Plagioclase nucleation and growth kinetics in a hydrous basaltic melt by decompression experiments

Abstract

Isothermal single-step decompression experiments (at temperature of 1075 °C and pressure between 5 and 50 MPa) were used to study the crystallization kinetics of plagioclase in hydrous high-K basaltic melts as a function of pressure, effective undercooling (ΔT eff ) and time. Single-step decompression causes water exsolution and a consequent increase in the plagioclase liquidus, thus imposing an effective undercooling (∆T eff ), accompanied by increased melt viscosity.Here we show that the decompression process acts directly on viscosity and thermodynamic energy barriers (such as interfacial-free energy), controlling the nucleation process and favoring the formation of homogeneous nuclei also at high pressure (low effective undercoolings). In fact, this study shows that similar crystal number densities (N a ) can be obtained both at low and high pressure (between 5 and 50 MPa), whereas crystal growth processes are favored at low pressures (5 -10 MPa).The main evidence of this study is that the crystallization of plagioclase in decompressed high-K basalts is more rapid than that in rhyolitic melts on similar timescales. The onset of the crystallization process during experiments was characterized by an initial nucleation event within the first hour of the experiment, which produced the largest amount of plagioclase. This nucleation event, at short experimental duration, can produce a dramatic change in crystal number density (N a ) and crystal fraction (φ), triggering a significant textural evolution in only 1 h. In natural systems, this may affect the magma rheology and eruptive dynamics on very short time scales.

Constraining pre-eruptive magma conditions and unrest timescales during the Monte Nuovo eruption (1538 ad; Campi Flegrei, Southern Italy): integrating textural and CSD results from experimental and natural trachy-phonolites

“…9d), nucleation was strongly favored. Our results show that nucleation is reduced at high pressure (Figs,5,8,9, and 10b), demonstrating that a dramatic increase of crystal number density can be induced at low pressure in trachytic melts, likely as a result of the effect of melt H 2 O content on nucleation process, as observed by Hammer (2004) for rhyolitic melts and by Arzilli et al (2015) for high-K basalts. Furthermore, our results show that the increase of crystallinity and N a can occur in a few hours (Figs.…”

Role of syn-eruptive plagioclase disequilibrium crystallization in basaltic magma ascent dynamics

“…This is a reasonable assumption since results from ref. 47 show that vesiculation of basaltic magmas happens over timescales of tens of seconds, whilst data from refs 26, 27 indicate that equilibrium crystallization is reached in about 2 h. Furthermore, the growth rates of vesicles (10 −4 –10 −2  cm s −1 )4748 are several order of magnitude higher than those of plagioclase (10 −7 –10 −8  cm s −1 )2627. These data suggest that the timescale of exsolution is shorter than crystallization.…”

“…Laboratory experiments of plagioclase crystallization show a very wide range of timescales for reaching equilibrium volume fraction, going from 2 to 20 h (refs 20, 21, 27). Comparison with our numerical results suggests that a longer timescale for crystallization and a shorter timescale for gas exsolution are most realistic, and that ∼100 min is required for plagioclase to reach equilibrium after a pressure change, with characteristic time of ∼20 min.…”

“…Considering a basaltic system characterized by water-saturated conditions and temperature of ∼1,100 °C, plagioclase crystallizes from 50/75 MPa (2/3 km) to the surface20252627, recording textural and chemical information of syn-eruptive conditions. Therefore, plagioclase can act as a sensitive indicator of relatively shallow disequilibrium processes.…”

Comb layering monitors decompressing and fractionating hydrous mafic magmas in subvolcanic plumbing systems (Fisher Lake, Sierra Nevada, USA)

“…Crystal growth rates calculated for slowly cooling systems are up to several orders of magnitude slower (10 −10 to 10 −12 cm/s) [ Barboni and Schoene , ; Cashman , ] than for systems prone to rapid disequilibrium by decompression, or cooling, which undergo moderate to high undercooling Δ T (10 −5 –10 −7 cm/s) [ Arzilli et al ., , ; Shea and Hammer , ; Waters et al ., ]. Using experimentally derived plagioclase crystal growth rates from mafic‐intermediate compositions [ Shea and Hammer , ], we can estimate that our smallest comb layer sequences of ~ 10 to 35 cm (Layers 564–590 and 321–391, respectively) formed in as little as 1–3 years, or as “slow” as 9–26 years, whereas the thick continuous sequence of plagioclase comb layers (Layers 1–90) would have formed in a minimum of 6 to 53 years (Table ).…”