Slow Ascent of Unusually Hot Intermediate Magmas Triggering Strombolian to Sub-Plinian Eruptions

Abstract

To assess whether magma ascent rates control the style of volcanic eruption, we have studied the petrography, geochemistry and size distribution of microlites of plagioclase and pyroxene from historical eruptions from Tongariro, Ruapehu and Ngauruhoe volcanoes located in the southern Taupo Volcanic Zone, New Zealand. The studied deposits represent glassy andesitic and dacitic tephra shards from the Mangamate, Mangatawai, Tufa Trig and the Ngauruhoe tephra formations, ranging in age from 11,000 years BP to 1996 AD. Covering a range in eruption styles and sizes from Strombolian to Plinian, these samples provide an excellent opportunity to explore fundamental volcanic processes such as pre-eruptive magma ascent processes. Our quantitative petrographic analysis shows that larger microlites (> 30 µm) display complex growth zoning, and only the smallest crystals (< 30 µm) have formed during magma ascent in the conduit. Using a combination of orthopyroxene geothermometry, plagioclase hygrometry, and MELTS modelling, we show that these microlites nucleated at maximum pressures of 550 MPa (c. 16.5 km) from hot andesitic magmas (1010-1130 ˚C) with low H2O content (0-1.5 wt%). Size distributions of a total of > 60,000 microlites, involving 22 tephras and 99 glass shards, yield concave-up curves, and the slopes of the pyroxene microlite size distributions, in combination with well-constrained orthopyroxene crystal growth rates from one studied tephra, indicate microlite population growth times of ∼3 ± 1 days, irrespective of eruption style. These data imply that microlites form in response to cooling of melts ascending at velocities of < 5 cm s-1 prior to H2O exsolution, which only occurs at < 33 MPa. Maximum magma ascent rates in the upper conduit, calculated using the exsolution of water during final decompression, range between 3 to 12 m s-1, i.e. at least an order of magnitude lower than the hypersonic vent velocities typical of Vulcanian or sub-Plinian eruptions (up to 400 m s-1). This implies that magma ascent from depths of an average of 4 km occurs in dykes, and that vent velocities at the surface are controlled by a reduction of conduit cross-section towards the surface (e.g. dyke transiting to cylindrical conduit).

Petrogenetic implications of chromite-seeded boninite crystallization experiments: Providing a basis for chromite-melt diffusion chronometry in an oxybarometric context

“…From a volcanological perspective, fast ascending magmas are extracted from their crustal domains and extruded onto the surface in as little as tens of minutes (Lloyd et al, 2016) when melts are hot (>1000℃) and volatile rich. Short timescales of the order of hours to days have also been inferred for volatile poor arc magmas (Lormand et al, 2020). In a rapidly ascending melt, a variety of concomitant processes may alter the physiochemical state of the melt volume, including syn-eruptive mixing (e.g.…”

Shallow magmatic processes revealed by cryptic microantecrysts: a case study from the Taupo Volcanic Zone

“…High-resolution investigations of microtextures can unravel the processes occurring at shallower depth in the leadup to eruption. For instance, recent studies focusing on crystal size distribution and numerical conduit model using microlites and microcrysts (Arzilli et al 2019;Lormand et al 2020;Moschini et al 2021) have contributed to our understanding of crystallisation kinetics during magma ascent in the conduit which revealed the links between undercooling and magma ascent rates. Although studies about microphenocryst compositional changes and textures have been reported since the late 1980s (e.g.…”

“…Martel 2012), the factors controlling eruption style (e.g. Lormand et al 2020), and to get a better understanding of degassing and crustal processes (e.g. Noguchi et al 2006).…”

“…In this study, we focus on the textural features of selected microlites to mesocrysts erupted in andesitic tephras from vents in the Tongariro Volcanic Centre (TgVC), which were recently subject to size characterisation (Lormand et al 2018) and CSD analysis (Lormand et al 2020). Crystal textures are used to explore the response of crystal cargo to the final stages of magma ascent, degassing and eruption in this active volcanic arc.…”

“…The analyses were performed using an accelerating voltage of 15 kV and a 10-µm-defocused beam with a current of 7 nA for glass, whereas the mineral analyses were performed using a fully focused beam with a current of 10 nA for plagioclases and 20 nA for pyroxenes. Plagioclase-liquid equilibrium was assessed based on the experimental calibration of the plagioclase hygrometer of Waters and Lange (2015) following the method in Lormand et al (2020), which calculated melt H 2 O content and temperature iteratively using the P-T-dependent plagioclase hygrometry (Waters and Lange, 2015), and the pyroxene-liquid thermobarometry, which, in turn, is affected by liquid H 2 O content (Putirka, 2008). Pyroxene-liquid equilibrium was assessed using the Fe-Mg exchange coefficient determined by Putirka (2008) for both orthopyroxene (Kd(Fe-Mg) opx−liq = 0.29 ± 0.06) and clinopyroxene (Kd(Fe-Mg) cpx−liq = 0.28 ± 0.08).…”