Didier Laporte - Academia.edu (original) (raw)

Papers by Didier Laporte

Geochemical Perspectives Letters, 2020

We present a novel application of Raman microtomography for quantitative characterisation of glas... more We present a novel application of Raman microtomography for quantitative characterisation of glass inclusion-hosted bubbles, which allows for the simultaneous identification and volumetric quantification of mineral and fluid phases filling the bubble. The combination of Raman microtomography with synchrotron XRF mapping and scanning electron microscopy provides a complete compositional and textural characterisation of the bubble. In the studied samples, minerals are systematically present on the walls of the bubbles: dominantly carbonates in samples from continental intraplate and hotspot volcanic provinces, and sulfates in the sample from subductionrelated settings. Along with fluid CO 2 , carbonates sequester 65 to 84 % of the CO 2 originally dissolved in the melt, while 18 to 60 % of the sulfur contained in the inclusion is stored in sulfides and/or sulfates. Thus, the total melt inclusion CO 2 and S contents can be underestimated (by up to ∼ 40 % and 60 %, respectively) if minerals in the bubbles are neglected. This study highlights the importance of 3D mapping of shrinkage bubbles hosted in glass inclusions for a better assessment of the bulk pre-eruptive contents of volatiles in magmas.

The mantle plume that generated the Deccan flood basalts 65 Ma ago, the Chagos-Maldive-Laccadive ... more The mantle plume that generated the Deccan flood basalts 65 Ma ago, the Chagos-Maldive-Laccadive Ridge and the Mascarene Plateau is presently located below the Reunion island (Indian Ocean). To understand the long-term variations in the geochemical dynamics of the Reunion plume, we studied lava samples from Mauritius and Reunion (Piton des Neiges and Piton de la Fournaise volcanoes) islands, which

In order to characterize the effect of ascent rate on the kinetics of bubble nucleation in a rhyo... more In order to characterize the effect of ascent rate on the kinetics of bubble nucleation in a rhyolitic magma, we performed three series of experiments decompressed at 1000, 167, or 27.8 kPa/s. The experiments were carried out in an externally heated pressure vessel at 800°C; the starting material was a crystal-free and bubble-free rhyolitic glass containing 7.0 wt.% H2O. In the three decompression series, homogeneous bubble nucleation began at 90 MPa, that is, ≡ 150 MPa below the water saturation pressure of the silicate liquid, 240 MPa. After a short nucleation event, the nucleation rate dropped and the bubble number density N reached a stationary value that was strongly sensitive to decompression rate: 6.8 mm 3 at 27.8 kPa/s, 470 mm 3 at 167 kPa/s, and 5800 mm 3 at 1000 kPa/s. This behaviour was dictated by a competition between nucleation and diffusive bubble growth, which depleted in water the surrounding liquid and so reduced the degree of volatile supersaturation. With increas...

L'intrusion d'Ile Rousse est une association intime de granitoides aux caracteristiques p... more L'intrusion d'Ile Rousse est une association intime de granitoides aux caracteristiques plus ou moins fortement contrastees, juxtaposes en lames submeridiennes a fort pendage. On y distingue des granitoides calcoalcalins magnesio-potassiques et des granitoides n'appartenant pas a l'association magnesiopotassiques dont les mieux types sont les granodivrites de Corbara. Analyse statistique de la sous-fabrique des megacristaux de Feldspath potassique accompagnee d'une modelisation mathematique

Contributions to Mineralogy and Petrology

The morphology of crystals in magmas strongly depends on the temperature regime of the system, in... more The morphology of crystals in magmas strongly depends on the temperature regime of the system, in particular the degree of undercooling and the cooling rate. To simulate low degrees of undercooling, we developed a new experimental setup based on thermal migration, in which large cylinders of forsterite (single crystals) immersed in haplobasaltic melt were subjected to a temperature gradient. As forsterite solubility is sensitive to temperature, the forsterite on the high-temperature side undergoes dissolution and the dissolved components are transported toward the low-temperature side where a layer of newly grown forsterite forms (up to 340 μm thick after 101 h). A striking feature is that the precipitation process does not produce a planar front of forsterite advancing at the expense of liquid: the growth front shows a fingered outline in planar section, with solid lobes separated by glass tubes that are perpendicular to the growth front. We ascribe this texture to cellular growth, a type of growth that had not been experimentally produced so far in silicate systems. We find that the development of cellular growth requires low degrees of undercooling (a few °C) and large crystal-liquid interfaces (~ 1 mm across or more), and that it occurs at a growth rate of the order of 10 −9 m/s. We found natural occurrences of cellular growth on the rims of olivines from basanites, but otherwise cellular textures are poorly documented in natural volcanic rocks. Melt inclusions were produced in our experiments, showing that they can form in olivine at relatively slow rates of growth (10 −9 m/s or lower).

Chemical Geology

Abstract The behavior of argon in H2O-CO2 bearing basaltic melts was experimentally investigated ... more Abstract The behavior of argon in H2O-CO2 bearing basaltic melts was experimentally investigated in the pressure range 1–5 GPa and at temperatures between 1350 and 1600 °C. Our experimental data simulate the partitioning of argon between an Ar-H2O-CO2 bearing fluid and a silicate melt occurring during magma ascent and degassing. The experimental results show several features: - At a pressure of 1 GPa the variation of H2O from 0.35 to about 2 wt.% (at constant CO2 content) does not induce any systematic variation of argon dissolved in basaltic melt. For higher water contents (> 2 wt.%) we observe a positive effect of water on argon solubility. - In the range of 3–5 GPa, we did not observe any systematic variation of the argon content in the basalt melt for water concentration from 0.35 to 5.3 wt.% and at CO2 content 0.5 wt.%, argon concentration in basalt melt decreases from about 3800 to 2400 ppm when the CO2 content increases from 0.5 to 0.8 wt.%. - At all pressures investigated in the present study, a negligible effect of CO2 for concentration - More importantly (despite all these variations) it seems that the effect of pressure in the range of 1 to 5 GPa is the dominant parameter on argon solubility in basaltic melt. Pressure has a positive effect on argon incorporation in the H2O-CO2 bearing basaltic melt reaching at 3 GPa a concentration of ~ 0.38 wt.%. This maximum of ~ 0.38 wt.% corresponds to 6.8 × 10− 5 cm3 g− 1 bar− 1 at standard temperature and pressure, a value of the same order of magnitude as that derived from volatiles free basaltic melts equilibrated with argon. The experimental data can be well described by a thermodynamic model assuming mixing of volatile species and oxygen in the silicate melt. The results can be applied for a better understanding of the fractionation noble gas/noble gas and noble gas/CO2 occurring in degassing processes during magma ascent.

Physics and Chemistry of Minerals

The structural parameters and the thermal behavior of a complete series of Ca–Mg carbonates synth... more The structural parameters and the thermal behavior of a complete series of Ca–Mg carbonates synthesized at high pressure and temperature (1–1.5 GPa, 1273–1373 K) in the range 0–50 mol% MgCO3 have been investigated by in situ powder synchrotron high-resolution X-ray diffraction at ambient and up to 1073 K under self-controlled CO2 partial pressure. The crystal structures are disordered Mg calcite in the range 1–41 mol% MgCO3, and Ca dolomite at 49 mol% MgCO3. New calibration curves of the cell parameters for the Mg content and thermal expansion from ambient to 1073 K are given. Short-range structural effects of cation substitution and ordering and their thermal behavior as a function of Mg content were identified from three sets of data: the peak broadening, the cell parameter strains and the Raman band enlargements. Both intra- and inter-crystalline levels of compositional heterogeneity are identified and allow splitting the Mg calcites into two groups: low- and high-Mg calcites. The low-Mg calcites (up to 22 mol% MgCO3) are homogeneous in Mg content with short-range ordering. High-Mg calcite (up to 41 mol% MgCO3) displays domains with different local ordering configurations and similar or slightly different Mg contents, and to which is added a compositional variation between crystals, as determined by EMP, of the order of ± 0.8 mol% MgCO3. The cation ordering in Ca-rich dolomites similarly occurs in high-Mg synthetic calcites. The role of (CO3)2− group ordering is shown to be an important factor in the formation of Ca–Mg carbonates.

European Journal of Mineralogy

American Mineralogist, 2016

Agu Fall Meeting Abstracts, Dec 1, 2010

Contributions to Mineralogy and Petrology, 2016

Contributions to Mineralogy and Petrology, Feb 28, 2009

Despite the growing interest for Li and B as geochemical tracers, especially for material transfe... more Despite the growing interest for Li and B as geochemical tracers, especially for material transfer from subducting slabs to overlying peridotites, little is known about the behaviour of these two elements during partial melting of mantle sources. In particular, mineral/melt partition coefficients for B and to a lesser extent Li are still a matter of debate. In this work, we re-equilibrated a synthetic basalt doped with *10 ppm B and *6 ppm Li with an olivine powder from a spinel lherzolite xenolith at 1 GPa-1,330°C, and we analyzed Li and B in the run products by secondary ion mass spectrometry (SIMS). In our experiment, B behaved as a highly incompatible element with mineral/melt partition coefficients of the order of 10-2 (D ol/melt = 0.008 (0.004-0.013); D opx/melt = 0.024 (0.015-0.033); D cpx/melt = 0.041 (0.021-0.061)), and Li as a moderately incompatible element (D ol/melt = 0.427 (0.418-0.436); D opx/melt = 0.211 (0.167-0.256); D cpx/melt = 0.246 (0.229-0.264)). Our partition coefficients for Li are in good agreement with previous determinations. In the case of B, our partition coefficients are equal within error to those reported by Brenan et al. (1998) for all the mineral phases analyzed, but are lower than other coefficients from literature for some of the phases (up to 5 times for cpx). Our measurements complement the data set of Ds for modelling partial melting of the upper mantle and basalt generation, and confirm that, in this context, B is more incompatible than previously anticipated. Keywords Lithium Á Boron Á Partition coefficients Á Upper mantle conditions Á SIMS Communicated by T.L. Grove.

ABSTRACT The transitions between densely welded and nonwelded ignimbrite are commonly abrupt, occ... more ABSTRACT The transitions between densely welded and nonwelded ignimbrite are commonly abrupt, occurring over a few centimeters to decimeters, and suggest that the welding process is rapid when the correct conditions are met. To explore the conditions of welding, we conducted experiments on a moderately well-sorted, vitric, natural ash finer than 500 microns, viz., from the nonwelded base of the Rattlesnake Tuff. Experiments were conducted in sealed gold capsules in an externally heated pressure vessel. Most runs were at 200 bar, 600oC and with water contents varying from 0.1 to 1.6 wt. % (the latter exceeds water saturation). Run times ranged from 1 hour to 1 month Densities of runs with 0.4 wt. % water increase with run time from 1.5 kg/m3 after 2 hours to 2.1 after 2 days, simulating the density range of natural tuff from "incipiently welded" to "partially welded with fiamme". About half of the density increase occurs between 1 and 2 days, consistent with the field observation that the degree of welding can change abruptly. From scanning electron imagery, we found that run products with densities of "incipiently welded" tuff lack fused grains and are slightly coherent owing to vapor-phase precipitates. After 4 hours, some grain contacts are fused and the charge has textural similarities with "partially welded tuff with pumice". Based on petrographic analysis of collapsed bubble shards and from alignment of platy shards, the flattening in the run products is greater than that in the natural samples, probably reflecting the greater pressure conditions of the experiments; the Rattlesnake Tuff is typically only a few tens of meters thick. While addition of small amounts of water promoted the degree of welding in charges, near and over-saturated experiments did not weld, presumably owing to counteraction of pore pressure and thus illustrating the importance of permeability in promoting welding. We conclude that there are three main contributions to what is termed welding: a mechanical densification that causes most of the loss of pore space, a sintering in which grain boundaries fuse, and precipitation of vapor phase minerals.

Journal of Geophysical Research, 2002

Rapid decompression experiments were performed to study homogeneous bubble nucleation in a crysta... more Rapid decompression experiments were performed to study homogeneous bubble nucleation in a crystal-free rhyolitic liquid at 800°C. Bubble nucleation was produced by lowering the pressure at 1-10 MPa s À1 from an initial value between 200 and 295 MPa to a final value below the volatile saturation pressure P Sat. Six volatile compositions with 4.1-7.7 wt % H 2 O and 10-1200 ppm CO 2 were investigated. For each composition we determined the critical pressure P HoN below which homogeneous nucleation can proceed. The samples quenched below P HoN showed a nucleated core with a large number of uniformly spaced bubbles. With decreasing pressure, bubble number densities increased from <10 11 m À3 (for samples quenched just below P HoN) to >10 15 m À3. The degree of supersaturation required for homogeneous nucleation, ÁP HoN (= P Sat À P HoN), increased with decreasing H 2 O content or increasing CO 2 content. Huge values of ÁP HoN , !135 to 310 MPa, were measured in the H 2 O-poor compositions (4.1-4.6 wt % H 2 O; 50-1100 ppm CO 2); much lower values from %60 to 160 MPa were obtained in the H 2 O-rich compositions (7.0-7.7 wt % H 2 O; 10-630 ppm CO 2). The high ÁP HoN in liquids with 4-5 wt % H 2 O should result in the buildup of large degrees of supersaturation during magma ascent, a very late nucleation event, and a rapid (explosive) vesiculation. By contrast, rhyolitic liquids with much larger water contents have higher saturation pressures and much lower ÁP HoN : bubble nucleation may therefore occur at depth in the volcanic conduit favoring a subsequent near-equilibrium degassing.

Contributions to Mineralogy and Petrology, Jun 16, 2008

We performed decompression experiments to simulate the ascent of a phenocryst-bearing rhyolitic m... more We performed decompression experiments to simulate the ascent of a phenocryst-bearing rhyolitic magma in a volcanic conduit. The starting materials were bubble-free rhyolites water-saturated at 200 MPa-800°C under oxidizing conditions: they contained 6.0 wt% dissolved H 2 O and a dense population of hematite crystals (8.7 ± 2 9 10 5 mm-3). Pressure was decreased from the saturation value to a final value ranging from 99 to 20 MPa, at constant temperature (800°C); the rate of decompression was either 1,000 or 27.8 kPa/s. In all experiments, we observed a single event of heterogeneous bubble nucleation beginning at a pressure P N equal to 63 ± 3 MPa in the 1,000 kPa/s series, and to 69 ± 1 MPa in the 27.8 kPa/s series. Below P N , the degree of water supersaturation in the liquid rapidly decreased to a few 0.1 wt%, the nucleation rate dropped, and the bubble number density (BND) stabilized to a value strongly sensitive to decompression rate: 80 mm-3 at 27.8 kPa/s vs.