Continuous mush disaggregation during the long-lasting Laki fissure eruption, Iceland

Abstract

Plagioclase textures were investigated in the products of the voluminous 1783-1784 CE Laki eruption from the Eastern Volcanic Zone (EVZ) of Iceland to establish whether mush disaggregation occurred solely at the onset of the eight-month eruption or throughout its whole duration. Phase proportions and plagioclase size distributions were determined using standard optical and manual techniques as well as automated approaches based on Quantitative Evaluation of Minerals by SCANing electron microscopy (QEMSCAN). Based on optical microscopy and the explicit combination of textural and compositional information in QEMSCAN images, plagioclase crystals were divided into two populations: small (<0.5 mm long), high-aspect ratio (length/width >4) microcrysts with low-anorthite (<An 80 ) cores; and large (>0.5 mm long), low-aspect ratio (length/width = 2-3) macrocrysts with high-anorthite (An 84 -An 92 ) cores. Small microcrysts grew from their carrier liquid during the final phase of pre-eruptive crystallization while large macrocrysts, which are out of geochemical equilibrium with their carrier liquids, were entrained from crystal mushes. Changes in phase proportions and plagioclase size distributions between eruptive episodes demonstrate that macrocryst entrainment efficiency varied substantially during the eruption; material erupted in later episodes contain proportionally more mush-derived material. Using stereologically corrected plagioclase size distributions, we estimate that the pre-eruptive residence times of microcrysts in the Laki carrier liquid were probably of the order of 2-20 days. Because microcryst crystallization was concurrent with macrocryst rim growth, these day-to-week residence times also indicate that macrocryst entrainment occurred on much shorter timescales than the eruption's eight-month duration. In line with constraints from independent geochronometers, macrocryst entrainment and mush disaggregation thus appears to have continued throughout the eruption. Magmas were assembled on an episode by episode basis, and the volume of eruptible magma in the plumbing system at any given time was probably closer to 1-2 km 3 than the final erupted volume of 15.1 km 3 .

Microstructural evolution of silicate immiscible liquids in ferrobasalts

“…4d') could result from either the heterogeneous nucleation of the droplet on the surface of a pre-existing pillar, or formation of the pillar by growth from a Fe-rich liquid after heterogeneous nucleation of the droplet on an originally planar plagioclase crystal face. If the former, then we might expect pillars to be a commonly observed feature of rapidly cooled basalts in which diffusion-limited growth occurs, but they are not (Helz and Wright 1992;Neave et al 2017). Consequently, following Philpotts (1981), we suggest that pillar formation post-dates the attachment of the Fe-rich droplets, and that the presence of a Fe-rich droplet destabilizes the plagioclase planar growth face, resulting in localised enhanced growth of plagioclase.…”

Unexpected large eruptions from buoyant magma bodies within viscoelastic crust

“…A widespread view is that these macrocrystals crystallised at middle to deep crustal levels and accumulated in a crystal mush layer from which they were picked up by their host/carrier melt during ascent. Well documented examples of Icelandic Holocene lavas, where prior studies have demonstrated the importance of such a magma mush layer, include the 8600 BP Þjórsárhraun lava 3 , the 1783 Laki lava 6,63 , and more recently the 2014-15 Holuhraun lava 7,35 . Consideration of elemental diffusion in these crystals and their melt inclusions demonstrates that timescales associated with the crystal entrainment process are likely to be short, or equivalent to timescales of these eruptions, i.e., a few weeks to months 3,7,64 .…”

“…In line with these observations, we suggest that an important process for formation of sustained magma channels is the addition of mush crystals to a magmatic liquid as it leaves a magma body and rises towards the surface. In this case, a near-continuous addition of mush crystals into the host/carrier melt 63 , in proportion to the rate of magma flow, leads to uniform crystal content throughout the erupted magma, despite variable rate of flow. We suggest that erosion of such a crystal mush layer may ultimately lead to formation of sustained openings, which are required for the extraction of a melt from a magma body.…”

Crystal Size Distribution (CSD) Analysis of Volcanic Samples: Advances and Challenges

“…Overwhelming documentation of the prevalence of antecrysts in magmatic systems (e.g., Bacon and Lowenstern, 2005;Humphreys et al, 2006;Smith et al, 2009;Viccaro et al, 2010;Neave et al, 2013Neave et al, , 2014Neave et al, , 2017van der Zwan et al, 2013;Witter et al, 2016) has been critical for the development of mushdominated paradigms of magmatic systems (e.g., Edmonds et al, 2019;Sparks et al, 2019). Entrained crystals can be identified by major element and/or isotopic compositions that are not in equilibrium with the transporting magma (e.g., Morgan et al, , 2007Berlo et al, 2007;Martin et al, 2010;Sakyi et al, 2012;Cashman and Blundy, 2013;Sides et al, 2014;Brenna et al, 2018;Shane et al, 2019) and by evidence of deformation in crystalrich mush zones (e.g., Kahl W. et al, 2017;Befus et al, 2019;Holness et al, 2019;Wieser et al, 2020).…”

“…Here I suggest an additional approach that complements the applications of isotope tagging and diffusion chronometry described above. When crystal cores and rims/groundmass differ in composition, the contribution of different crystal components to the overall sample crystallinity can be assessed using compositional maps made directly from BSE images or element maps (e.g., Muir et al, 2012), including automated analysis such as QEMSCAN (e.g., Neave et al, 2014Neave et al, , 2017. To illustrate the use of compositional information in BSE images, I use an example from Mount St. Helens, where the diversity of the plagioclase population has been well documented (e.g., Berlo et al, 2007;Cashman and Blundy, 2013).…”