The aim of the work presented in this paper is to acquire experimental knowledge of the growth kinetics of alkali feldspar in Phlegraean Fields trachytic melts, with emphasis on magmas erupted in the Monte Nuovo event of 1538, to constrain system time scales of magmatic processes. Data on alkali feldspar crystallization kinetics in trachytic melts are totally absent from the available scientific literature. Starting from a trachytic composition coming from Phlegrean Fields, cooling and decompression experiments were performed at different condition of T and P and with different duration (crystallization time): experimental pressure varies between 30 and 200 MPa, temperature between 750 and 860°C, time between 7200 and 57600 seconds and fO2 = NNO + 0.8 (Di Matteo et al., 2004). For every series of decompression experiment with constant ∆T, we always performed an experiment where there was no decompression, to allow a comparison between the growth rate in decompressed experiments with the growth rate in the constant PH2O conditions. To obtain the dimensions of alkali feldspar crystals backscattered images were collected with the Scanning Electron Microscope (SEM) and were used to measure the length and the width of the crystals with Photoshop 7.0. A reasonable approximation for growth rate is to average the half-length of the ten longest alkali feldspar crystals in any experiment, as employed by Fenn (1977), Hammer and Rutherford (2002) and Couch (2002). The ratio between half-length and crystallization time (that is the duration of the experiment) is used to calculate the growth rate G. The crystal growth rate varies for cooling experiments from 3,6x10-8 to 3,9x10-7 cm/s, while for decompression experiments it varies from 1.4x10-8 to 1.6x10-7 cm/s. In both cases G is higher in experiments with lower duration, while it trends to decrease with increasing experimental duration. From the comparison between cooling and decompression growth rates we can see that the former are slightly lower than the latter. This is because the amount of dissolved water in the melt depends on the experimental starting pressure. In decompression experiments pressure is lowered and consequently the amount of dissolved water in the melt lowers as well, implying a lower mobility of cations and consequently a lower crystal growth rate. The estimate of alkali feldspar growth rate together with the study of the CSD (Crystal Size Distribution) is useful to determine residence and ascent times (t = -1/ Gs, where s is the slope of the CSD) of trachytic phlegraean magmas to forecast eruptive dynamics of volcanic events that can involve the area. The values of G presented here are the same (10-8 cm/s) or differ at the most of an order of magnitude (10-7 cm/s) from the growth rates used by Piochi et al., (2005) for Monte Nuovo, so the ascent time that will be calculated will be similar or lower than those estimated by Piochi et al., (2005).

Study of growth rate of alkali feldspar in Phlegraean Fields trachytes

CALZOLAIO, Marta
2009-01-01

Abstract

The aim of the work presented in this paper is to acquire experimental knowledge of the growth kinetics of alkali feldspar in Phlegraean Fields trachytic melts, with emphasis on magmas erupted in the Monte Nuovo event of 1538, to constrain system time scales of magmatic processes. Data on alkali feldspar crystallization kinetics in trachytic melts are totally absent from the available scientific literature. Starting from a trachytic composition coming from Phlegrean Fields, cooling and decompression experiments were performed at different condition of T and P and with different duration (crystallization time): experimental pressure varies between 30 and 200 MPa, temperature between 750 and 860°C, time between 7200 and 57600 seconds and fO2 = NNO + 0.8 (Di Matteo et al., 2004). For every series of decompression experiment with constant ∆T, we always performed an experiment where there was no decompression, to allow a comparison between the growth rate in decompressed experiments with the growth rate in the constant PH2O conditions. To obtain the dimensions of alkali feldspar crystals backscattered images were collected with the Scanning Electron Microscope (SEM) and were used to measure the length and the width of the crystals with Photoshop 7.0. A reasonable approximation for growth rate is to average the half-length of the ten longest alkali feldspar crystals in any experiment, as employed by Fenn (1977), Hammer and Rutherford (2002) and Couch (2002). The ratio between half-length and crystallization time (that is the duration of the experiment) is used to calculate the growth rate G. The crystal growth rate varies for cooling experiments from 3,6x10-8 to 3,9x10-7 cm/s, while for decompression experiments it varies from 1.4x10-8 to 1.6x10-7 cm/s. In both cases G is higher in experiments with lower duration, while it trends to decrease with increasing experimental duration. From the comparison between cooling and decompression growth rates we can see that the former are slightly lower than the latter. This is because the amount of dissolved water in the melt depends on the experimental starting pressure. In decompression experiments pressure is lowered and consequently the amount of dissolved water in the melt lowers as well, implying a lower mobility of cations and consequently a lower crystal growth rate. The estimate of alkali feldspar growth rate together with the study of the CSD (Crystal Size Distribution) is useful to determine residence and ascent times (t = -1/ Gs, where s is the slope of the CSD) of trachytic phlegraean magmas to forecast eruptive dynamics of volcanic events that can involve the area. The values of G presented here are the same (10-8 cm/s) or differ at the most of an order of magnitude (10-7 cm/s) from the growth rates used by Piochi et al., (2005) for Monte Nuovo, so the ascent time that will be calculated will be similar or lower than those estimated by Piochi et al., (2005).
2009
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11581/401930
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