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MAL27

Robert Wilhelm Bunsen Medal Lecture by Janne Blichert-Toft & Arne Richter Award Lecture for Outstanding ECS Lecture by Fabian B. Wadsworth
Convener: Marian Holness
Presentations
| Tue, 24 May, 19:00–20:00 (CEST)
 
Room D2

Tue, 24 May, 19:00–20:00

Chairperson: Marian Holness

19:00–19:05
Introduction and citation

19:05–19:20
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EGU22-13600
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Arne Richter Award for Outstanding ECS Lecture
Fabian B. Wadsworth et al.

The controls on the style of silicic eruptions – hazardously explosive, more gently effusive, or hybrid explosive-effusive – are poorly constrained. Current models invoke escape of gas through a connected foam, or through fractures, as the primary mechanism for the transition from explosive to effusive eruption. We propose a new model, in which hybrid and effusive silicic eruptions are typically explosive at depth, but the clastic products of this 'cryptic fragmentation' sinter and weld in the conduit to produce coherent lava at the surface. Drawing on numerous case studies of natural textures within eruptive products and dissected conduits, we show that effusive silicic eruptions are best interpreted as being the welded, squeezed-out remnant of ongoing or recent subsurface explosive behaviour. We demonstrate that effusively erupted lavas have microtextures diagnostic of a welding/sintering genesis and are comparable with those found in rheomorphic and welded ignimbrites. All eruptive products share pore network geometries and associated mechanical and hydraulic property-porosity relationships that are consistent with models for sintered materials. We conclude that silicic lava is generally clastogenic, and that, after it is sinter-assembled, it may undergo gas-driven fracturing that produces lava plug-cutting tuffisites (closed fractures filled with sintered particles), and sintered pyroclasts (from ash- to bomb-sized). At some sites (e.g. Volcán Chaitén 2008), the first material to be extruded from the vent is a pyroclastic rubble similar texturally to the volcanic bombs from the same site. We propose therefore that the shallow conduit is filled with pyroclastic and lithic rubble; a volume that variably compacts over time to produce a plug of densified lava. Envisaging the shallow conduit as a compacting rubble pile instead of a coherent magma-filled pipe or crack leads us to posit that the explosive-effusive transition is a blurred behavioural switch controlled by the competition between material supply at the underlying fragmentation front, and shallow particle capture, welding and lava production above. This framework has broad top-down implications for geochemical and geophysical predictions of shallow silicic volcanism, which we will explore in this presentation.

How to cite: Wadsworth, F. B., Llewellin, E. W., Vasseur, J., Gardner, J. E., and Tuffen, H.: A reappraisal of explosive-effusive silicic eruption dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13600, https://doi.org/10.5194/egusphere-egu22-13600, 2022.

19:20–19:25
Questions and answers

19:25–19:30
Introduction and citation

19:30–19:55
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EGU22-1035
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Robert Wilhelm Bunsen Medal Lecture
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On-site presentation
Janne Blichert-Toft et al.

The geochemical toolkit is pertinent to fields other than that of Earth and Planetary Sciences for which it is traditionally and commonly used. Here we show two recent examples of its application to numismatics, archaeology, and ancient history. High-precision Pb isotopes treated by novel statistical tools were used to provide data-based answers to important research questions revolving around the sources of silver used for money, jewelry, and other valuable artefacts in the ancient world.

In the first example, we studied remnants of the silver making up the largest treasure of precious metals reported in ancient Western history, namely that of Alexander the Great which he looted in his conquest of the Persian Empire, by analyzing a large set of ancient silver coins (alexanders, sigloi, Greek coins, and early Indian pseudo-coinage) for their Pb isotopic compositions. The high-precision data were treated using a new statistical approach in the form of calculated Pb model ages combined with cluster analysis and convex-hull theory, which allows the tracking of silver provenance with greater accuracy and precision than was previously possible when using only raw Pb isotope ratios and manually comparing artefacts with known ores on a one-to-one basis. Based on the Pb isotopic compositions of the analyzed silver coins compared with a ca. 6700-entry Pb isotope database on ores that we have compiled from the literature and our own work, we established that the bulk of the silver sources can be traced to the southern Aegean, Macedonia, and Thrace [1]. These origins had so-far only been the subject of speculation by numismatists, archaeologists, and historians, whereas now they are supported by high-precision isotope data and objective data analysis. Furthermore, we were able to confidently exclude India as a source [1], thereby putting to rest a long-standing debate around a possible Indian silver contribution to the Persian treasury.

In the second example [2], we measured high-precision Pb isotopes on pieces of hoarded Hacksilber (irregularly cut silver bullion) in the southern Levant, which facilitated trade and transactions from the beginning of the second millennium BCE until the late fourth century BCE. In a similar fashion to the first example, we treated the data using cluster analysis and convex-hull theory applied to Pb model ages calculated from measured high-precision Pb isotopic compositions. We found that exchanges between the Levant and the Aegean world continued at least intermittently from the Late Bronze Age through to the Iron Age III. Importantly, contrary to common belief that silver trade had come to an end following the Late Bronze Age collapse, we demonstrated that despite the Aegean world dominating silver supply during the Iron Age, exchanges between the eastern and the western Mediterranean did not cease altogether. People around the Mediterranean remained connected with silver flowing to the Levant possibly as a result of trade or plunder.

[1] Blichert-Toft, J., de Callatay, F., Télouk, P., Albarède, F., submitted. J. Archaeo. Meth. Theo.

[2] Gentelli, L., Blichert-Toft, J., Davis, G., Gitler, H., Albarède, F., 2021. J. Archaeo. Sci. 134, Article 105472.

How to cite: Blichert-Toft, J., Gentelli, L., Davis, G., Gitler, H., de Callataÿ, F., and Albarède, F.: When Geochemistry encounters Archaeology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1035, https://doi.org/10.5194/egusphere-egu22-1035, 2022.

19:55–20:00
Questions and answers