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REE- and HFSE-minerals in peralkaline syenites: crystal-chemistry and petrogenetic significance
No full textAl-in-amphibole barometry in calcalkaline magma: application to subvolcanic active systems
No full textAl-in-amphibole barometry of calcalkaline magma: assessment of active subvolcanic systems.
No full textCa-amphiboles (hornblende-hastingsite-pargasite solution) have been historically tested by many authors in order to discern their physic-chemical stability and the evolution of calcalkaline magmas at subduction-related systems. In general these amphiboles show direct proportional stability curves in P-T diagrams and, at high-T, their crystallization involves high fO2 and H2O contents of the melt.
Several mechanisms such as Edenite-reaction, Tschermak-reaction and Fe3+ = [6]Al exchange are inferred to drive the Al content of amphibole by the variation of T, P and fO2, respectively. Althought the degree of these exchanges have not been experimentally verified, the influence of T and fO2 on the Al-in-amphibole is considered to be negligible compared with pressure. In this framework, several Al-barometers were calibrated for subalkaline low-T granitoids (T < 800°C) and seem to fit well (±1 kbar) in the range of 2-12 kbar (i.e. Johnson & Rutherford, 1989; Thomas & Ernst, 1990). By contrast, these barometers tested with amphiboles synthesized at higher T, from calcalkaline basaltic andesite-rhyolite rocks (Johnson & Rutherford, 1989; Martel et al., 1999; Scaillet & Evans, 1999; Pichavant et al., 2002; Klimm et al., 2003; Rutherford & Devine, 2003) demonstrate to be inaccurate with errors up to ±2.1 kbar (±7.5 km of granitic-equivalent crust).
Using the above published data on Ca-amphiboles mainly synthesized by “crystallization methods” from calcalkaline rocks, we calibrated two new barometers suitable for basaltic andesite-andesite (BAAB) and dacite-rhyolite (DRB) series. BAAB is a 2nd order polynomial equation, i.e. P = 1.3701Al2 - 1.8457Al + 1.6116 (R2 = 0.95), valuable at high-T (825-1000°C) and fO2 (ΔNNO between +0.4 and +2.2) accounting for a maximum error of ±0.61 kbar (~2.2 km). The DRB calibrated at lower T (700-834°C) and between -0.2 and +2.0 ΔNNO, works even better (±0.49 kbar, ~1.8 km) and is characterized by a relation which accounts for the tetrahedral aluminium only (P = 3.3629[4]Al3 - 7.0947[4]Al2 + 3.8369[4]Al + 1.9063; R2 = 0.98). This is probably due to the removal of the fO2 dependence (i.e. Fe3+ = [6]Al) which should play an important role in the high-viscosity (dacite-rhyolite) magmas.
The BAAB applied to the amphiboles within the November 2002 calcalkaline products (early andesite pumice falls and late basaltic andesite-andesite lavas) of El Reventador volcano (Ecuador) allowed to constrain the magma chamber location between 8 km (pumice phenocrysts) and 11 km (lava poikilitic crystals). The poikilitic crystal depth fit well with the 10-11 km deep hypocenter earthquake swarm occurred ~1 month before the eruption, which should represent the mafic intrusion event at the bottom of the magma chamber (Ridolfi et al., submitted).
The same calculation on amphibole phenocrysts (i.e. Mg-hastingsite; Menna, 2000) within high-K calcalkaline andesites of the Petrazza pyroclastics (85-60 ka; Paleostromboli I, Italy) emphasizes crystallization depths of 12-14 km. This calculation fairly agree with the data on the early fluid inclusions within quartzite xenoliths of the Strombolicchio (200 ka) and Paleostromboli II (60 ka) extrusives, which suggest significant magma rest at depths of ~11 km (Vaggelli et al., 2003).
The Al-in-amphibole is strongly dependent on both P and composition of the magma and it is worth to note the use of inappropriate amphibole barometers could lead to blunders in locating magma chambers up to 9.5 kbar as shown by the pressure difference between BAAB and DRB calculations on the Stromboli amphiboles
REFERENCES
Johnson, M.C., Rutherford, M.J., 1989: Geology 17, 837-841.
Klimm, K., Holtz, F., Johannes, W., King, P. L., 2003: Precam. Res. 124, 327-341.
Martel, C., Pichavant, M., Holtz F., Scaillet, B., Bourdier, J.L., Traineau, H., 1999: J. Geoph. Res. 104, 29453-29470.
Menna, M., 2000: Unpublished Degree Thesis, Univ. Urbino, IT, pp. 109.
Pichavant, M., Martel, C., Bourdier, J. L., Scaillet, B., 2002: J. Geoph. Res. 107, B5, 2093, 10.1029/2001JB000315.
Ridolfi, F., Puerini, M., Renzulli, A., Menna, M., Toulkeridis, T.: J. Volc. Geoth. Res., submitted.
Rutherford, M.J., Devine, J.D., 2003: J. Petrol. 44, 1433-1454.
Scaillet, B., Evans, B.W., 1999. J. Petrol. 40, 381-411.
Thomas, W.M., Ernst W.G. 1990: Geochem. Soc., Spec. Publ. 2, 59-63.
Vaggelli, G., Francalanci, L., Ruggeri, G., Testi, S. 2003: Bull. Volcanol. 65, 385-404
The feeding system of calc-alkaline volcanoes as inferred from new amphibole-thermobarometric formulations: reconciling petrological and geophysical evidence
No full textStability and chemical equilibrium of amphibole in calc-alkaline magmas: an overview, new thermobarometric formulations and application to subduction-related volcanoes
No full textUnder review, accepted manuscrip
On the stability of magmatic cordierite and new thermobarometric equations for cordierite-saturated liquids
No full textIn this work, we have reviewed a large compositional dataset (571 analyses) for natural and experimental glasses to understand the physico-chemical andcompositional conditions of magmatic cordierite crystallization. Cordierite crystallizes in peraluminous liquids (A/CNK ≥1) at temperatures ≥750 °C, pressures ≤700 MPa, variable H2O activity (0.1–1.0) and relatively low fO2 conditions (≤NNO - 0.5). In addition to A/CNK ratio ≥1, a required condition for cordierite crystallization is a Si + Al cation value of the rhyolite liquid of 4 p8O (i.e. calculated on the 8 oxygen anhydrous basis), which is consistent with low Fe3+ contents and the absence or low content of non-bridging oxygens (NBO). This geochemical condition is strongly supported by the rare, if not unique, structure of cordierite where the tetrahedral framework is composed almost exclusively of Si and Al cations the sum of which is equal to 4 p8O [i.e. (Mg,Fe)8/9 Al16/9 Si20/9 O8], indicating that aluminium (and cordierite) saturation is limited by rhyolite liquids with Al = 4 - Si. Indeed, synthetic or natural systems with Al > 4 - Si always show metastable glass-in-glass separation or crystallization of refractory minerals such as corundum (Al16/3 O8) and aluminosilicates (Al16/5 Si8/5 O8). Multivariate regression analyses of literature data for experimental glasses coexisting with magmatic cordierite produced two empirical equations to independently calculate the T (±13 °C; ME, maximum error = 29 °C) and P (±16 %; ME% = 27 %) conditions of cordierite saturation. The greatest influence on the two equations is exerted by H2Omelt and Al concentrations, respectively. Testing of these equations with other thermobarometric constraints (e.g. feldspar-liquid, GASP, Grt–Bt and Grt–Crd equilibria) and thermodynamic models (NCKFMASHTO and NCKFMASH systems) was successfully performed for Crd-bearing rhyolites and residual enclaves from San Vincenzo (Tuscany, Italy), Morococala Field (Bolivia) and El Hoyazo (Spain). The reliability of each calculated P–T pair was graphically evaluated using the minimum and maximum P–T–H2O relationships for peraluminous rhyolite liquids modified after the metaluminous relationships in this work. Both P–T calculations and checking can be easily performed with the attached user-friendly spreadsheet (i.e. Crd-sat_TB)
Textures and crystallisation of agpaitic minerals of the syenitic clasts of São Miguel (Azores) and Tenerife (Canary).
No full textStability and chemical equilibrium of amphibole in calc-alkaline magmas: an overview, new thermobarometric formulations and application to subduction-related volcanoes
No full textIron-bearing chlor-fluorapatites in crustal xenoliths from the Stromboli volcano (Aeolian Islands, Southern Italy): an indicator of fluid processes during contact metamorphism
No full textRefinement and application of calcic amphibole thermobarometry for igneous rocks: Amp-TBX.xlsx
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