Uniform “water” content in quartz phenocrysts from silicic pyroclastic fallout deposits – implications on pre-eruptive conditions

Mátyás Hencz,Dávid Karátson,Károly Németh,Zoltán Pécskay,Alexandru Szakács,Tamás Biró,István János Kovács,Roland Stalder,Zsófia Pálos

doi:10.5194/ejm-33-571-2021

Abstract

Abstract. Structural hydroxyl content of volcanic quartz phenocrysts was investigated with unpolarized Fourier-transform infrared spectroscopy. The phenocrysts originated from five pyroclastic fallout deposits from the Bükk Foreland Volcanic Area (BFVA), Hungary, and two from the AD 1314 Kaharoa eruption (KH eruption), Okataina Volcanic Complex (Taupo Volcanic Zone), New Zealand. All investigated quartz populations contain structural hydroxyl content in a narrow range with an average of 9.3 (±1.7) wt ppm. The earlier correlated horizons in the BFVA had the same average structural hydroxyl content (within uncertainty). Thus, it can be concluded that the structural hydroxyl content does not depend on the geographical distance of outcrops of the same units or the temperature or type of the covering deposit. The rare outlier values and similar structural hydroxyl contents show that the fallout horizons cooled fast enough to retain their original structural hydroxyl content. The similarity of the structural hydroxyl contents may be the result of similar P, T, and x (most importantly H2O and the availability of other monovalent cations) conditions in the magmatic plumbing system just before eruption. Therefore, we envisage common physical–chemical conditions, which set the structural hydroxyl content in the quartz phenocrysts and, consequently, the water content of the host magma (∼ 5.5 wt %–7 wt % H2O) in a relatively narrow range close to water saturation.

Highlights

Water content significantly affects the physical properties of magma and has a large impact on the magnitude and type of the volcanic eruptions especially by moderating explosivity (Sparks, 1978; Dixon and Stolper, 1995; Dingwell, 1996; Popa et al, 2019, 2020, 2021; Allabar et al, 2020)
We report AlOH structural hydroxyl concentrations of quartz phenocrysts from five Miocene pyroclastic fallout deposits of the Bükk Foreland Volcanic Area (BFVA hereafter), northern Hungary, Carpathian–Pannonian region (Fig. 1), and two pyroclastic fallout deposits of the AD 1314 Kaharoa eruption, Tarawera, New Zealand (KH eruption hereafter)
Syn-eruptive hydrogen loss (Wallace et al, 2003; Jollands et al, 2020b); presumably their AlOH structural hydroxyl content can be used to infer the original water content of the host magma. In this way, we investigate whether the structural hydroxyl content of quartz phenocrysts depends on any volcanological conditions

Summary

Introduction

Water content significantly affects the physical properties (density, viscosity) of magma and has a large impact on the magnitude and type of the volcanic eruptions especially by moderating explosivity (Sparks, 1978; Dixon and Stolper, 1995; Dingwell, 1996; Popa et al, 2019, 2020, 2021; Allabar et al, 2020). This diffusivity is fast enough to produce diffusion profiles, which are applicable for the determination of timescales of magma ascent rates and eruption history (Myers et al, 2018, 2019; Tollan et al, 2019; Jollands et al, 2020b) This rapid diffusion is sufficient to lose a significant portion of the original H content of the quartz phenocrysts exposed to high temperatures (300–700 ◦C) for time intervals ranging from days to months under decreasing water activity (e.g., degassing, slow cooling at the surface; Biró et al, 2017; Stalder et al, 2017). Other elements (e.g., major elements, like Na) possibly do not play a significant role in charge balancing of Al in competition with H because they diffuse much more slowly (Cherniak and Dimanov, 2010; Zhang et al, 2010)

Methods

Findings

Discussion

Conclusion