Evolution of product phase assemblages during thermal decomposition of muscovite under strong disequilibrium conditions

http://www.minsocam.org/MSA/AmMin/TOC/Abstracts/2006_Abstracts/FM06_Abstracts/Devineau_p413_06.pdfWe investigated the thermal decomposition of muscovite in natural granite powders heated to 1175°C for durations from 5 min to 68 h, at 1 bar, paying special attention to the early stages of decomposition. This study shows that muscovite is completely transformed after 5 min. Muscovite pseudomorphs consist of glass, mullite, and Al-rich oxides. For short durations (5 and 40 min), the Al-rich phase was identiÞ ed by XRD, electron diffraction, and TEM microanalysis as γ-Al2O3 containing 4.8 wt% FeO (total Fe), probably a few weight percents of MgO, and possibly up to 10 wt% SiO2. Faint superstructure spots and diffuse streaks observed in electron-diffraction patterns suggest vacancy or trace elements ordering in the γ-Al2O3 defect spinel structure. γ-Al2O3 displays an unexpected acicular morphology, elongated along three directions at 120° in the basal (001)musc planes and parallel to lateral faces of the former muscovite. Mullite forms rods elongated in the basal (001)musc planes along a direction at 90° from one set of γ-Al2O3 needles. The γ-Al2O3 structure appears to be a metastable phase that is replaced by corundum for longer durations

Textural and geochemical analysis of a pumice polisher with grooves from the Magdalenian site of Duruthy (Sorde, Landes, France)

Magdalenian archaeological sites often contain grooved polishers used to polish other implements (for example, Capitan and Peyrony 1928). These objects are generally made from sandstone but a few are made from pumice. Bibliographic research revealed that only four sites have yielded pumice stone polishers (fig. 1, fig. 2, tab. 1). These are the sites of Gourdan (Piette 1873), Isturitz (Passemard 1944), Aitzbitarte IV (Barandiaran 1965) and Arancou (Chauchat et al. 1999). Several polishers were found in the Duruthy rock-shelter, Sorde-L'abbaye, Landes. Most of these polishers are in sandstone (fig. 3) but one of them is different and has the appearance of pumice (fig. 4). The textural study and geochemical analysis (fig. 5, fig. 6, tab. 2) of this polisher indicate that it was made from rhyolitic pumice. The geochemical characteristics of this material indicate that it is not a product of French volcanism. The fact that all the sites where pumice polishers were found are located near the Atlantic Ocean suggests that floated pumices may have been collected along the ocean shores. Such rhyolitic pumices could have been produced during the eruption of a volcano on an island in the Atlantic Ocean. Although we have not yet identified the volcano responsible for this eruption, this remains the most likely hypothesis. In this respect, these artefacts offer a new approach to the study of the exploitation of the littoral zone by the last Palaeolithic hunters in southwest France and northern Spain

The petrology of a hazardous volcano: Calbuco (Central Southern VolcanicZone, Chile)

peer reviewedThe recurrent explosive eruptions of Calbuco (Andean Southern Volcanic Zone (SVZ)) threat a rapidly expanding tour- istic and economic region of Chile. Providing tighter constraints on its magmatic system is therefore important for better monitoring its activity. Calbuco is also distinguished by hornblende-bearing assemblages that contrast with the anhydrous parageneses of most Central SVZ volcanoes. Here we build on previous work to propose a detailed petrological model of the magmatic system beneath Calbuco. Geochemical data acquired on a hundred samples collected in the four units of the volcano show no secular compositional change indicating a steady magmatic system since ~ 300 ka. A tholeiitic Al2O3-rich (20 wt. %) basalt (Mg# = 0.59) is the parent magma of a differentiation trend straddling the tholeiitic/calc-alkaline fields and displaying a narrow compositional Daly gap. Amphibole crystallization was enabled by the higher H2O content of the basalt (3–3.5 wt. % H2O at 50 wt. % SiO2) compared to neighboring volcanoes. This characteristic is inherited from the primary mantle melt and possibly results from a lower degree of partial melting induced by the mantle wedge thermal structure. Although macrocrysts are not all in chemical equilibrium with their host rocks and were thus presumably unlocked from the zoned crystal mush and transported in the carrier melt, the bulk-rock trend follows both experimental liquid lines of descent and the chemical trend of calculated melts in equilibrium with amphibole (AEMs). These contradictory observations can be reconciled if minerals are transported in near cotectic proportions. The AEMs overlap the Daly gap revealing that the missing liquid compositions were present in the storage region. Geothermobarometers all indicate that the chemical diversity from basalt to dacite was acquired at a shallow depth (210–460 MPa). We suggest that differentiation from the primary magma to the parental basalt took place either in the same storage region or at the MOHO.CDR J.00066.14, PDR T.0079.18, Odysseus grant (ON

Calbuco (Central Southern Volcanic Zone, Chile): a hazardous volcano

CDR J.00066.14, PDR T.0079.18, Odysseus grant (ON

Growth Kinetics and Distribution of Trace Elements in Precious Corals

The concentration and spatial distribution of major (Ca, Mg) and trace elements (Na, Sr, S, Li, Ba, Pb, and U) in different Corallium skeletons (C. rubrum, C. japonicum, C. elatius, C. konojoi) have been studied by electron microprobe (EMP) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). EMP data show positive Na-Mg and negative Na-S and Mg-S correlations in all skeletons. LA-ICPMS data display additional Sr-Mg, Li-Mg, and U-Mg positive correlations. Medullar zones in the skeletons, corresponding to fast growing zones, are systematically richer in Mg, Na, Sr, Li, and U and poorer in S than the surrounding slow growing zones. These spatial distributions are mostly interpreted in terms of growth kinetics combined with steric effects influencing the incorporation of impurities in biogenic calcites. This interpretation is in agreement with available experimental data on kinetic effects on the incorporation of elements in calcite. At a different scale, annual growth rings in annular slow growing zones show oscillations in Mg, Na, Sr, and S. These chemical oscillations probably result from growth rate variations: fast growth would produce rings enriched in Mg, Sr, and Na, while slow growth would produce rings enriched in Ca, S and organic matter. From previous studies in C. rubrum, the Mg-rich rings would develop during the spring to fall period while the S-rich rings would form immediately after (late fall and winter). Analytical traverses performed in annular zones of different Corallium skeletons indicate that Mg, Na, Sr, Li, and U decrease from core to rim. This observation indicates that radial growth rate decreases as the colony gets older. Contrary to Mg, Na, Sr, Li, S, and U, barium and lead concentrations are identical in medullar and annular zones and appear independent of growth kinetics. Thus, concentrations in Corallium skeletons could provide indications on Ba and Pb contents in the oceans. Barium and lead concentrations are higher in Mediterranean than in Pacific precious corals, these two elements can be used to discriminate C. rubrum from C. japonicum, and contribute enforcing regulations on the trade of precious corals

Experimental melting of phlogopite-peridotite in the garnet stability field

International audienceMelting experiments have been performed at 3 GPa, between 1150 and 1450 °C, on a phlogopite-peridotite source in the garnet stability field. We succeeded to extract and determine the melt compositions of both phlogopite-bearing lherzolite and harzburgite from low to high degrees of melting (ϕ = 0.008–0.256). Accounting for the presence of small amounts of F in the mantle, we determined that phlogopite coexists with melt >150 °C above the solidus position (1150–1200 °C). Fluorine content of phlogopite continuously increases during partial melting from 0.2 to 0.9 wt% between 1000 and 1150 °C and 0.5 to 0.6 wt% between 1150 and 1300 °C at 1 and 3 GPa, respectively. The phlogopite continuous breakdown in the lherzolite follows the reaction: 0.59 phlogopite + 0.52 clinopyroxene + 0.18 garnet = 0.06 olivine + 0.23 orthopyroxene + 1.00 melt. In the phlogopite-harzburgite, the reaction is: 0.93 phlogopite + 0.46 garnet = 0.25 olivine + 0.14 orthopyroxene + 1.00 melt. Melts from phlogopite-peridotite sources at 3 GPa are silica-undersaturated and are foiditic to trachybasaltic in composition from very low (0.8 wt%) to high (25.6 wt%) degrees of melting. As observed at 1 GPa, the potassium content of primary mantle melts is buffered by the presence of phlogopite, but the buffering values are higher, from 6.0 to 8.0 wt% depending on the source fertility. We finally show that phlogopite garnet-peridotite melts are very close to the composition of the most primitive post-collisional lavas described worldwide

On the interpretation of closed system mineral dissolution experiments: comment on ''Mechanism of kaolinite dissolution at room temperature and pressure part II: kinetic study'' by Huertas et al. (1999)

International audienceHuertas et al. (1999) reported the results of a series of constant-pH kaolinite dissolution experiments performed at 25°C in closed system reactors. The authors of this study chose to interpret these experiments assuming that the dissolution rates of kaolinite in their reactors, after a certain initial elapsed time, were time independent. The purposes of this comment are to (a) recall evidence available in the literature demonstrating that constant-pH kaolinite dissolution does not attain a constant rate in closed system reactors; (b) present a simple equation describing solution composition evolution during closed system multioxide mineral or glass dissolution experiments; and (c) demonstrate that Huertas et al.'s (1999) data are consistent with kaolinite rates that are proportional to cAlnAl where cAl refers to the concentration of aqueous aluminum, and nAl denotes a constant

Apatite solubility in carbonatitic liquids and trace element partitioning between apatite and carbonatite at high pressure

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Trace element partitioning during partial melting of carbonated eclogites

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