Alessandro Fabbrizio

New experimental data on biotite + magnetite + sanidine saturated phonolitic melts and application to the estimation of magmatic water fugacity

Abstract New experimental data are presented that allows the biotite-magnetite-sanidine equilibrium to be used for estimating water fugacity (fH₂O) in hydrous phonolitic magmas. It is also demonstrated that the partly ionic model gives the best estimate for the annite activity (aannite). Crystallization experiments were carried out on a representative sample of peralkaline, phonolitic obsidian of Montaña Blanca (MB) pumice deposit, Tenerife, Canary Islands. Experiments were performed from 720.810 °C and 50.250 MPa. Redox conditions were varied between NNO (nickel + nickel oxide) + 1 (±0.2) and FMQ (fayalite + magnetite + quartz). The majority of the experiments were done under H₂O saturation conditions (Pwater = Ptotal). Several experiments were done using a mixed H2O-CO2 fluid phase whereas in other experiments 10 or 20 wt% powdered alkali feldspar was added to the starting material. The pre-eruptive fH₂O of the Montaña Blanca magma is estimated at 676 ± 200 bars. The pre-eruptive fH₂O for the Fish Canyon tuff (753.2978 bars) and Bishop tuff rhyolite (1065.2440 bars) were also calculated, as well as fH₂O for metamorphic biotite from Au Sable Forks (≈ 130 bars). The results of this study suggest that this geohygrometer can be used in any magmatic system in which biotite-magnetite-sanidine is a stable assemblage.

Substitution mechanisms and implications for the estimate of water fugacity for Ti-rich phlogopite from Mt. Vulture, Potenza, Italy

Abstract The crystal chemistry of Ti-rich phlogopite from deposits of the oldest activity (~740 ka) of Mt. Vulture, Potenza, Italy, was investigated to identify the substitution mechanisms in Ti-rich phlogopite and to determine its potential as a geohygrometer. Substitution mechanisms were determined by electron probe microanalysis (EPMA), single-crystal X‑ray diffraction (SCXRD), Mössbauer spectroscopy, and micro-Fourier transform infrared (FTIR) spectroscopy. Magnetite and sanidine from the mica-rich host rocks were also analyzed. Use of the biotite-magnetite-sanidine geohygrometer in these volcanics (essentially trachytic-phonolitic ignimbrites) is exploited. All investigated phlogopite samples consist of the 1M polytype, with 5.3275(2) ≤ a ≤ 5.3635(4) Å, 9.2211(4) ≤ b ≤ 9.2958(8) Å, 10.1211(5) ≤ c ≤ 10.281(1) Å, and 99.980(3) ≤ β ≤ 100.097(2)°. Structure refinements in space group C2/m converged to 2.42 ≤ R1 ≤ 4.00% and 2.04 ≤ wR2 ≤ 4.50%. VIFe3+/Fetot from Mössbauer analyses ranged from 34(1)-89(1)%. The main bands in the OH-stretching region are 3709, 3682, and 3658 cm-1, and were assigned to 3Mg-OH--K-OH-, 3Mg-OH--K-O2-, and 2MgFe3+- OH--K-O2- local configurations, respectively. The overall crystal chemical features are compatible with the M3+ -Tschermak substitutions (VIM2+ + IVSi4+ ↔ VIM3+ + IVAl, with M3+ = Al,Fe3+), M3+-oxy [VIM2+ + (OH)- ↔ M3+ + (O)2- + ½H2↑] and Ti-oxy substitutions [VIM2+ + 2(OH)- → VITi4+ + 2(O)2- + H2↑]. The magnetite composition varies between Mt82Uv18 and Mt93Uv07, whereas the proportion of orthoclase with respect to albite plus anorthite is 0.77 ≤ XOr ≤ 0.82. A partly ionic model was used to estimate annite activity, starting from experimentally determined substitutions in phlogopite. The resulting water fugacity was an order of magnitude lower than that calculated using stoichiometry to determine the phlogopite formula. The results are consistent with the occurrence of a well-developed hydrothermal system that altered the magmatic oxygen fugacity conditions at Mt. Vulture.

Estimation of pre-eruptive magmatic water fugacity in the Phlegrean Fields, Naples, Italy

We estimated the water fugacity (fH(2)O) in the trachytic magma of Phlegrean Fields using the biotite-magnetite-sanidine equilibrium. We confirm that the partly ionic model is the most appropriate to estimate annite activity (a(annite)) for the experimental biotite. Crystallization experiments were carried out on a representative sample of trachytic Breccia Museo eruption, Naples, Italy. Experiments were performed in the temperature and pressure range or 725-870 degrees C and 50-200 MPa, and redox conditions ranging from NNO (nickel- nickel oxide) + 1 (+/- 0.2) to FMQ (fayalite + magnetite + quartz). Most experiments were done under conditions of H2O saturation (P-water = P-total). Few experiments were done using a mixed H2O-CO2 fluid phase. The pre-eruptive fH(2)O of the Phlegrean Fields magma is estimated at 735 bars, which compares well with available melt inclusion constraints. Our results confirm that this geohygrometer can be used in any magmatic systems bearing the worked assemblage.

Sulfur and REE zoning in apatite: the example of the Colli Albani magmatic system

We investigate the distribution of major and trace elements in apatite crystals hosted in granular alkaline rocks composed mainly of leucite and clinopyroxene, representative of the hypabyssal crystallization of a magma body in the Quaternary ultra-potassic Colli Albani Volcanic District (CAVD), which was emplaced into thick limestone units along the Tyrrhenian margin of Italy. Results show that the analyzed crystals are the SrO-richest (up to 4.6 wt%) fluorapatite (F =2.6-3.7 wt%) of the Italian alkaline rocks. The strontium enrichment is caused by the lack of other Sr-compatible mineral phases, such as plagioclase, alkali feldspar and melilite, in these leucite- and clinopyroxene-bearing rocks. The studied samples show core-rim zoning with rims enriched in Si, S, and REE whereas the cores are enriched in Ca and P. The LREE-oxides contents of apatite, reaching 4.2 wt%, represent more than 95% of the total REE budget; SiO2 contents range from 1.3 to 3.6 wt%, and SO3 concentrations between 0.6 and 1.4 wt%.