Bjoern O. Mysen

Anionic Constitution of 1-Atmosphere Silicate Melts: Implications for the Structure of Igneous Melts

A structural model is proposed for the polymeric units in silicate melts quenched at 1 atmosphere. The anionic units that have been identified by the use of Raman spectroscopy are SiO44– monomers, Si2O76– dimers, SiO32– chains or rings, Si2O52– sheets, and SiO2 three-dimensional units. The coexisting anionic species are related to specific ranges of the ratio of nonbridging oxygens to tetrahedrally coordinated cations (NBO/Si). In melts with 2.0 < NBO/Si < ∼ 4.0, the equilibrium is of the type [See equation in the PDF file]. In melts with NBO/Si ∼ 1.0 to 2.0, the equilibrium anionic species are given by [See equation in the PDF file]. In alkali-silicate melts with NBO/Si <~ 1.3 and in aluminosilicate melts with NBO/T < 1.0, where T is (Si + Al), the anionic species in equilibrium are given by [See equation in the PDF file]. In multicomponent melts with compositions corresponding to those of the major igneous rocks, the anionic species are TO2, T2O5, T2O6, and TO4, and the coexisting polymeric units are determined by the second and third of these disproportionation reactions.

Source and H2O content of high-MgO magmas in island arc settings: an experimental study of a primitive calc-alkaline basalt from St. Vincent, lesser antilles arc

Liquidus phase relationships have been determined for a high-MgO basalt (STV301: MgO=12.5 wt%, Ni=250 ppm, Cr=728 ppm) from Black Point, St Vincent (Lesser Antilles arc). Piston-cylinder experiments were conducted between 7.5 and 20 kbar under both hydrous and oxidizing conditions. AuPd capsules were used as containers. Compositions of supraliquidus glasses and mass-balance calculations show that Fe loss is < 10% in the majority of experiments. Two series of water concentrations in melt were investigated: (i) 1.5 wt% and (ii) 4.5 wt% H2O, as determined by SIMS analyses on quenched glasses and with the by difference technique. The Fe3+/Fe2+ partitioning between Cr-Al spinel and melt and olivine-spinel equilibria show that oxidizing fO2 were imposed (NNO + 1.5 for the 1.5 wt% H2O series, NNO + 2.3 for the 4.5 wt% H2O series). For both series of water concentrations, the liquid is multiply-saturated with a spinel lherzolite phase assemblage on its liquidus, at 1235°C, 11.5 kbar (1.5 wt% H2O) and 1185°C, 16 kbar (4.5 wt% H2O). Liquidus phases are homogeneous and comparable to typical mantle compositions. Mineral-melt partition coefficients are generally identical to values under anhydrous conditions. The modal proportion cpx/opx on the liquidus decreases from the 1.5 wt% to the 4.5 wt% H2O series. The experimental data are consistent with STV301 being a product of partial melting of lherzolitic mantle. Conditions of multiple saturation progressively evolve toward lower temperatures and higher pressures with increasing melt H2O concentration. Phase equilibria constraints, i.e., the necessity of preserving the mantle signature seen in high-MgO and picritic arc basalts, and glass inclusion data suggest that STV301 was extracted relatively dry ( 2 wt% H2O) from its mantle source. However, not all primary arc basalts are extracted under similarly dry conditions because more hydrous melts will crystallize during ascent and will not be present unmodified at the surface. From degrees of melting calculated from experiments on KLB-1, extraction of a 12.5 wt% MgO melt with 2 wt% H2O would require a H2O concentration of 0.3 wt% in the sub-arc mantle. For mantle sources fluxed with a slab-derived hydrous component, extracted melts may contain up to 5.5 wt% H2O.

Enthalpies of formation of CaAl4O7 and CaAl12O19 (hibonite) by high temperature, alkali borate solution calorimetry

Abstract The enthalpies of formation of CaAl4O7 and CaAl12O19 (hibonite), by alkali borate solution calorimetry at 1063 K are: ΔH 0 f,CaAl 4 O 7 , (1063 K; from oxides ) = −[25.6 ± 4.7] kj g.f.w. and ΔH 0 f,CaAl 12 O 19 (1063 K; from oxides ) = −[33.0 ± 9.7] kj g.f.w. Using experimental enthalpy data for CaAl4O7 and estimated values for CaAl12O19, the standard enthalpies of formation of these compounds from the elements at 298 K are: ΔH 0 f,CaAl 4 O 7 (298 K; from elements ) = −4,007 ± 5.2 kj g.f.w. and ΔH 0 f,CaAl 12 O 19 (298 K; from elements ) = −10,722 ± 12 kj g.f.w. Comparison with high-temperature galvanic cell data for the Gibbs energy of formation of CaAl12O19 allows a calculation of the standard entropy of hibonite, S 0 hibonite (1100 K) = 1280.2 J K g.f.w. This value is only about 2% larger than the oxide sum. Hence it is inferred that the standard entropy of hibonite at 298 K is probably only slightly larger than the oxide sum value of 343.7 J K g.f.w. The present data were used to extrapolate Kumar and Kays (1985) data for the Gibbs energies of formation of hibonite and CaAl4O7 to the temperature range 1500–1700 K. These data were then used in equilibrium thermodynamic calculations of the condensation of a gas of solar composition. Contrary to calculations of Kornacki and Fegley (1984), who used thermodynamic data of Allibert et al. (1981) for calcium aluminates, our results show no stability field for CaAl4O7 in a gas of solar composition at 10−3 to 10−5 atm total pressure. At 10−3 atm pressure, hibonite forms by reaction of corundum with the gas at 1725 K, begins to react with the gas to form gehlenite at 1607 K and disappears completely in a reaction to form spinel at 1494 K. The absence of CaAl4O7 from hibonite-, spinel-rich inclusions in carbonaceous chondrites cannot be used as an argument against a condensation origin for these objects.

The effect of temperature and bulk composition on the solution mechanism of phosphorus in peraluminous haplogranitic magma

Abstract Solution mechanisms of P in peraluminous glasses and melts in the system CaO-Na2O-K2O-Al2O3- SiO2-P2O5 have been examined with in-situ microRaman spectroscopy from ambient temperature to near 1200 °C. The principal aim was to examine the relative stabilities of phosphate complexes as functions of P content, peraluminosity, and temperature. Increasing peraluminosity was accomplished by increasing the proportions of Al3+ and Ca2+ of constant SiO2 content. The molar ratio Al2O3/ (CaO+Na2O+K2O) (A/CNK) ranged from ~1 to ~1.3. In all compositions, P5+ is bonded to Al3+ to form AlPO4 complexes. In addition, there is evidence for pyrophosphate complexing (P2O7). In melts with the highest (Ca+Na+K)/P, there is probably also a small fraction of orthophosphate complexes present. The relative importance of AlPO4-like complexes is correlated positively with peraluminosity (A/CNK), P2O5 content, and increasing temperature at temperatures above that of the glass transition. These structural relationships among phosphate complexes are coupled with decreasing polymerization of the aluminosilicate melts.