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- Berg, S., et al.
(författare)
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Petrogenesis of Voluminous Silicic Magma in Northeast Iceland
- 2012
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Konferensbidrag (refereegranskat)abstract
- Neogene silicic volcanic complexes in the greater Borgarfjörður eystri area, NE-Iceland, are the focus of a petrological and geochemical investigation. The region contains the second-most voluminous occurrence of silicic rocks in Iceland, including caldera structures, inclined sheet swarms, extensive ignimbrite sheets, sub-volcanic rhyolites and silicic lava flows. Despite the relevance of these rocks to understand the generation of evolved magmas in Iceland, the area is geologically poorly studied [c.f. 1, 2, 3].The voluminous occurrence of evolved rocks in Iceland (10-12 %) is very unusual for an ocean island or a mid-oceanic ridge, with a typical signal of magmatic bimodality, often called “Bunsen-Daly” compositional gap [e.g. 4, 5, 6]. The Bunsen-Daly Gap is a long-standing and fundamental issue in petrology and difficult to reconcile with continuous fractional crystallization as a dominant process in magmatic differentiation [7]. This implies that partial melting of hydrothermally altered crust may play a significant role. Our aim is to contribute to a solution to this issue by unravelling the origin, timing and evolution of voluminous evolved rhyolites in NE-Iceland.We use a combined petrological, textural, experimental and in-situ isotope approach on a comprehensive sample suite of intrusive and extrusive rocks, ranging from basaltic to silicic compositions. We are performing major, trace element and Sr-Nd-Hf-Pb-He-O isotope geochemistry, as well as U-Pb geochronology and Ar/Ar geochronology on rocks and mineral separates. Zircon oxygen isotope analysis will be performed in conjuction with zircon U-Pb geochronology for further assessment of the role of processes such as partial melting of hydrated country rock and/or fractional crystallization in generating Icelandic rhyolites. In addition, high pressure-temperature partial melting experiments aim to reproduce and further constrain natural processes. Using the combined data set we intend to produce a comprehensive and quantitative analysis of rhyolite petrogenesis, and of the temporal, structural and geochemical evolution of silicic volcanism in NE-Iceland. The chosen field area serves as a good analogue for active central volcanoes in Iceland, such as Askja and Krafla, where interaction of basaltic and more evolved magma has led to explosive eruptions. [1] Gústafsson (1992) PhD dissertation, Berlin University. [2] Martin & Sigmarsson (2010) Lithos 116, 129–144. [3] Burchardt, Tanner, Troll, Krumbholz & Gustafsson (2011) G3 12 (7), Q0AB09. [4] Bunsen (1851) Annalen der Physik und Chemie 159 (6), 197-272. [5] Daly (1925) Proceedings of the American Academy of Arts and Sciences 60 (1), 3-80. [6] Barth, Correns & Eskola (1939) Die Entstehung der Gesteine. Springer Verlag, Berlin. [7] Bowen (1928) The evolution of the igneous rocks. Princeton University Press.
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- Berg, S., et al.
(författare)
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Silicic Magma Genesis in Neogene Central Volcanoes in Northeast Iceland
- 2012
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Konferensbidrag (refereegranskat)abstract
- We report on a geological expedition to NE Iceland in August 2011. A comprehensive sample suite of intrusive and extrusive rocks, ranging from basaltic to silicic compositions, was collected from the Neogene silicic central volcanic complexes in the region between Borgarfjörður eystri and Loðmundarfjörður. The area contains the second-most voluminous occurrence of silicic rocks in Iceland, including caldera structures, inclined sheet swarms, extensive ignimbrite sheets, sub-volcanic rhyolites and silicic lava flows. Yet it is one of Iceland's geologically least known areas (c.f. Gústafsson, 1992; Martin & Sigmarsson, 2010; Burchardt et al., 2011). The voluminous occurrence of evolved rocks in Iceland (10-12 %) is very unusual for an ocean island or a mid-oceanic ridge, with a typical signal of magmatic bimodality, often called "Bunsen-Daly" compositional gap (e.g. Bunsen, 1851; Daly, 1925; Barth et al., 1939). The Bunsen-Daly Gap is a long-standing fundamental issue in petrology and difficult to reconcile with continuous fractional crystallization as a dominant process in magmatic differentiation (Bowen, 1928), implying that hydrothermal alteration and crustal melting may play a significant role. Our aim is to contribute to a solution of this issue by unravelling the occurrence of voluminous evolved rhyolites in NE Iceland. We will use a combined petrological, textural, experimental and in-situ isotope approach. We plan to perform major, trace element and Sr-Nd-Hf-Pb-He-O isotope geochemistry, as well as U/Pb and Ar/Ar geochronology on rocks and mineral separates. In addition, high pressure-temperature partial melting experiments aim to reproduce and further constrain natural processes. Using the combined data set we intend to produce a comprehensive and quantitative analysis of rhyolite petrogenesis, and of the temporal, structural and geochemical evolution of the silicic volcanism in NE Iceland. The chosen field area serves as a good analogue for active central volcanoes in Iceland, such as Askja and Krafla, where a close interaction of basaltic and more evolved magma has led to explosive eruptions.
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- Blythe, L. S., et al.
(författare)
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Viscosity controlled magma-carbonate interaction : a comparison of Mt. Vesuvius (Italy) and Mt. Merapi (Indonesia)
- 2012
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Konferensbidrag (refereegranskat)abstract
- Magma-carbonate interaction is increasingly seen as a viable and extremely important cause of magma contamination, and the generation of a crustally sourced CO2 phase (Goff et al., 2001; Freda et al., 2010). Even though the process is well recognized at certain volcanoes e.g. Popocatépetl, (Mexico); Merapi, (Indonesia); and Colli Albani, (Italy) (Goff et al., 2001; Deegan et al., 2010; Freda et al., 2010), neither the kinetics of carbonate assimilation nor its consequences for controlling the explosivity of eruptions have been constrained. Here we show the results of magma-carbonate interaction experiments conducted at 1200 °C and 0.5 GPa for varying durations (0 s, 60 s, 90 s and 300 s) for the Mt. Merapi (Indonesia) and Mt. Vesuvius (Italy) volcanic systems. We performed experiments using glassy starting materials specific to each volcano (shoshonite for Mt. Vesuvius, basaltic-andesite for Mt. Merapi) with different degrees of hydration (anhydrous vs hydration with ~ 2 wt % water) and using carbonate fragments of local origin; see Deegan et al., (2010) and Jolis et al., (2011). Experimental products include a gas phase (CO2-rich) and two melt phases, one pristine (Ca-normal) and one contaminated (Ca-rich) separated by a 'contamination front' which propagates outwards from the carbonate clast. Vesicles appear to nucleate in the contaminated glass and then migrate into the pristine one. Both contamination front propagation and bubble migration away from the carbonate are slower in anhydrous basaltic-andesite (Merapi anhydrous series) than in hydrated basaltic-andesite and shoshonite (Merapi and Vesuvius hydrated series), suggesting that assimilation speed is strongly controlled by the degree of hydration and the SiO2 content, both of which influence melt viscosity and hence diffusivity. As the carbonate dissolution proceeds in our experiments, initially dissolved and eventually exsolved CO2 builds up in the contaminated Ca-rich melt phase. Once melt volatile oversaturation is achieved, the reaction can only progress further if vesicles are efficiently removed from the contaminated melt phase. Viscosity, which controls the vesicle migration efficiency, thus ultimately determines the progression and rate of the contamination reaction. Our results show that characteristics of magma-carbonate interaction at different volcanic systems are likely to differ as a result of a volcanos' individual magma properties, especially viscosity, which determines the speed at which gaseous reaction products (i.e. CO2) can be removed from the reaction site.
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