Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
The equilibrium phase relations of a mafic durbachite (53 wt.% SiO2) from the TA (TM) ebi pluton, representative of the Variscan ultrapotassic magmatism of the Bohemian Massif (338-335 Ma), have been determined as a function of temperature (900-1,100A degrees C), pressure (100-200 MPa), and H2O activity (1.1-6.1 wt.% H2O in the melt). Two oxygen fugacity ranges were investigated: close to the Ni-NiO (NNO) buffer and 2.6 log unit above NNO buffer (a dagger NNO + 2.6). At 1,100A degrees C, olivine is the liquidus phase and co-crystallized with phlogopite and augite at 1,000A degrees C for the whole range of investigated pressure and water content in the melt. At 900A degrees C, the mineral assemblage consists of augite and phlogopite, whereas olivine is not stable. The stability field of both alkali feldspar and plagioclase is restricted to low pressure (100 MPa) at nearly water-saturated conditions (< 3-4 wt.% H2O) and T < 900A degrees C. A comparison between experimental products and natural minerals indicates that mafic durbachites have a near-liquidus assemblage of olivine, augite, Ti-rich phlogopite, apatite and zircon, followed by alkali feldspar and plagioclase, similar to the mineral assemblage of minette magma. Natural amphibole, diopside and orthopyroxene were not reproduced experimentally and probably result from sub-solidus reactions, whereas biotite re-equilibrated at low temperature. The crystallization sequence olivine followed by phlogopite and augite reproduces the sequence inferred in many mica-lamprophyre rocks. The similar fractionation trends observed for durbachites and minettes indicate that mafic durbachites are probably the plutonic equivalents of minettes and that K- and Mg-rich magmas in the Bohemian Massif may have been generated from partial melting of a phlogopite-clinopyroxene-bearing metasomatized peridotite. Experimental melt compositions also suggest that felsic durbachites can be generated by simple fractionation of a more mafic parent and mixing with mantle-derived components at mid crustal pressures.
ISSN: 0010-7999

hal-00618418
https://hal.archives-ouvertes.fr/hal-00618418
DOI : 10.1007/s00410-009-0430-5

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
The composition of S-rich apatite, of volatile-rich glass inclusions in apatite, and of interstitial glasses in alkaline xenoliths from the 1949 basanite eruption in La Palma has been investigated to constrain the partitioning of volatiles between apatite and alkali-rich melts. The xenoliths are interpreted as cumulates from alkaline La Palma magmas. Apatite contains up to 0.89 wt% SO3 (3560 ppm S), 0.31 wt% Cl, and 0.66 wt% Ce2O3. Sulfur is incorporated in apatite via several independent exchange reactions involving (P5+, Ca2+) vs. (S6+, Si4+, Na+, and Ce3+). The concentration of halogens in phonolitic to trachytic glasses ranges from 0.15 to 0.44 wt% for Cl and from < 0.07 to 0.65 wt% for F. The sulfur concentration in the glasses ranges from 0.06 to 0.23 wt% SO3 (sulfate-saturated systems). The chlorine partition coefficients (D (Cl) (apatite/glass) ) range from 0.4 to 1.3 (average D (Cl) (apatite/glass) = 0.8), in good agreement with the results of experimental data in mafic and rhyolitic system with low Cl concentrations. With increasing F in glass inclusions D (F) (apatite/glass) decreases from 35 to 3. However, most of our data display a high partition coefficient (similar to 30) close to D (F) (apatite/glass) determined experimentally in felsic rock. D (S) (apatite/glass) decreases from 9.1 to 2.9 with increasing SO3 in glass inclusions. The combination of natural and experimental data reveals that the S partition coefficient tends toward a value of 2 for high S content in the glass (> 0.2 wt% SO3). D (S) (apatite/glass) is only slightly dependent on the melt composition and can be expressed as: SO3 apatite (wt%) = 0.157 * ln SO3 glass (wt%) + 0.9834. The phonolitic compositions of glass inclusions in amphibole and hauyne are very similar to evolved melts erupted on La Palma. The lower sulfur content and the higher Cl content in the phonolitic melt compared to basaltic magmas erupted in La Palma suggest that during magma evolution the crystallization of hauyne and pyrrhotite probably buffered the sulfur content of the melt, whereas the evolution of Cl concentration reflects an incompatible behavior. Trachytic compositions similar to those of the (water-rich) glass inclusions analyzed in apatite and clinopyroxene are not found as erupted products. These compositions are interpreted to be formed by the reaction between water-rich phonolitic melt and peridotite wall-rock.
ISSN: 0010-7999

hal-00644803
https://hal.archives-ouvertes.fr/hal-00644803
DOI : 10.1007/s00410-011-0606-7

Éditeur(s) : HAL CCSD Mineralogical Society
Résumé : International audience
Almost all magmas contain sulfide- or sulfate-bearing phases. In most natural samples the sulfur-bearing phase is a sulfide, which typically is pyrrhotite (Fe1−xS) or pyrite (FeS2) although chalcopyrite (CuFeS2), pentlandite ((Fe,Ni)9S8), sphalerite (ZnS) or molybdenite (MoS2) may be present as well. Sulfate minerals are rare at magmatic conditions, and anhydrite (CaSO4) is the most common. Other magmatic SO4-bearing minerals include the sodalite group minerals (haüyne simplified formula: (Na,Ca)4-8(Al6Si6(O,S)24)(SO4,Cl)1-2), scapolite minerals (silvialite: (Ca,Na)4Al6Si6O24(SO4,CO3)), and S-bearing apatite (Ca5(PO4)3(F, Cl, OH)). Barite (BaSO4) has been mentioned in rare cases (Marchev 1991). Sulfur-bearing minerals usually constitute a negligible fraction of the mineral assemblage in magmatic rocks and thus can be classified as accessory minerals. The crystallization of sulfide, sulfate, and S-bearing minerals strongly depends on melt composition, temperature and pressure, and the S speciation in melt which, in turn, is strongly dependent on the prevailing oxygen fugacity (Baker and Moretti 2011, this volume; Wilke et al. 2011, this volume). Irrespectively of the low abundance of S-bearing minerals, the evolution of sulfur in magmas may be evaluated from the occurrence of sulfides and sulfates. These minerals are critical for estimating the activity of various sulfur-bearing species in magmas and can be used to constrain the oxygen fugacity and the S concentration in the melt (e.g., pre-eruptive sulfur concentration in melts). The presence of either sulfide or sulfate in silicate melt indicates that the predominant dissolved sulfur species in the melt are S2− or S6+, respectively. Typically one of these species is dominant, but there are conditions where S2− ...
ISSN: 1529-6466

hal-00644580
https://hal.archives-ouvertes.fr/hal-00644580
DOI : 10.2138/rmg.2011.73.10

Éditeur(s) : HAL CCSD Mineralogical Society of America
Résumé : International audience
Crystallization experiments were conducted at 200 MPa to determine the effect of small amounts of H2O on the liquidus temperature of basaltic melts in which plagioclase is the liquidus phase. The H2O concentrations in the quenched glasses, determined by infrared spectroscopy and Karl-Fischer titration, ranged from 0.02 to 4.2 wt% H2O. The dry liquidus temperature at 200 MPa was estimated from experiments at 1 atm (H2O-free) and from the known pressure dependence of plagioclase crystallization temperature. The effect of water (expressed as wt% H2O) on the plagioclase liquidus temperature is nonlinear and diminishing with increasing melt H2O concentrations. According to our new experimental data, it can be empirically predicted with following equation: Formula where CH2O is the water concentration in the melt (wt%), TDRY, and TWET are plagioclase crystallization temperatures in water-free and water-bearing systems, respectively. The relationship between CH2O and liquidus temperature worked out in this study is valid for a range of basaltic compositions, ranging from high-alumina basalts to basaltic andesites. The combination of the empirical equation predicting the liquidus depression of plagioclase with previous models predicting the olivine liquidus curve is useful to determine the liquidus temperature in various H2O-bearing basaltic systems in which either plagioclase or olivine is the liquidus phase.
ISSN: 0003-004X

hal-00757600
https://hal.archives-ouvertes.fr/hal-00757600
DOI : 10.2138/am.2012.4100