Rapid Assembly and Eruption of a Shallow Silicic Magma Reservoir, Reyðarártindur Pluton, Southeast Iceland. (8th November 2021)

• Record Type: Journal Article
• Authors: Rhodes, E. L.; Barker, A. K.; Burchardt, S.; Hieronymus, C. F.; Rousku, S. N.; McGarvie, D. W.; Mattsson, T.; Schmiedel, T.; Ronchin, E.; Witcher, T.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 22:Number 11(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-11-08
• Subjects: volcanic‐plutonic connection -- magma mixing -- magma chamber -- eruption trigger -- conduit; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2021GC009999
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: Although it is widely accepted that shallow silicic magma reservoirs exist, and can feed eruptions, their dynamics and longevity are a topic of debate. Here, we use field mapping, geochemistry, 3D pluton reconstruction and a thermal model to investigate the assembly and eruptive history of the shallow Reyðarártindur Pluton, southeast Iceland. Primarily, the exposed pluton is constructed of a single rock unit, the Main Granite (69.9–77.7 wt.% SiO2 ). Two further units are locally exposed as enclaves at the base of the exposure, the Granite Enclaves (67.4–70.2 wt.% SiO2 ), and the Quartz Monzonite Enclaves (61.8–67.3 wt.% SiO2 ). Geochemically, the units are related and were likely derived from the same source reservoir. In 3D, the pluton has a shape characterized by flat roof segments that are vertically offset and a volume of >2.5 km 3 . The pluton roof is intruded by dikes from the pluton, and in two locations displays depressions associated with large dikes. Within these particular dikes the rock is partially to wholly tuffisitic, and rock compositions range from quartz monzonite to granite. We interpret these zones as eruption‐feeding conduits from the pluton. A lack of cooling contacts throughout the pluton indicates rapid magma emplacement and a thermal model calculates the top 75 m would have rheologically locked up within 1, 000 years. Hence, we argue that the Reyðarártindur Pluton was an ephemeral part of the wider plumbing system that feeds a volcano, and that timeframes from emplacement to eruption were rapid. Plain Language Summary: Scientists who monitor active volcanoes are interested in the pathways that magma takes on its journey through the crust to eruption. They use this information to aid in forecasting eruptions. Specifically, in Iceland it is known that magma often moves from chambers deep in the crust (>20 km) to shallow levels (<5 km) where the magma is stored for a time before it erupts (or does not erupt). Because scientists cannot physically "observe" active magma chambers in the crust, we turn to older, now fossil magma chambers, to examine what processes occurred during their construction. In this paper, we examined a shallow fossil magma chamber in Iceland and used geological mapping and geochemical analysis to reconstruct its history. We found that it has a "castle‐like" shape with a minimum volume of 2.5 km 3, and was assembled in pulses of magma over a rapid timeframe. The magmas are silicic in composition, which is important from an eruption hazard perspective because silicic magmas have the potential to cause larger, explosive, eruptions. We also found evidence that the chamber erupted from at least two locations, and used a cooling model to determine that it could have been active for up to 1, 000 years. Key Points: The pluton was constructed in batches from silicic magmas that were all sourced from the same underlying reservoir Silicic magma injection may have been a trigger for eruption‐feeding conduits in the roof The pluton was rapidly assembled and had a maximum eruptible lifetime of around 1, 000 years