma reservoir was growing by the addition and mixing of two or more trachytic melts, only slightly different in composition. A volatile‐rich cap eventually separated from the lava‐forming zone and became compositionalIy zoned by liquid state processes. In late pfecaldera times, trachyte magma was able to penetrate into the cap zone, resulting in the eruption of mixed magma. The first Menengai ash flow tuff was erupted from a compositionally zoned magma chamber which showed strong roofward enrichment in Fe, Mn, Cs, Hf, Nb, Ni, Pb, Rb, Ta, Th, U, Y, Zn, Zr, and the REE (including Eu) and probably also Na, Cl, and F, and roofward depletion in Al, Mg, Ca, K, Ti, P, Ba, and Sc. Observed enrichment factors of up to 5 make this one of the most strongly zoned ash flow units yet recorded. Zonation was achieved by liquid state differentiation, probably involving volatile transfer and thermodiffusion, and minor crystal fractionation. After a period of rehomogenization of the magma remaining in the upper parts of the chamber, the second Menengai ash flow tuff was erupted, with formation of the present caldera. This unit is also compositionalIy zoned, although with lower observed enrichment factors than the first sheet. Caldera collapse was followed by convective overturn within the magma chamber and the rise to the roof zone of a Ba‐rich magma from a level not tapped by the as flow. Enrichment of volatiles in this zone resulted in the establishment of a stable density interface between an upper, tuff‐producing zone and the lower, lava‐forming zone. In the latter, 25% crystallization of syenite took place against the side walls. Crystallization in the tuff‐forming zone was more extensive (75%) and produced a series of chemically evolved tuffs. At some critical stage, liquid state processes became dominant in both zones and compositional variations comparable to those in the ash flow sheets were established. Some mixing of the upper and lower zone magmas may have occurred relatively recently. The Menengai volcano provides evidence of the development by liquid state mechanisms of extensive compositional zonations through thicknesses in excess of 102 m in times of 102–103 years. It also gives unique information on the senses and amounts of elemental enrichments and depletions during liquid state differentiation of trachytic magma.

Some effects of viscosity on the dynamics of replenished magma chambers

Some aspects of the dynamical behavior of magma chambers, replenished from below with hotter but denser magma, have been modeled in a series of laboratory experiments. In previously reported work the fluids used were aqueous solutions of comparable viscosity, and thus the results should be applicable to basaltic magma chambers, in which the magmas do not vary greatly in viscosity. In that case, the lower layer cools by convective heat transfer to the fluid above, and crystallization causes the density of the residual liquid in the lower layer to decrease. When the density becomes equal to that in the upper layer, sudden overturning and intimate mixing take place. The present paper reports experimental results that allow us to extend the application to systems in which there is a large viscosity ratio between the resident and the injected fluid, for example, to calcalkaline magmas, where magma viscosity can vary by as much as 5 orders of magnitude. The largest viscosity ratio in our experiments (about 3000) was achieved using cold glycerine for the upper layer, above a hot denser KNO3 solution. The most striking new feature with the very viscous upper layer is that now less dense fluid is released immediately and continuously from the interface and rises as plumes through the upper layer. Further crystallization occurs in the plumes, and the crystals fall out, but there is little mixing, and a layer of depleted KNO3 solution is eventually deposited at the top. The transfer process between the layers is dominated by interfacial effects, with the high‐viscosity upper layer acting as a nearly rigid lid that allows buoyant fluid to accumulate just below the interface and then rise in localized plumes across the interface into the viscous layer. This physical picture is supported by a series of experiments in which the viscosity ratio is varied systematically; the mixing behavior changes gradually between that described above for a large viscosity ratio and the sudden over‐turning characteristic of layers with comparable viscosity. The importance of the viscosity ratio, rather than just an increase in viscosity, is confirmed by experiments in which both viscosities are increased by the same factor; the overturning process is then slower, but symmetrical. Other variations suggested by previous experiments are also described: the release of gas by a chemical reaction, to model the release of volatiles following an overturning event in a magma chamber; the effect of a cold, immiscible layer above the cooling crystallizing fluid; the influence of two viscous layers with a density step between them; and the constraining effects of a density (with corresponding viscosity) gradient in the upper region. The experiments indicate that whatever the stratification, whether it be in layers or continuous, the form of the initial motion in the upper fluid is determined by the viscosity ratio between the two fluids immediately adjacent to the interface. Geological applications are not examined in detail in this paper, but the experiments suggest that both sudden overturning (characteristic of magmas of nearly equal viscosity) and continuous release (when the upper layer is much more viscous) are viable mechanisms for magma mixing in the appropriate circumstances.

Coordination of Fe, Ga and Ge in high pressure glasses by Mössbauer, Raman and X-ray absorption spectroscopy, and geological implications

Mössbauer spectra of glasses of NaFeSi 3O 8 and 3NaAlSi 2O 6 · NaFeSiO 4 starting compositions consist of a dominant Fe 3+ and subordinate Fe 2+ quadrupole-split doublet, in agreement with previous work. Fe 3+ is assigned to tetrahedral coordination. Pressure-induced coordination changes are not observed in the pressure range 1 bar to 30 kbar. A gradual increase in isomer shift of the Fe 3+ doublet with increase in pressure is attributed to steric effects. Raman spectra of GeO 2, NaGaSi 3O 8 and NaGaSiO 4 glasses are dominated by network structure vibrations. There is no detectable change in the nearest-neighbor coordination of Ge 4+ in GeO 2 from 1 bar to 14 kbar, of Ga 3+ in NaGaSi 3O 8 from 1 bar to 28 kbar and of Ga 3+ in NaGaSiO 4 from 1 bar to 25 kbar. However, some structural reorganization outside of the first coordination sphere occurs in the high pressure glasses. XANES and EXAFS spectra on powdered samples of 1 bar and 25 kbar NaGaSiO 4 glasses and crystalline NaGaSiO 4 were obtained from K edge absorption spectra at the Stanford Synchrotron Radiation Laboratory using a double crystal monochromator equipped with Si(220) crystals. The XANES spectra indicate that Ga 3+ has a similar extended coordination geometry in both glasses. The EXAFS spectra reveal that Ga 3+ is four-coordinated with oxygen in all three samples with a Ga 3+-O distance of about 1.83 Å. The radial distribution functions of the two glasses are virtually identical. However, both XANES and EXAFS spectra indicate significant structural differences between crystalline NaGaSiO 4 (nepheline-type structure) and vitreous NaGaSiO 4 beyond the first coordination shell of Ga 3+. Thus, X-ray absorption spectroscopy independently confirms the Raman results on the unchanged coordination of Ga 3+ in NaGaSiO 4 glasses with pressures up to 25 kbar. Glass compositions were selected in anticipation that larger and/or lower charged cations would exhibit pressure-induced coordination changes at lower pressures than Al 3+ and Si 4+. The present null result suggests that the stabilizing features of open network structures in the liquid state (large entropy and minimized cation-cation repulsion) more than compensate for large molar volume in the pressure range accessible to experimentation. It appears that network structures in natural magmas should remain stable throughout the upper mantle. Consequently, the densities of magmas at high pressures which are calculated from compressibility data and the appropriate equation of state will be only slightly underestimated, due to the effect of minor structural changes beyond the first coordination sphere.

Symmetrical and segmented variation of physical and geochemical characteristics of the central american volcanic front

Abstract The regional variation of physical and geochemical characteristics of Central American volcanoes occurs in two fundamentally different patterns. The first pattern is symmetrical about Nicaragua. Crustal thickness, silica contents of mafic lavas and volcanic edifice heights are lowest in Nicaragua and increase smoothly toward Costa Rica to the south and Guatemala to the north. Magma density is maximum in Nicaragua and decreases smoothly outward. The regional variation in crustal thickness is just enough so that magma densities, calculated at appropriate Moho pressures, are the same at the base of the crust throughout the region. This is consistent with magma ponding at the base of the crust. The bulk compositions of Central American basalts show the same symmetrical variation. Suites of Nicaraguan basalts plotted in pseudo-ternary CMAS projections indicate large olivine and plagioclase primary-phase volumes. Toward Costa Rica and Guatemala the olivine and plagioclase fields inferred from suites of basaltic lavas are smaller, which is consistent with fractionation at increasing depth. The second pattern is the segmentation of the volcanic front and the plate margin in general. The segmentation strongly affects the spacing and size of volcanic centers. At segment boundaries volcanic centers are generally small and unusually widely spaced. Toward segment interiors volcano spacing and size increase systematically. The LIL element contents of lavas strongly reflect this pattern. For lavas with similar silica contents the larger the volcano, the higher the LIL element contents. The relationships between segmentation, volcano spacing and volcano size are compatible with diapiric rise of magma accumulated in narrow ribbons near the upper surface of the underthrust slab. The relationship between volcano volume and LIL element content is qualitatively in agreement with an open-system fractionation model.

Density changes during the fractional crystallization of basaltic magmas: fluid dynamic implications

The dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes. In this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization. Removal of olivine and pyroxene, whose values of fractionation density are larger than the density of the magma, causes the density of residual liquid to decrease. Removal of plagioclase, with fractionation density less than the magma density, can cause the density of residual liquid to increase. During the progressive differentiation of basaltic magma, density decreases during fractionation of olivine, olivine-pyroxene, and pyroxene assemblages. When plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum. Calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects. In the closed-system evolution of basaltic magma, several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization, as well as on the geometry of the chamber. In an early stage of the evolution, where olivine and/or pyroxenes are the fractionating phases, compositional stratification can occur due to side-wall crystallization and replenishment by new magma, with the most differentiated magma tending to accumulate at the roof of the chamber. When plagioclase becomes a fractionating phase a zone of well-mixed magma with a composition close to the density minimum of the system can form in the chamber. The growth of a zone of constant composition destroys the stratification in the chamber. A chamber of well-mixed magma is maintained while further differentiation occurs, unless the walls of the chamber slope inwards, in which case dense boundary layer flows can lead to stable stratification of cool, differentiated magma at the floor of the chamber. In a basaltic magma chamber replenished by primitive magma, the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma. Successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber. In a chamber containing well-mixed, plagioclase — saturated magma, the primitive magma can be either denser or lighter than the resident magma. In the first case, the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma. Mixing then occurs between magmas of different temperatures and compositions. In the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma.

Evolution, growth and stabilization of the Precambrian lithosphere

Modern plate tectonic processes do not appear to be very efficient in continent-building since they have contributed little to the total volume of the continental lithosphere over the last 900 Ma. The much higher crust-formation rate in the Archaean cannot be ascribed to faster plate motion or frequent collision of many small plates during that time but appears to be related to extensive melting in the upper mantle and underplating of dense magmas that could not penetrate the overlying continental crust. These magmas provided a reservoir for most of the typical Archaean magmatic rock associations found in greenstone-grainite-gneiss terrains. It is argued that the predominantly biomodal volcanic suites of Archaean greenstone belts and the genetically related tonalite-trondhjemite-granite plutons did not originate through growth on intra-oceanic island arcs or in Andean-type arcs and marginal basins but in intracontinental rift settings. Magmatic underplating led to non-linear growth and differentiation of Archaean continental crust and is also seen as the prime cause for early lithospheric thickening and stabilization, allowing large intracontinental basins to form since 3.5 Ga ago. The end-Archaean global crust-forming event generated the first large cratons whose subsequent stability may be the result of subcrustal lithospheric thickening through melt extraction and depletion, underplating and carbonic granulite metamorphism in the lower crust. Magmatic underplating continued to remain an important process in the generation of volcanic sequences in the Lower Proterozoic intracontinental basins although there appears to be first evidence for local horizontal crustal accretion in magmatic arcs at about 2 Ga ago. Most of the widespread Proterozoic mobile belts lack typical Wilson-cycle signatures and are interpreted by a model involving extensive crustal thinning, subcrustal lithosphere delamination and crust restacking during basin closure and orogeny. New and fertile, but initially mechanically weak lithosphere is accreted to the crust from below during this process. By the end of the Precambrian horizontal accretion of juvenile continental crust became widespread, but vertical accretion may still be important in Phanerozoic crust-forming events. It is concluded that vertical rather than horizontal growth of lithosphere has dominated global evolution during the first 3.5 Ga of earth's history and that this mechanism also accounts for the observed magmatic rock assemblages as well as for lithospheric stabilization and cratonization of continental crust.

The J-M platinum-palladium reef of the Stillwater Complex, Montana; II, Origin by double-diffusive convective magma mixing and implications for the Bushveld Complex

Sequences of cumulates form concurrently by downdip accretion from a column of liquid layers that are primarily separated by diffusive interfaces. The low cumulates grow in advance of the upper cumulates in accord with their higher crystallization temperatures. An important observation is that mixing of the two parent liquids lowers their crystallization temperatures and, consequently, should produce unsaturated hybrids that could dissolve earlier cumulates. The repetition of cyclic units above the J-M Reef is explained by having periodic additional injections of dense magma beneath the subsiding column of crystallizing liquid layers. Neither the J-M Reef nor the Bushveld ore zones should ever have been continuous across the floor of their respective magma chambers; and, for Bushveld, the circumstances resulted in zones that are actually minable.--Modified journal abstract.

On the lateral variations in chemical composition and volume of quaternary volcanic rocks across Japanese arcs

The Fe/Mg+Fe) ratios (X Fe) of the Quaternary basalts (SiO 2 < 53 wt.%) in the Japanese arcs were examined. The X X Fe of relatively magnesian basalts decreases from the volcanic front toward the Japan Sea across the arcs. Based on the partition coefficient of Mg-Fe 2+ between olivine and liquid, it is suggested that all the basalts near the volcanic front, which are mostly tholeiitic basalts, are significantly fractionated, whereas many basalts near the Japan Sea, which are mostly alkali basalts, are little fractionated. The K 2 O content in the primary basalt magmas increases toward the Japan Sea. Combining the X Fe and K 2 O data, it is suggested that relatively large amounts of tholeiitic magmas are produced near the volcanic front, but they fractionate during their ascent, whereas smaller amounts of alkali basalt magmas are formed near the Japan Sea, but they can ascend with less fractionation. The density of primary tholeiite magma is significantly larger than that of primary alkali basalt magmas. It is most likely that primary tholeiite magmas cannot ascend beyond the upper crust and would fractionate to produce less dense tholeiitic magmas near the volcanic front, whereas primary alkali basalt magmas can ascend through the upper crust without fractionation, as far as buoyancy is the principal ascending force. In the Japanese arcs, the stress field may be less compressional near the Japan Sea than near the volcanic front, so that magmas can ascend more rapidly in the latter region than in the former. These two factors may be responsible for the above mentioned chemical variations of basalt magmas across the arcs. The variation in volume of the Quaternary volcanic rocks across the arcs can be explained by the presence of a melt-rich zone above but nearly parallel to the subducted slab.

A Model for the Origin of the Platinum-Rich Sulfide Horizons in the Bushveld and Stillwater Complexes

The sulfides of the Merensky Reef and the UG–2 chromite seam of the Bushveld Complex and the H.P. Reef of the Stillwater Complex have concentrated platinum group elements (PGE) with remarkable efficiency. Attempts to model these sulfides have exposed two major problems: (i) very high Nernst distribution coefficients (D) are required and (ii) a mass balance problem. The second problem, that of mass balance, stems from the first. If the D value is high, the sulfides must reach equilibrium with a large mass of silicate melt in order to take full advantage of this high D value. In practice R, the silicate/sulfide mass ratio, must be greater than D. It is suggested that the Merensky Reef, UG–2 and H.P. Reef were formed by major new pulses of picritic magma. Factors which control the value of R depend on the injection process and on the mechanism of sulfide liquation. During the early stages of fractional crystallization, when olivine and bronzite are the fractionating phases, the density of the magma in the chamber decreases with increased fractionation. A new pulse of magma entering the chamber will be denser than the magma in the chamber and will spread out across the floor. If the new pulse enters the chamber with sufficient momentum it will initially jet upwards into the overlying magma then fall back on itself when negative buoyancy overcomes the jet's initial momentum. During jetting the new pulse mixes with the host magma to form a hybrid melt which collects at the floor of the chamber, producing a gravity stratified layer with warm primitive magma overlain by cooler, more fractionated magma. On cooling the gravity stratified zone breaks up into a number of double diffusive convection cells. Once plagioclase becomes a cumulus phase the density of a tholeiitic magma increases with increased fractionation. Eventually the density of the magma in the chamber becomes greater than that of the parent magma. A new pulse entering the magma chamber at a stratigraphic level above the density crossover point will rise as a plume to the top of the chamber. During ascent the new pulse will mix turbulently with the host magma to produce a density stratified hybrid layer at the top of the chamber. This time hot primitive magma will overlie cooler, more fractionated magma. Magma mixing is a process that can produce sulfide liquation. The turbulence associated with either jets or plumes can produce a very high R value. The actual value of R is probably controlled by the size of the new pulse; the larger the pulse the higher the R value. If sulfide liquation is temperature-controlled, stratigraphic factors and the relative densities of the two magmas may have an important influence on R. The most favorable condition for producing a high value for R is when a new pulse of hot magma, slightly lighter than the host magma, enters a density stratified chamber. Under these conditions, the new pulse will rise to its own density level, then spread laterally to form a density stratified intermediate layer which will lose heat rapidly to the cooler overlying host magma. If the density difference between the original magmas was small, the density stratification forming within the hybrid layer will destabilize as it cools, allowing any immiscible sulfides which form to equilibrate with a large volume of silicate melt. If the density difference is large, cooling will not destabilize the density stratification and the sulfides will have a low R value.

Density constraints on the formation of the continental Moho and crust

The densities of mantle magmas such as MORB-like tholeiites, picrites, and komatiites at 10 kilobars are greater than densities for diorites, quartz diorites, granodiorites, and granites which dominate the continental crust. Because of these density relations primary magmas from the mantle will tend to underplate the base of the continental crust. Magmas ranging in composition from tholeiites which are more evolved than MORB to andesite can have densities which are less than rocks of the continental crust at 10 kilobars, particularly if they have high water contents. The continental crust can thus be a density filter through which only evolved magmas containing H2O may pass. This explains why primary magmas from the mantle such as the picrites are so rare. Both the over-accretion (i.e., Moho penetration) and the under-accretion (i.e., Moho underplating) of magmas can readily explain complexities in the lithological characteristics of the continental Moho and lower crust. Underplating of the continental crust by dense magmas may perturb the geotherm to values which are characteristic of those in granulite to greenschist facies metamorphic sequences in orogenic belts. An Archean continental crust floating on top of a magma flood or ocean of tholeiite to komatiite could have undergone a major cleansing process; dense blocks of peridotite, greenstone, and high density sediments such as iron formation could have been returned to the mantle, granites sweated to high crustal levels, and a high grade felsic basement residue established.

Effects of volatiles on mixing in calc-alkaline magma systems

The intimate mixing between different magmas of disparate densities, characteristic of some calc-alkaline magma systems, is explained by a new mechanism involving the emplacement of a layer of wet undersaturated mafic magma at the base of a magma chamber containing more differentiated magma. Turbulent transfer of heat between the mafic magma and the overlying magma leads to crystallization and exsolution of volatiles in the lower layer. For an initial water content of a few per cent the bulk density of the mafic magma can become equal to that of the overlying magma causing overturning and intimate mixing of the magmas. The mechanism is most probable in the low pressure environment of a high level magma chamber.

Sedimentary features of the Nunarssuit and Klokken syenites, S Greenland

SUMMARY: The sediment-like features of two layered syenites are described and reviewed in the light of recent suggestions that layering in gabbros maybe produced by in situ crystal growth in a near-rigid medium. In the Nunarssuit Syenite igneous layering is normally graded (sharp mafic bases becoming more feldspathic upwards) and channel structures, some with breccias and extreme concentrations of mafic phases, are evidence of magmatic currents. Contorted bedding provides evidence of c. 15 m of crystal mush. In the chemically similar Klokken Laminated Syenite the layers are inversely graded with well-sorted horizons of hedenbergite or fayalite at the top of graded units. Current bedding and rare channels are superimposed o n this inverse grading, and striking load structures occur at junctions with interbedded horizons of Granular Syenite, demonstrating the presence of &gt;3 m of crystal mush. The Klokken Laminated Syenites are slightly more fractionated than Nunarssuit and approach the water-rich termination of fractionation of olivine, clinopyroxene and ternary feldspar. Cumulus phases in Nunarssuit layers are chemically less fractionated than those in ‘normal’ rock and occur with cumulus chevkinite and zircon, consistent with separation of suspended crystals from melt, but cumulus minerals at Klokken do not show this type of chemical contrast. Calculated magma densities and viscosities show that crystal settling of all phases was feasible in both complexes if the magmas were hydrous. Druses at Klokken suggest that the intercumulus liquid was water saturated during load structure formation. Sorting of locally derived crystals by currents accounts for the Nunarssuit layering, but at Klokken the crystal supply was controlled by order-of-nucleation and growth rate effects, although crystal settling occurred.