George W. Bergantz

Underplating and Partial Melting: Implications for Melt Generation and Extraction

The quantitative assessment of underplating and concomitant partial melting of continental crust requires the use of geologically reasonable melt fraction distributions as a function of temperature. Conductive modeling indicates that simple underplating of metapelite by basalt can yield magma bodies with melt fractions above the rheological limit of extraction for almost any reasonable geotherm. Convection and subsequent homogenization are likely in these bodies. Granitic and tonalitic protoliths do not appear to yield substantial amounts of extractable magma. These results indicate that underplating involves repeated intrusion or occurs in deep crust.

Petrology of the Chilliwack batholith, North Cascades, Washington: generation of calc-alkaline granitoids by melting of mafic lower crust with variable water fugacity

Calc-alkaline granitoid rocks of the Oligocene-Pliocene Chilliwack batholith, North Cascades, range from quartz diorites to granites (57–78% SiO2), and are coeval with small gabbroic stocks. Modeling of major element, trace element, and isotopic data for granitoid and mafic rocks suggests that: (1) the granitoids were derived from amphibolitic lower crust having REE (rare-earth-element) and Sr-Nd isotopic characteristics of the exposed gabbros; (2) lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting. The main effect of fH2O variation is to change the relative proportions of plagioclase and amphibole in the residuum. The REE data for intermediate granitoids (quartz diorite-granodiorite; Eu/Eu*=0.84–0.50) are modeled by melting with fH2O<1 kbar, leaving a plagioclase + pyroxene residuum. In contrast, data for leucocratic granitoids (leuco-granodiorites and granites; Eu/Eu* =1.0–0.54) require residual amphibole in the source and are modeled by melting with fH2O=2–3 kbar. Consistent with this model, isotopic data for the granitoids show no systematic variation with rock type (87Sr/86Sri =0.7033–0.7043; εNd(0)=+3.3 to +5.5) and overlap significantly with data for the gabbroic rocks (87Sr/86Sri =0.7034–0.7040; εNd(0)=+3.3 to +6.9). The fH2O variations during melting may reflect additions of H2O to the lower crust from crystallizing basaltic magmas having a range of H2O contents; Chillwack gabbros document the existence of such basalts. One-dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large-scale melting of amphibolitic lower crust, provided that ambient wallrock temperatures exceed 800°C. Based on lithologic and geochemical similarities, this model may be applicable to other Cordilleran batholiths.

The Magma Reservoirs That Feed Supereruptions

The vigor and size of volcanic eruptions depend on what happens in magma reservoirs in the Earth’s crust. When magmatic activity occurs within continental areas, large reservoirs of viscous, gas-rich magma can be generated and cataclysmically discharged into the atmosphere during explosive supereruptions. As currently understood, large pools of explosive magma are produced by extracting interstitial liquid from long-lived “crystal mushes” (magmatic sponges containing >50 vol% of crystals) and collecting it in unstable liquid-dominated lenses.

Rejuvenation of the Fish Canyon magma body: A window into the evolution of large-volume silicic magma systems

Voluminous, unzoned, phenocryst-rich pyroclastic deposits, considered as erupted batholiths, provide a unique opportunity to investigate magmatic processes in silicic magmas. The Fish Canyon Tuff, a well-documented example of these monotonous ignimbrites, displays evidence for simultaneous dissolution of feldspars 1 quartz and crystallization of hydrous phases during gradual near-isobaric reheating from ;720 to 760 8C. These observations, along with a high crystallinity (45%) and near-solidus mineral assemblage, suggest that the Fish Canyon magma cooled to a rigid crystal mush before being partly remelted prior to eruption. Rejuvenation was triggered by intrusion of water-rich mafic magmas at the base of the Fish Canyon mush, but the mechanisms of heat transfer remain poorly understood. The growth of amphibole during reheating requires addition of mafic components, but the absence of any measurable gradients and the paucity of mafic enclaves in the Fish Canyon magma rule out a reheating event dominated by convective mixing with a mafic magma. Closed-system processes, such as heat conduction and convective self-mixing, could not account for the transport of externally derived mafic components. We performed numerical simulations of upward percolation of a hot, low-density H2O-CO2 fluid phase (gas sparging) through a crystalline framework saturated with rhyolitic melt to assess the efficiency of such a process in rejuvenating silicic mushes in open systems. Sparging by ;20‐40 km 3 of gas extracted from ;3000 km 3 of mafic magma is capable of reheating 7500 km 3 of silicic crystal mush by .40 8C in 150‐200 k.y. Moreover, the vertical thermal gradient after 150 k.y. in most of the mush is small (;25 8C in the upper 65%). Gas sparging also produces an increase in the internal pressure of silicic crystal mushes and may lead to the formation of crystal-poor rhyolites by expelling interstitial melt. However, our simulations predict that filter pressing driven by sparging of externally derived gas could not solely account for the generation of the most voluminous rhyolites.

A rapid mechanism to remobilize and homogenize highly crystalline magma bodies

The largest products of magmatic activity on Earth, the great bodies of granite and their corresponding large eruptions, have a dual nature: homogeneity at the large scale and spatial and temporal heterogeneity at the small scale. This duality calls for a mechanism that selectively removes the large-scale heterogeneities associated with the incremental assembly of these magmatic systems and yet occurs rapidly despite crystal-rich, viscous conditions seemingly resistant to mixing. Here we show that a simple dynamic template can unify a wide range of apparently contradictory observations from both large plutonic bodies and volcanic systems by a mechanism of rapid remobilization (unzipping) of highly viscous crystal-rich mushes. We demonstrate that this remobilization can lead to rapid overturn and produce the observed juxtaposition of magmatic materials with very disparate ages and complex chemical zoning. What distinguishes our model is the recognition that the process has two stages. Initially, a stiff mushy magma is reheated from below, producing a reduction in crystallinity that leads to the growth of a subjacent buoyant mobile layer. When the thickening mobile layer becomes sufficiently buoyant, it penetrates the overlying viscous mushy magma. This second stage rapidly exports homogenized material from the lower mobile layer to the top of the system, and leads to partial overturn within the viscous mush itself as an additional mechanism of mixing. Model outputs illustrate that unzipping can rapidly produce large amounts of mobile magma available for eruption. The agreement between calculated and observed unzipping rates for historical eruptions at Pinatubo and at Montserrat demonstrates the general applicability of the model. This mechanism furthers our understanding of both the formation of periodically homogenized plutons (crust building) and of ignimbrites by large eruptions.

Reconciling Pyroclastic Flow and Surge: the Multiphase Physics of Pyroclastic Density Currents.

Abstract Two end-member types of pyroclastic density current are commonly recognized: pyroclastic surges are dilute currents in which particles are carried in turbulent suspension and pyroclastic flows are highly concentrated flows. We provide scaling relations that unify these end-members and derive a segregation mechanism into basal concentrated flow and overriding dilute cloud based on the Stokes number ( S T ), the stability factor ( Σ T ) and the dense–dilute condition ( D D ). We recognize five types of particle behaviors within a fluid eddy as a function of S T and Σ T : (1) particles sediment from the eddy, (2) particles are preferentially settled out during the downward motion of the eddy, but can be carried during its upward motion, (3) particles concentrate on the periphery of the eddy, (4) particles settling can be delayed or ‘fast-tracked’ as a function of the eddy spatial distribution, and (5) particles remain homogeneously distributed within the eddy. We extend these concepts to a fully turbulent flow by using a prototype of kinetic energy distribution within a full eddy spectrum and demonstrate that the presence of different particle sizes leads to the density stratification of the current. This stratification may favor particle interactions in the basal part of the flow and D D determines whether the flow is dense or dilute. Using only intrinsic characteristics of the current, our model explains the discontinuous features between pyroclastic flows and surges while conserving the concept of a continuous spectrum of density currents.

On the dynamics of magma mixing by reintrusion: implications for pluton assembly processes

Many plutons have formed by repeated intrusion, with complex internal contacts characterized by sharp and diAuse kinematic and compositional domains. We performed numerical experiments of mixing following magma chamber recharge that explicitly considers the fluid dynamics of multiphase mixtures. In the limit of zero diAusivity of intensive scalar quantities and low Stokes number, mingling by fluid instability is equivalent to deformation, and persistent fluid structures are kinematic ‘attractors’. Three distinct regimes are exemplified, and can be described by their multiphase Reynolds (or Grashof) number. For a Reynolds number greater than about 100, an internal intrusive contact will collapse by internal wave-breaking, and chaotic magma mingling and mixing yield a nearly chamber-wide stratification. This flow may scour mushy regions at the walls and widely distribute previously crystallized material. For Reynolds numbers from 10 to 100, the internal wave does not break, and the stratification occupies less of the chamber, leaving islands of unmixed material. For Reynolds numbers around 1, internal slumping and folding can occur, and is the most likely to be preserved by a mineral fabric. Internal, sub-vertical contacts require a high absolute viscosity in the resident magma, representing a time-break between episodes of reintrusion. The shape of perched mafic domains captures the progress of sinking from higher levels into regions of increasing strength and crystallinity. 7 2000 Elsevier Science Ltd. All rights reserved.

Regional granulite facies metamorphism in the Ivrea zone: Is the Mafic Complex the smoking gun or a red herring?

One widely accepted paradigm for the development of continental lower crust is that regional granulite facies metamorphism is caused by intrusion of mafic magma beneath or into the crust (magmatic accretion). The amphibolite to granulite facies supracrustal section exposed in the Ivrea zone (southern Alps, northern Italy) is commonly cited as a classic example establishing this postulated genetic relationship. Our interpretation of the pattern of metamorphic isograds, compositional trends in high-grade metasedimentary rocks, and textural evidence in metapelite, however, indicates that final emplacement of the mafic plutonic rocks (the Mafic Complex) occurred subsequent to the regional thermal maximum. Field and petrographic relations suggest that a spatially restricted contact-melting event in crustal rocks accompanied the emplacement of the Mafic Complex. This inference is consistent with leucosome compositions in migmatites and a low-pressure, high-temperature metamorphic overprint recorded by mineral assemblages in wall rocks proximal to the intrusion. Therefore, evidence of anatexis and metamorphism of crustal rocks associated unequivocally with emplacement of the Mafic Complex is found only within an ~2-km-wide contact aureole overlying the intrusion. The narrow aureole associated with emplacement of the Mafic Complex demonstrates that, in some cases, emplacement of large volumes of mafic magma within the crust does not inexorably lead to regional-scale granulite facies metamorphism and large ion lithophile element depletion by melt loss.

Wavelet-based correlation (WBC) of zoned crystal populations and magma mixing

Magma mixing is a common process and yet the rates, kinematics and numbers of events are difficult to establish. One expression of mixing is the major, trace element, and isotopic zoning in crystals, which provides a sequential but non-monotonic record of the creation and dissipation of volumes of distinct chemical potential. We demonstrate a wavelet-based correlation (WBC) technique that uses this zoning for the recognition of the minimum number of mixing, or open-system events, and the criteria for identifying populations of crystals that have previously shared a mixing event. When combined with field observations of the spatial distribution of crystal populations, WBC provides a statistical link between the time-varying thermodynamic and fluid dynamic history of the magmatic system. WBC can also be used as a data mining utility to reveal open-system events where outcrop is sparse. An analysis of zoned plagioclase from the Tuolumne Intrusive Suite provides a proof of principle for WBC. 1 2002 Elsevier Science B.V. All rights reserved.

A numerical study of sedimentation by dripping instabilities in viscous fluids

Abstract A common instability in metallurgy and geophysics is the dripping of negatively-buoyant, solid–liquid mixtures. We conducted a numerical study of the finite amplitude evolution of multiphase Rayleigh–Taylor instabilities. For systems with a density-weighted average viscosity of less than 0.2, two time scales of sedimentation were observed. Initially, plumes form and merge, and solids disperse throughout the cavity. Final clarification of the carrier phase by hindered Stokes settling then occurs. The ratio of the time to clear the mixture to the time for near-uniform dispersal of the solids can be two or three orders of magnitude. Roof sedimentation in more viscous liquids demonstrates a complex time dependence. The solid volume fraction distribution becomes non-topological with features of both viscous and inertially dominated conditions at a given time step. Cyclic sedimentation occurs as the potential energy associated with the initially unstable layer does not decay in a temporally uniform manner.