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

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 (ST), the Stability factor (ET) and the Dense-Dilute condition (DD). We recognize five types of particle behaviors within a fluid eddy as a function of ST and ET : (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 DD 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

Numerical simulations of the mingling caused by a magma intruding a resident mush

Currently, our ability to interpret the mechanics of magma mingling and mixing is limited by an incomplete understanding of the modes of mixing across all melt fractions and compositions. Here, we present numerical simulations of the emplacement of crystal-free magma in crystal-rich reservoirs employing a computational fluid dynamics and discrete element method (CFD–DEM). We performed two runs corresponding to the emplacement of basalt into two end-member types of magmas mush (basaltic and dacitic). We found that the intruded volumes have similar shapes and are surrounded by a halo where the crystal volume fraction of the mush is lower. The dynamics of intruded melt are, however, different. Importantly, the mingling of the intruded and host materials starts after emplacement and consists in the incorporation of mush material into the intruded magma. Our findings imply that purely thermo-mechanical processes controlled by grain-scale dynamics are sufficient to explain fundamental aspects of recharge

Double-diffusive boundary layer convection in a porous medium : implications for fractionation in magma chambers

M.S.Robert P. Lowel

Time scales of crystal mixing in magma mushes

International audienceMagma mixing is widely recognized as a means of producing compositional diversity and preconditioning magmas for eruption. However, the processes and associated time scales that produce the commonly observed expressions of magma mixing are poorly understood, especially under crystal-rich conditions. Here we introduce and exemplify a parameterized method to predict the characteristic mixing time of crystals in a crystal-rich magma mush that is subject to open-system reintrusion events. Our approach includes novel numerical simulations that resolve multiphase particle-fluid interactions. It also quantifies the crystal mixing by calculating both the local and system-wide progressive loss of the spatial correlation of individual crystals throughout the mixing region. Both inertial and viscous time scales for bulk mixing are introduced. Estimated mixing times are compared to natural examples and the time for basaltic mush systems to become well mixed can be on the order of 10 days

Ultrafast Magmatic Buildup and Diversification to Produce Continental Crust During Subduction

The processes and fluxes that produce the distinct compositional structure of Earth’s continental crust by subduction remain controversial. The rates of oceanic crust production, in contrast, are well quantified and are generally believed to be faster than those responsible for building magmatic systems in subduction settings. Here we show that a recently recognized crustal section, the 30-km-thick Ordovician Sierra Valle Fértil–Sierra Famatina complex in Argentina, was built magmatically within only ∼4 m.y. More than half of the crustal section represents additions from the mantle, and is preserved as mafic igneous rocks and mafic-ultramafic cumulates; the remainder is tonalite to granodiorite with evidence for widespread assimilation from highly melted metasedimentary units. U-Pb zircon geochronology reveals that the construction of the arc was not a simple bottom-up construction process. This continuous exposure of the arc crust allows the quantification of field constrained magmatic addition rates of 300–400 km3 km–1m.y.–1. These rates are similar to those determined for modern slow-spreading mid-ocean ridges and are of the same magnitude as magmatic addition rates required to build certain large segments of the continental masses such as the Arabian-Nubian shield, among others. The implication is that significant convective removal of arc roots is required over time in order to build the modern continental crust via subduction-related magmatism

A MASH zone revealed: The mafic complex of the Sierra Valle Fértil

The Sierra Valle Fértil Complex of west-central Argentina represents a section of the Ordovician (~470 Ma) Famatinian arc and exposes a continuous, tilted crustal arc section ranging in depth from ~12 to 32 km (~4-8 kbar pressure). This arc section exposes the complete compositional architecture from ultramafic and mafic rocks to upper crustal granodiorites. Field and compositional data are presented to document the deep (~6-8 kbar) mafic complex of the Sierra Valle Fértil. The mafic complex is composed of many tens to hundreds of plutonic cumulate bodies in a complex and non-regular arrangement. There is no simple compositional, kinematic or age relationship between neighboring plutons throughout the section, as expressed by cumulate compositions, emplacement horizon, size, composition, texture or style of contact. Amphibole gabbronorites and mafic tonalites dominate, but norites, amphibole websterites, troctolites and minor anorthosites are present. Amphibole is common but always as a replacement phase, and is never observed undergoing subsequent dehydration melting. Hence there is no evidence that voluminous tonalites were produced by dehydration melting of mafic precursors. A field-based, cumulate-removal fractionation model is presented that produces the observed compositional variations in five steps. Isotopic compositions of Sr and Nd deviate significantly from primitive mantle values, indicating a crustal contribution; however, this hybridization appears to have played a minor role in the major element evolution of the mafic complex. We interpret this isotopic and elemental decoupling as a byproduct of prolonged, punctuated MASH (melting, assimilation, storage, homogenization) processes in the lower crust. Isotopes may be the only residual evidence of assimilation within the mafic zone. This requires that melt removal from the cumulates was extraordinarily efficient.Fil: Walker, Barry A.. University of Washington; Estados UnidosFil: Bergantz, George W.. University of Washington; Estados UnidosFil: Otamendi, Juan Enrique. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Fisicoquímicas y Naturales. Departamento de Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Ducea, Mihai N.. University of Arizona; Estados UnidosFil: Cristofolini, Eber Ariel. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Fisicoquímicas y Naturales. Departamento de Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Seismic volcanic risk assessment, Puna geothermal prospect area, Hawaii

Prepared for Thermal Power Company, Dillingham, AMFAC"This report summarizes the geologic, volcanologic, and seismologic data that relate to evaluation of the Seismic-Volcanic Risk Assessment (SVRA) for the Puna Geothermal Prospect (PGP) of the Lower East Rift Zone (LERZ) on the volcano, Kilauea, Hawaii. The basis for the assessment sections is developed in detail in the Geology Section (3.0), with the volcanic and seismic assessments discussed in Sections 4.0 and 5.0, respectively. The Volcanic Risk Assessment leads to conclusions that the hazards and risk from volcanic activity and associated fissuring and faulting are high and diverse, but the risk to engineered structures and installations can be mitigated by proper procedures in siting and design. Recommendations are made as to the methods and factors that should be considered for mitigation. The Seismic Risk Assessment demonstrates that the hazard and risk from ground motion is of three general sources: frequent low magnitude volcanic earthquakes with epicenters near the site, infrequent earthquakes of deep focus, low to moderate magnitudes and generally from Kilauea, and tectonic earthquakes of about 6.5 to 7.2 magnitude from the Hilina fault zone with epicenters at distances of from 20 to 50 km (12 to 30 mi)."2 maps : black and whiteMaps: Plate 1 / Epicenter map of earthquakes of ML>4.0 from 1960 to 1980 with focal depths greater than about 25 km. -- Plate 2 / Epicenter map of earthquakes of of ML>4.0 from 1929 to 1980 with focal depths less than about 25 km.Thermal Power Compan

Reworking subducted sediments in arc magmas and the isotopic diversity of the continental crust: The case of the Ordovician Famatinian crustal section, Argentina

Since the onset of plate tectonics, continents have evolved through a balance between crustal growth, reworking, and recycling at convergent plate margins. The term “reworking” involves the re-insertion of crustal material into pre-existing crustal volumes, while crustal growth and recycling respectively represent gains from and losses to the mantle. Reworking that occurs in the mantle wedge (“source” contamination from slab material) or within the upper plate (“path” contamination), will have contrasting effects on crustal evolution. However, due to limited access to deep crustal and mantle rocks, quantifying source vs. path contamination remains challenging. Based on the 4-dimensional record of the fossil (Ordovician) Famatinian continental arc (Argentina), we demonstrate that source contamination plays a dominant role in imprinting mafic to granitic rocks with crustal oxygen-hafnium (O-Hf) isotopic compositions. We argue that source contamination at convergent plate margins significantly increased the diversity of O-Hf isotopic signatures of continents over geologic time. Our interpretation implies that crustal evolution models attributing this isotopic diversity dominantly to intra-crustal reworking may be over-simplistic and may underestimate continental growth in the last 2.5 billion years.ISSN:0012-821XISSN:1385-013

Reconstruction of the Early Ordovician Famatinian arc through thermobarometry in lower and middle crustal exposures, Sierra de Valle Fértil, Argentina

The crustal structure of the Famatinian paleoarc is reconstructed by determining the metamorphic crystallization P–T conditions from metasedimentary rocks at various structural levels in the Valle Fértil section. The bulk section exhibits a 15-km-thick arc crustal section. Thermobarometry shows that nested tonalitic and granodioritic plutons constructed the arc crust at depths 6.6 km/s is located at deeper depths in the Famatinian arc than in Talkeetna arc. The thickness of a crustal layer dominated by plutonic rocks with low seismic wave velocities (< 6.2 km/s) is 10 km thinner than the crustal layer with similar physical properties in the Sierra Nevada batholith. A putative model for the whole Famatinian arc suggests a total crustal thickness between 30 and 35 km with three distinct layers.Fil: Tibaldi, Alina María. Universidad Nacional de Río Cuarto; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Otamendi, Juan Enrique. Universidad Nacional de Río Cuarto; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Cristofolini, Eber Ariel. Universidad Nacional de Río Cuarto; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Baliani, Ignacio. Universidad Nacional de Río Cuarto; ArgentinaFil: Walker, Barry A.. University of Washington; Estados UnidosFil: Bergantz, George W.. University of Washington; Estados Unido