Ca-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.

Al-in-amphibole barometry of calcalkaline magma: assessment of active subvolcanic systems

RIDOLFI, FILIPPO;RENZULLI, ALBERTO
2007

Abstract

Ca-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.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11576/2539777
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