Ariel Provost

Topology in isotopic multispace and origin of mantle chemical heterogeneities

Abstract In this paper we present a simple technique for multidimensional treatment of isotopic data, which allows a global and rigorous correlation between the various radiogenic tracers. This technique is based on the determination of the eigenvectors of the data matrix, allowing a geometric description of the inertia ellipsoid corresponding to the cluster of experimental data points. The relationships between the various sets of samples can be analyzed using the projections on the main elongation planes. When processing the Pb Sr Nd data for the oceanic mantle (OIB + MORB) with this technique we find that at least four different end-members are needed to define the “mantle array” which thus cannot be a plane surface. Samples from island arcs (IAB) show the contribution of a component clearly out of the oceanic domain and very similar to terrigenous sediments. Continental tholeiites (CFB) also show some sort of contamination (but distinct from that of IAB) by the continental crust. They also show a domain overlapping with that of the oceanic islands corresponding to the compositions of the “Dupal anomaly”. Multispace analysis also permits a rigorous comparison of relationships between the various isotopic tracers. In particular, we demonstrate that a U Pb fractionation independent from the correlated Sr Nd, Th U and Th Sr ones does exist. Consequently, a three-dimensional analysis performed only with lead isotopes yields by itself the main information that can be inferred from the five Pb Sr Nd dimensions. Helium also yields independent information decoupled with respect to the other tracers, adding one fifth end-member (Loihi, Hawaii islands) to the OIB + MORB array.

Wetting Angles, Equilibrium Melt Geometry, and the Permeability Threshold of Partially Molten Crustal Protoliths

The permeability of a partially molten rock is one of the primary factors controlling the melt segregation rate. With decreasing melt percentage, permeability becomes increasingly sensitive to the grain-scale geometry of partial melt. At low melt percentages, the ratio of grain-boundary energy to solid-melt interfacial energy, [γ ss ]/[γ sl ], is the fundamental physical property that determines the equilibrium melt geometry, including the wetting angle θ at a solid-solid-melt triple junction, the area-tovolume ratio s/v of melt pockets at grain corners, and the permeability threshold φ c (φ c , is the volume melt percentage at which melt interconnection is established). The trends of increasing θ and φ c or decreasing s/v with decreasing [γ ss ]/[γ sl ] are well established in the case of a monomineralic system with isotropic interfacial energies. We argue that these general trends must apply as well in natural systems where solid-melt interfacial energies are essentially anisotropic.

An improved diagram for isochron data

Abstract Among all possible isochron diagrams, one is shown to display optimal properties: 1. (1) Like the conventional isochron diagram (CID), the best isochron diagram (BID) shows the initial ratio as the left-hand border intercept. Unlike on the CID, the initial ratio is at a scale where its precision can be easily seen. 2. (2) Unlike the CID, the BID also displays the radiometric age as an intercept (on the right-hand border), again on an appropriate scale. 3. (3) On the BID, highly radiogenic samples are not unduly emphasized. There is no need for inserts nor enlargements. In particular, it is possible to plot data for whole rocks and minerals on the same graph. 4. (4) The whole height of the BID is available for displaying the analytical errors and deviations from the (nearly horizontal) isochron: there is no need for a Papanastassiou-Wasserburg insert. Due to these optimal properties, the BID allows one to simultaneously visualize the experimental data and their analytical precision, to judge the quality of the linear fit, to appraise the accuracy of the age and initial ratio results, and to quantitatively appreciate the influence of any existing or expected datum.

Mineralogical and geochemical evolution of the 1982-1983 Galunggung eruption (Indonesia)

Pyroclastic deposits from the 1982–1983 eruption of Galunggung volcano (Java, Indonesia) reflect preeruptive magmatic evolution which is of interest because of: (1) its duration of nine months, compared to a few hours or days for most historical eruptions; (2) the diversity of eruptive styles, from ash and scoria flows to phreatomagmatic explosions, and to the strombolian activity that marked the end of the eruption; and (3) the progressive variation in chemical composition with time, from andesite (58 wt.% SiO2) to high-Mg basalt (47 wt.% SiO2). The 1982–1983 Galunggung basalts are rather primitive: 10 to 12 wt% MgO, 180 to 200 ppm Ni and 550 to 700 ppm Cr. Despite the presence of about 40% phenocrysts, they may represent the most primitive basalts recognized in western Java. Basalts contain phenocrysts of olivine (Fo90-80), diopside-salite, and plagioclase (An95-75). Andesites contain plagioclase (An80–60), augite, hypersthene (En67-64), and titanomagnetite. The distribution of mineral compositions in each petrographic type is nearly unimodal, although scarce plagioclase and olivine xenocrysts have been observed. Abundance of gabbroic cumulates associated with the pyroclastic flows and evolution of mineral compositions from high-Mg basalts to andesites support crystal fractionation as the main differentiation mechanism, although magma mixing of basaltic andesite and andesite cannot be excluded. Major and trace element trends, which display rough decreases of MgO, CaO, Ni, Cr with increasing degree of differentiation and also linear positive correlations of hygromagmaphile elements, are compatible with both processes. However, some discrepancies are observed between major and trace element modelling, which may be explained to some extent by the influence of in situ crystallization and/or magma mixing. The constancy of 143Nd/144Nd (0.51286±3), 230Th/232Th (0.65±0.02), Th/U (4.08±0.07) ratios, and to a lesser extent δ18O values (+5.8 to +6.4 % SMOW) and 87Sr/86Sr ratios (0.70440 to 0.70468) is compatible with a magmatic evolution through fractional crystallization without significant crustal contamination. Nevertheless low-18O and high 87Sr/86Sr values in basaltic andesites may be due to the introduction of meteoric fluids into the Galunggung magma.

The Grain-Scale Distribution of Silicate, Carbonate and Metallosulfide Partial Melts: a Review of Theory and Experiments

In a partially molten rock containing a low melt fraction, the permeability and consequently the dynamics of melt segregation are strongly sensitive to the distribution of the melt at the grain scale. Melt distribution is controlled by a variety of factors such as the minimization of interfacial energies, the stress regime and different aspects of the melting reaction (melting rate, volume change on melting, spatial distribution of the reactants). Due to the long duration of large-scale melting events, an equilibrium melt configuration corresponding to a minimum total interfacial energy per unit volume should commonly be approached. In this chapter, we review the theoretical and experimental studies devoted to the equilibrium distribution of melt in a partially molten rock.

Equilibrium geometry of a fluid phase in a polycrystalline aggregate with anisotropic surface energies: Dry grain boundaries

The equilibrium distribution of a fluid phase in a rock is controlled by the ratio of grain boundary energy to solid-fluid interfacial energy (“surface energy”). A strong dependence of interfacial energy on interface orientation (anisotropy) is the rule for solid-fluid surfaces of geological interest. We investigated the effects of surface energy anisotropy on the equilibrium interface configuration at the junction of two crystals with a fluid in a two-dimensional solid-fluid system. The two major effects are to promote the development of planar solid-fluid interfaces parallel to crystallographic planes of minimum energy and to lead to large variations of the dihedral angle from one triple junction to the other (as a function of the orientation of crystalline lattices relative to the grain boundary). Despite these large variations, the frequency distributions of equilibrium dihedral angles remain unimodal; also the relationship between the mean fluid dihedral angle and the ratio of grain boundary to surface energy is very close to the isotropic law. Even in the most anisotropic systems, the fluid dihedral angle is therefore a parameter of primary importance for predicting the grain-scale geometry of a low-volume fraction of fluid and its mobility.

Thermochemical dynamics of magma chambers: A simple model

A magma chamber may be considered as an open, non adiabatic chemical reactor that continuously receives new liquid, crystallizes, and erupts lava. Heat is transferred to the walls, and crystals settle on the chamber floor. Considering, for simplicity, the chamber magma as a thermally and chemically homogeneous two-component liquid in which the solidification rate is a bell-shaped function of temperature and composition, the system is governed by two coupled differential equations for the evolution of temperature and composition with time. For constant external factors, such as fresh-magma input flow or heat loss flux, the magma chamber tends to a steady state. For a certain domain of external factor values, there are two stable steady state solutions instead of one. When the external factors vary slowly, the magma chamber state also drifts continuously, except when the external factors leave the bistability domain. Then all characteristics of the volcanic system undergo a sudden jump, for example, from basic to felsic composition and from high to low lava output flow when the fresh-magma input flow of a waning volcano declines. The jump takes place over a time of the order of the magma residence time and is thus short compared to the lifetime of the chamber. Such a magmatic catastrophe, which may occur for a small and continuous evolution of the external conditions, results in a radical change in eruption style and volcanic products and may trigger an exceptionally large or violent eruption. This kind of event may explain the widespread observation of compositional gaps in lava series (e.g., the Daly gap), a number of radical changes in eruptive style, and certain eruptions which appear unique in the lifetime of some composite volcanoes.

Process identification and search for optimal differentiation parameters from major element data. General presentation with emphasis on the fractional crystallization process

Abstract Different igneous differentiation processes result in various evolution trends in a certain composition space Rn. Almost every genetic process can be described as a single two-pole mixing process, which leads to a straight evolution line in Rn; or as a series of two-pole mixing processes, which in the general case leads to a broken curved evolution line. When a fundamental two-pole mixing process clearly prevails over one or several secondary processes in the genesis of a volcanic suite, a ‘best’ mixing line in Rn can be computed with due attention to experimental uncertainties. A number of statistical tools are available for testing the data-uncertainties-linear model consistency and for pointing out a few dubious data, or the influence of a few secondary processes. The best mixing line and its associated error volumes serve to quantify the trend, and then can be used to get some ‘best’ estimate (or competitive acceptable estimates) of the mineralogical parameters that control the genetic process. Any mineral assemblage defines a continuous composition family in Rn which can be characterized by its gaussian distance to the best mixing line. In the framework of a fractional crystallization model, that distance serves as a test for accepting or rejecting the mineral assemblage as a likely candidate for representing the crystallizing solid. In favorable situations the minimization procedure used to compute the distance also determines both the mineral proportions in the crystallizing solid and their compositions. In less favorable situations it may leave us with some indeterminacy among a small number of acceptable interpretations. The mixing line calculation and the mineralogical inversion procedure are applied to the classical Kilauea Iki Lava Lake (Hawaii) example, where olivine and chromite crystallization with incomplete solid-liquid separation is shown to be a likely explanation for the observations, along with near-surface iron oxidation as a secondary process. The origin of volcanic rocks from Terceira Island (Azores) is less clear. It is shown that the simultaneous crystallization of olivine, Ti-augite and plagioclase can be retained as an acceptable interpretation for the least differentiated rocks, but that substantial secondary processes are also involved.

CaO1bMgO1bAl2O31bSiO21bNa2O (CMASN) at 1 bar from low to high Na2O contents: Topology of an analogue for alkaline basic rocks☆

One-atmosphere synthetic systems, notably the system CaO1bMgO1bAl2O31bSiO21bNa2O (CMASN), have long been used in the study of alkaline rock petrogenesis. However, although much is known about phase relations, few studies provide quantitative data on liquid compositions. Systems that were considered previously, for example larnite-nepheline-forsterite-silica, were limited to compositions with high Na/Al ratios. Thus, some important end-members, like anorthite, could not be shown. This study fills a part of the gap between CMAS and the high Na/Al compositions of CMASN. In the composition range investigated, 7 phases have been encountered: liquid, forsterite, diopside, plagioclase, spinel, melilite and nepheline (liq, fo, di, pl, sp, mel and ne, respectively). The phase relations are such that 3 invariant points ([di], [sp], [ne]) are stable, and 3 ([fo], [pl], [mel]) are metastable. Twelve univariant (5-phase) assemblages are also present. Along with their divariant counterparts, they constitute most of the parageneses found in silica-undersaturated alkaline rocks. The liquidus equilibria in CMASN at 1 bar provide a very useful framework for discussing the petrogenetic problems of alkaline rocks, like the pl-mel incompatibility dilemma.

Mass Eruption Rates of Tephra Plumes During the 2011–2015 Lava Fountain Paroxysms at Mt. Etna From Doppler Radar Retrievals

Real-time estimation of eruptive source parameters during explosive volcanic eruptions is a major challenge in terms of hazard evaluation and risk assessment as these inputs are essential for tephra dispersal models to forecast the impact of ash plumes and tephra deposits. Between 2011 and 2015, Etna volcano has produced 49 paroxysms characterized by lava fountains generating tephra plumes that reached up to 15 km a.s.l.. We analyzed these paroxysms using the 23.5 cm wavelength Doppler radar (VOLDORAD 2B) signals along with visible camera images of the monitoring network of the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo. Range gating of the radar beam allows the identification of the active summit craters in real-time, no matter the meteorological conditions. The radar echoes help to mark (i) the onset of the paroxysm when unstable lava fountains, progressively taking over Strombolian activity, continuously supply the developing tephra plume, then (ii) the transition to stable fountains (climax), and (iii) the end of the climax with a waning phase, therefore providing paroxysm durations. We developed a new methodology to retrieve in real-time a Mass Eruption Rate (MER) proxy from the radar echo power and maximum Doppler velocity measured near the emission source. The increase in MER proxies is found to precede by several minutes the time variations of plume heights inferred from visible and X-Band radar imagery. A calibration of the MER proxy against ascent models based on observed plume heights leads to radar-derived climax MER from 2.96 × 104 to 3.26 × 106 kg s-1. The total erupted mass (TEM) of tephra was computed by integrating over beam volumes and paroxysm duration, allowing quantitative comparisons of the relative amounts of emitted tephra among the different paroxysms. When the climactic phase can be identified, it is found to frequently release 76% of the TEM. Calibrated TEMs are found to be larger than those retrieved by satellite and X-band radar observations, deposit analyses, ground-based infrared imagery or dispersion modeling. The radar-derived mass load parameters therefore represent a very powerful all-weather tool for the quantitative monitoring and real-time hazard assessment of tephra plumes at Etna.