Dominique Massare

High-temperature experiments on silicate melt inclusions in olivine at 1 atm: inference on temperatures of homogenization and H2O concentrations

Abstract A series of heating stage experiments, at ambient atmosphere and high temperature, were conducted in order to detail the change in the homogenization temperature and the behavior of H 2 O of melt inclusions in olivine phenocrysts (Fo 82–89 ). The samples were collected at different volcanic sites (Piton de la Fournaise [PdF], Reunion Island; Stromboli, Aeolian Islands; FAMOUS zone) expected to have a range of magmatic water contents. The melt inclusions vary in composition from basaltic to shoshonitic, with H 2 O content from 0.14 to 2.9 wt.%. Temperatures of homogenization ( T h ) of melt inclusions systematically increase with time during heating experiments, regardless of their major element composition and their H 2 O. It is proposed that T h changes with time in response to the deformation of the host crystal and change in the volume of the cavity. The FTIR spectra successively acquired on the same inclusion repeatedly heated at constant temperature clearly demonstrated that the relative absorbance (Abs. n /Abs. 0 ) measured at 3535 cm −1 corresponding to hydroxyl group and molecular H 2 O decreases with time. The H 2 O-rich melt inclusions may have lost from 20% to 80% H 2 O after the first homogenization and almost total dehydration of melt inclusions may occur within few hours or less, at 1 atm. Water loss driven out off olivine-hosted melt inclusions possibly exists to some extent in natural large dunitic bodies or in mantle xenoliths.

Primitive basaltic melts included in podiform chromites from the Oman Ophiolite

Abstract In an attempt to characterize the composition of the parental melts of ophiolitic chromitites and their tectonic setting, we have undertaken a study of polymineralic solid inclusions trapped in chromites from the Oman Ophiolite (Sumail nappe). High-temperature experiments performed on inclusions show that they result from post-entrapment crystallization of homogeneous basaltic melts with primitive compositions (Mg# = 63.5–66.8). The primary nature of the inclusions, demonstrated by their distribution outlining the crystallographic zones of mineral growth, indicates that the trapped melts represent small amounts of the parental liquids of the host chromites. Homogenised melt inclusions show depleted trace element patterns, with significant Nb depletions, characteristic of a subduction-related origin. These observations indicate that chromite deposits from Oman harzburgitic ophiolites may have formed in a geodynamic setting akin to present-day back-arc basins.

Low-degree partial melting trends recorded in upper mantle minerals

The study of glass inclusions inside mantle minerals provides direct information about the chemistry of naturally occurring mantle-derived melts and the fine-scale complexity of the melting process responsible for their genesis. Minerals in a spinel lherzolite nodule from Grande Comore island contain glass inclusions which, after homogenization by heating, exhibit a continuous suite of chemical compositions clearly distinct from that of the host basanitic lava. The compositions range from silicic, with nepheline–olivine normative, 64 wt% SiO2 and 11 wt% alkali oxides, to almost basaltic, with quartz normative, 50 wt% SiO2 and 1–2 wt% alkali oxides. Within a single mineral phase, olivine, the inferred primary melt composition varies from 54 to 64 wt% SiO2 for MgO content ranging from 8 to 0.8 wt%. An experimental study of the glass and fluid inclusions indicates that trapped melts represent liquids that are in equilibrium with their host phases at moderate temperature and pressure (T≈1230°C and P≈1.0 Gpa for melts trapped in olivine). Quantitative modelling of the compositional trends defined in the suite shows that all of the glasses are part of a cogenetic set of melts formed by fractional melting of spinel lherzolite, with F varying between 0.2 and 5%. The initial highly silicic, alkali-rich melts preserved in Mg-rich olivine become richer in FeO, MgO, CaO and Cr2O3 and poorer in SiO2, K2O, Na2O, Al2O3 and Cl with increasing melt fractions, evolving toward the basaltic melts found in clinopyroxene. These results confirm the connection between glass inclusions inside mantle minerals and partial mantle melts, and indicate that primary melts with SiO2 >60 wt%, alkali oxides >11%, FeO <1 wt% and MgO <1 wt% are generated during incipient melting of spinel peridotite. The composition of the primary melts is inferred to be dependent on pressure, and to reflect both the speciation of dissolved CO2 and the effect of alkali oxides on the silica activity coefficient in the melt. At pressures around 1 GPa, low-degree melts are characterized by alkali and silica-rich compositions, with a limited effect of dissolved CO2 and a decreased silica activity coefficient caused by the presence of alkali oxides, whereas at higher pressures alkali oxides form complexes with carbonates and, consequently, alkali-rich silica-poor melts will be generated.

Micro X-ray absorption near edge structure at the sulfur and iron K-edges in natural silicate glasses

Both sulfur and iron chemical environments were studied in natural compounds; sulfides, sulfates and silicate basaltic glasses which are supposed to have variable oxidation states, using XANES (X-ray absorption near edge structure) spectroscopy. Although X-ray absorption methods are well suited for such investigations, the size of natural glass inclusions trapped in volcanic minerals, ranging from a few micrometers to several tenths of micrometers, makes micro-spectroscopy necessary. Hence, we present the μ-XANES spectra at the sulfur and iron K-edges of olivine-hosted glass inclusions containing from 1000 to 1650 ppm S, and between 5.5 and 7.8 wt.% Fe. By combining both high energy and spatial resolutions, we demonstrate the ability of the μ-XANES to produce quantitative determination of sulfur and iron valence states. We have identified various species for sulfur, specifically SII−, SVI and possibly SIV, in basaltic glass inclusions hosted in olivine grains. We propose a method to calibrate the proportion of Fe dissolved as Fe3+ in basaltic glasses for which the Fe3+/ΣFe ratio varies between 0.05 and 0.48 with a relative precision of less than 10%.

Iron oxidation states in silicate glass fragments and glass inclusions with a XANES micro-probe

Abstract The iron oxidation states and its environment in volcanic silicate glasses have been studied by combining micro-X-ray absorption near edge structure (XANES) experiments at the iron K-edge in silicate glass fragments and glass inclusions trapped during the crystal growth. Experiments were performed at the ESRF (ID22 beam-line) – a third generation synchrotron – using a focused beam (2×10 μm 2 ) together with a high brilliance and a good energy resolution. The selected silicate glasses, basaltic in composition, contain between 5.5 and 8 wt% total Fe. In all XANES spectra, pre-peaks exhibit a multi-component structure, which is the convolution of information related to the valence state of iron (2+,3+), the site geometry and transition types. The Fe3+/ΣFe ratio of silicate glass fragments deduced from the XANES spectra is correlated to values determined by chemistry on bulk samples within the range 0.05–0.85. XANES spectra from glass inclusions hosted in natural olivine crystals may be interpreted considering the glass fragments as reference samples.

In situ mapping of high-pressure fluids using hydrothermal diamond anvil cells

We present new results combining high pressures and temperatures attainable in a diamond anvil cell with in situ synchrotron radiation induced micro-X-ray fluorescence measurements. Hydrothermal diamond anvil cells experiments have been performed by measuring the partitioning of Pb between aqueous fluids (pure water or NaCl-enriched water) and hydrous silicate melts of haplogranite composition using synchrotron X-ray fluorescence. The in situ measurements were performed in the range 0.3–1.2 GPa and 730–850 °C both in the aqueous fluid and in the silicate melts being in equilibrium. Pb is strongly partitioned into high-pressure–temperature hydrous melts when Cl is present in either the hydrous melt or the aqueous fluid. Moreover, our comparisons of in situ results with post-mortem results show that significant changes take place during rapid quenching especially when samples are small (few hundred of microns in diameter). Water exsolution is induced by the quench in the silicate melt showing the high mobility of Pb which immediately partitions into the water vapor phase during the quench. The current in situ approach offers thus a pertinent complementary method to the classical experimental petrology investigations.

The partitioning of barium and lead between silicate melts and aqueous fluids at high pressures and temperatures

Abstract The origin of subduction-related magmas is still a matter of debate in the Earth Sciences. These magmas are characterised by their distinctive trace element compositions compared to magmas from other tectonic settings, e.g. mid-ocean ridges or rifts. The distinct trace element composition of these magmas is generally attributed to alteration of the source region by a contaminating agent: either a silicate melt or a hydrous fluid, possibly chlorine-enriched. In this study, we have used μPIXE (proton induced X-ray emission) to analyse synthetic samples obtained from a micro-experimental petrology study that aims to determine the partitioning behaviour of two key elements, Ba and Pb, between silicate melt and both pure water and saline fluids. Our experiments were performed at high-pressure (>0.34–1.53 GPa) and high-temperature (697–1082 °C) in a hydrothermal diamond anvil cell, that was used as a transparent rapid quench autoclave. We observed that at high pressure and temperature, in the presence of pure water, Ba and Pb are not strongly fractionated into one phase or the other. The partition coefficient of Pb is ranging from 0.46 to 1.28. Results from one experiment performed at 0.83 GPa and 847 °C, in the presence of a saline fluid indicate that the presence of Cl induces strong fractionation of Pb and moderate fractionation of Ba both into the silicate melt. In addition, our data indicate that Cl is strongly partitioned into the fluid phase.

In situ study of magmatic processes: a new experimental approach

We present an internally heated autoclave, modified in order to permit in situ studies at pressure up to 0.5 GPa and temperature up to 1000 °C. It is equipped with transparent sapphire windows, allowing the observation of the whole experiment along the horizontal axis. In the experimental cell, the sample is held between two thick transparent plates of sapphire or diamond, placed in the furnace cylinder. The experimental volume is about 0.01 cm3. Video records are made during the whole experiment. This tool is developed mainly to study the magmatic processes, as the working pressures and temperatures are appropriate for subvolcanic magma reservoirs. However, other applications are possible, such as the study of subsolidus phase equilibria as we have used well-known phase transitions, such as the system of AgI, to calibrate the apparatus with respect to pressure and temperature. The principle of the apparatus is detailed. Applications are presented, such as studies of melt inclusions at pressure and temperature and an in situ study of magma degassing through the investigation of nucleation and growth processes of gas bubbles in a silicate melt during decompression.