Michel Pichavant

Evidence for mantle metasomatism by hydrous silicic melts derived from subducted oceanic crust.

The low concentrations of niobium, tantalum and titanium observed in island-arc basalts are thought to result from modification of the sub-arc mantle by a metasomatic agent, deficient in these elements, that originates from within the subducted oceanic crust. Whether this agent is an hydrous fluid or a silica-rich melt has been discussed using mainly a trace-element approach and related to variable thermal regimes of subduction zones. Melting of basalt in the absence of fluid both requires high temperatures and yields melt compositions unlike those found in most modern or Mesozoic island arcs. Thus, metasomatism by fluids has been thought to be the most common situation. Here, however, we show that the melting of basalt under both H2O-added and low-temperature conditions can yield extremely alkali-rich silicic liquids, the alkali content of which increases with pressure. These liquids are deficient in titanium and in the elements niobium and tantalum and are virtually identical to glasses preserved in mantle xenoliths found in subduction zones and to veins found in exhumed metamorphic terranes of fossil convergent zones. We also found that the interaction between such liquids and mantle olivine produces modal mineralogies that are identical to those observed in metasomatized Alpine-type peridotites. We therefore suggest that mantle metasomatism by slab-derived melt is a more common process than previously thought.

Fluid immiscibility in natural processes: Use and misuse of fluid inclusion data: II. Interpretation of fluid inclusion data in terms of immiscibility

Abstract Phase equilibrium analysis of the relevant systems together with the application of the principles of chemical equilibrium put severe constraints on the interpretation of fluid inclusion data in terms of immiscibility (in Part I). Following from that point, the major limits on the accuracy, and even the validity, of fluid inclusion quantitative data and their interpretation in terms of fluid composition and density are briefly discussed. The practical implications of the general constraints (temperature, pressure, topology of the fluid systems) are envisaged. Emphasis is laid on some important consequences such as: the use of isochore intercepts (and the possible resulting interpretation of fluid mixing rather than unmixing), the case of highly saline inclusions, the identification and interpretation of heterogeneous trapping. The composition and density constraints on coexisting fluids are presented, and illustrated by natural examples. Taking into account all the measurable parameters in fluid inclusions (volume, temperature and nature of phase transitions, more or less complete individual spectroscopic analyses), all the available experimental data, and all the theoretical constraints, may be long and difficult. However, it is most generally very informative and productive although part of these data is often sufficient to deny unmixing. Nevertheless, a final example on metamorphic fluids demonstrates how such an approach can “prove”; and also characterize a fluid unmixing during a geologic process.

Carbonatite Melts and Electrical Conductivity in the Asthenosphere

Electrically conductive regions in Earths mantle have been interpreted to reflect the presence of either silicate melt or water dissolved in olivine. On the basis of laboratory measurements, we show that molten carbonates have electrical conductivities that are three orders of magnitude higher than those of molten silicate and five orders of magnitude higher than those of hydrated olivine. High conductivities in the asthenosphere probably indicate the presence of small amounts of carbonate melt in peridotite and can therefore be interpreted in terms of carbon concentration in the upper mantle. We show that the conductivity of the oceanic asthenosphere can be explained by 0.1 volume percent of carbonatite melts on average, which agrees with the carbon dioxide content of mid-ocean ridge basalts.

Fluid immiscibility in natural processes: Use and misuse of fluid inclusion data: I. Phase equilibria analysis — A theoretical and geometrical approach

Abstract Many occurrences of fluid immiscibility in natural geologic systems have been reported recently, most often from fluid inclusion studies. However, the interpretation of fluid inclusion data in terms of immiscibility sometimes suffers from ambiguity of the vocabulary, insufficient knowledge of the immiscibility constraints and insufficient knowledge of the topology (in the TPX space) of natural fluid systems. For such reasons some authors have been misled to erroneous interpretations. The term “chemical immiscibility” is unambiguously redefined as a multiphase multicomponent equilibrium. The consequences of this definition are directly derived from the phase rule and concern the possible equations that relate the various parameters (temperature, pressure, volumes, compositions) to each other. These equations already put constraints on the topology of the phase equilibria in fluid systems. A particular expression of the phase rule is proposed, which takes into account the multiphase-constant bulk volume-constant bulk composition constraints in fluid inclusions. The consequences of such an expression are of major importance in fluid inclusion studies. The phase relations of some simple systems that approximate quite efficiently the natural complex fluids are then detailed: H2ONaCl, CO2CH4, H2OCO2, H2OCO2NaCl. The effects of these topologies and of the supplementary constraints (constant bulk composition and constant bulk volume) assumed for fluid inclusions (isopleth-isochoric systems) are discussed.

Redox control of sulfur degassing in silicic magmas

Explosive eruptions involve mainly silicic magmas in which sulfur solubility and diffusivity are low. This inhibits sulfur exsolution during magma uprise as compared to more mafic magmas such as basalts. Silicic magmas can nevertheless liberate large quantities of sulfur as shown by the monitoring of SO 2 in recent explosive silicic eruptions in arc settings, which invariably have displayed an excess of sulfur relative to that calculated from melt degassing. If this excess sulfur is stored in a fluid phase, it implies a strong preference of sulfur for the fluid over the melt under oxidized conditions, with fluid/melt partition coefficients varying between 50 and 2612, depending on melt composition. Experimentally determined sulfur partition coefficients for a dacite bulk composition confirm this trend and show that in volcanic eruptions displaying excess gaseous sulfur, the magmas were probably fluid-saturated at depth. The experiments show that in more reduced silicic magmas, those coexisting only with pyrrhotite, the partition coefficient decreases dramatically to values around 1, because pyrrhotite locks up nearly all the sulfur of the magma. Reevaluation of the sulfur yields of some major historical eruptions in the light of these results shows that for oxidized magmas, the presence of 1-5 wt % fluid may indeed account for the differences observed between the petrologic estimate of the sulfur yield and that constrained from ice core data. Explosive eruptions of very large magnitude but involving reduced and cool silicic magmas, such as the Toba or the Bishop events, release only minor amounts of sulfur and could have consequently negligible long-term (years to centuries) atmospherical effects. This redox control on sulfur release diminishes as the melt composition becomes less silicic and as temperature increases, because both factors favor more efficient melt sulfur degassing owing to the increased diffusivity of sulfur in silicate melts under such conditions.

An experimental study of the effect of boron on a water saturated haplogranite at 1 Kbar vapour pressure

An experimental study of the effect of boron in the water saturated Q-Or-Ab-B2O3-H2O system has been performed at P=1 Kbar to provide experimental data and explain the role of boron in some late magmatic and early hydrothermal events. Experiments were conducted between 500° C and 800° starting from a gel, or a previously crystallized gel, and variable amounts of boron (0 to 18% B2O3) added to water. The phases obtained were: quartz, sanidine, albite, silicate liquid quenched to glass, and aqueous vapour phase. Boric acids, borates and isotropic low index materials were found in the quenched vapour phase. An aluminium silicate-like mineral, not yet fully identified, is also present.The solidus temperature of the Q-Or-Ab composition is lowered by 60° C when 5 wt. % B2O3 is added and by more than 130° C when 17wt. % B2O3 is added. Compositions of equilibrated silicate melts and vapours were obtained between 780° C and 750° C for various B2O3 concentrations. The vapour phase is B and Si rich. It is also enriched in Na with respect to K, and in alkalis with respect to Al. Its silicate solute content is higher than in experiments with pure water. The solubility of water is increased by the addition of boron in Q-Or-Ab melts. Microprobe data show that the melts equilibrated with vapour phases become hyperaluminous and more potassic than sodic. The partition coefficient of boron is in favour of the vapour (kD=B2O3% in melt/B2O3% in vapour=0.33±0.02). The effect of the interaction between the silicate phases and the vapour is discussed. Comparison is made between the behaviour of boron and that of chlorine and fluorine. Geological applications are also provided, which concern the influence of boron on minimum melting, on muscovite stability and on the hypersolvus-subsolvus transition.

Effects of f O2 and H2O on andesite phase relations between 2 and 4 kbar

Experimental phase equilibria have been investigated on three medium-K silicic andesite (60–61 wt % SiO2) samples from Mount Pelee at 2–4 kbar, 850–1040°C, under both vapor-saturated CO2-free and vapor-saturated CO2-bearing conditions. Most experiments were crystallization experiments using dry glasses prepared from the natural rocks. Both normal- and rapid quench experiments were performed. Two ranges of oxygen fugacity (fO2) were investigated: NNO (Ni-NiO buffer) to NNO + 1 and NNO + 2 to NNO + 3. At 2 kbar for moderately oxidizing conditions, plagioclase (pl) and magnetite (mt) are the liquidus phases, followed by low-Ca pyroxene (opx); these three phases coexist over a large temperature (T)-H2O range (875–950°C and 5–7 wt % H2O in melt). Amphibole (am) is stable under near vapor-saturated CO2-free conditions at 876°C. At 900°C, ilmenite (ilm) is found only in experiments less than or equal to NNO. Upon increasing pressure (P) under vapor-saturated CO2-free conditions, pl + mt is replaced by am + mt on the liquidus above 3.5 kbar. For highly oxidizing conditions, mt is the sole liquidus phase at 2 kbar, followed by pl and opx, except in the most H2O-rich part of the diagram at 930°C, where opx is replaced by Ca-rich pyroxene (cpx) and am. Compositions of ferromagnesian phases systematically correlate with changingfO2 Experimental glasses range from andesitic through dacitic to rhyolitic, showing systematic compositional variations with pl + opx + mt fractionation (increase of SiO2 and K2O, decrease of Al2O3, CaO, FeOt, and MgO). FeO*/MgO moderately increases with increasing SiO2. For fO2 conditions typical of calk-alkaline magmatism (approximately NNO + 1), magnetite is either a liquidus or a near-liquidus phase in hydrous silicic andesite magmas, and this should stimulate reexamination for the mechanisms of generation of andesites by fractionation from basaltic parents.

Phase equilibrium constraints on the viscosity of silicic magmas: 1. Volcanic‐plutonic comparison

By using recently determined experimental phase equilibria we show that the viscosity of granitic magmas emplaced at upper crustal levels is approximately constant at ∼104.5 Pa s, irrespective of their temperature and level of emplacement. Magmas crystallizing as granitic plutons are not water-poor and thus not more viscous than their extrusive equivalents. Instead, comparison between pre-eruption magma viscosities of extrusive silicic-intermediate and intrusive granitic magmas shows that the former are on average slightly more viscous. Given the typical strain rates in silicic magma chambers, magma rheological behavior is expected to be dominantly Newtonian, bubbles having a minor rheological influence at depth although exceptions can exist. Thus whether a silicic-intermediate magma is erupted or frozen at depth depends primarily on the rheological properties of surrounding terranes or on external tectonic factors, but not on the rheology of the magma itself. However, preemptive viscosities of extrusive magmas rarely exceed 106 Pa.s, which suggests that crystal-melt mushes with higher viscosities cannot leave the magma storage regions beneath volcanoes. The narrow range of viscosities displayed by silicic-intermediate magmas results from both the strong control that pressure exerts on volatile solubilities in silicate melts and thermal limitations required to produce acid magmas. Considerations of the relationships between magma crystallinities, bulk SiO2, and preemptive melt H2O contents show that the higher the melt H2O content is the higher the maximum crystallinity that a given magma will be while still being potentially erupted. An empirical correlation is proposed that enables us to estimate preemptive melt H2O contents of erupted magmas by knowing their crystallinity and bulk SiO2.

Petrogenesis of tourmaline granites and topaz granites; the contribution of experimental data

Abstract Fluorine and boron are generally only minor components of granitic rocks, but may be found highly concentrated in certain tourmaline granites and topaz granites (up to 1 wt.% B2O3 and 3.2 wt.% F, respectively). Studies of various examples of these granite types, which occur as small bodies emplaced at shallow levels (P lithostatic Textural and petrographic data indicate that the tourmaline and topaz granite magmas crystallized at low to moderate temperatures (below ∼ 700°C) and that a fluid phase may have been present during the crystallization interval. Both granites varieties are peraluminous (Co up to 5%), characterized by the presence of white micas (lithium micas or muscovite), aluminium silicates and other Al-rich minerals (garnet, cordierite). Boron and fluorine are hosted by tourmaline (schorl-rich member of the schorl-dravite series), topaz and mica, respectively. These granites have low Ca, Fe, Mg and Ti contents (each The crystallization of tourmaline granite and topaz granite magmas is discussed using experimental data derived from studies of granitic systems with added boron and fluorine. Both elements act as a flux, enabling boron and fluorine enriched melts to persist to temperatures as low as 650°C at 1 kbar. In addition the effect of increasing fluorine content displaces the minimum in QAbOr at PH2O = 1 kbar toward the albite apex. Plots of fluorine-rich topaz granites coincide with this trend, suggesting that the role of fluorine in magmatic processes may have affected the rock composition as well as crystallization temperatures. Boron-rich granites plot near the low PH2O minima in QzAbOr, suggesting that boron has less of an influence on minimum melt compositions. The low temperatures of crystallization permits phases such as muscovite or andalusite, topaz, and two alkali feldspars to crystallize directly from F and B enriched melts at low pressures. In addition, the limited available experimental data are used to compare the effects of fluorine and boron on major and trace element distribution during melt-vapour interaction.

Apatite solubility in peraluminous liquids: Experimental data and an extension of the Harrison-Watson model

Abstract New experimental data at 750–1000°C, 2–5 kbar, and for peraluminous melt compositions show a dramatic increase of apatite solubilities and phosphorus melt contents when compared to metaluminous or peralkaline compositions ( Harrison and Watson , 1984). The formation of alumino-phosphate units in peraluminous melts qualitatively explains our data. A new apatite solubility model, incorporating the data for the peraluminous region, is presented. Application of this model to examples of peraluminous felsic suites is discussed.