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Magmatic evolution and textural development of the 1739 CE Pietre Cotte lava flow, Vulcano, Italy

Magmatic evolution and textural development of the 1739 CE Pietre Cotte lava flow, Vulcano, Italy
Magmatic evolution and textural development of the 1739 CE Pietre Cotte lava flow, Vulcano, Italy

Textural evidence from occurrences of mingled magmas in lava flows often yields insights into chemical and thermal disequilibrium between multiple magma batches at depth. An understanding of these interactions is key as they can occur on short timescales and may act as eruption triggers, particularly important in very active volcanic settings. This paper focuses on the Pietre Cotte lava flow (Vulcano, Aeolian Islands, Italy), a short (<1 km in length), texturally-heterogeneous rhyolitic extrusion on the northern slope of the active Fossa Cone. The occurrence of (i) multiple magma compositions, (ii) distinct magmatic cumulates (as glomerocrysts) and (iii) mineral resorption textures within glomerocrysts and isolated feldspar phenocrysts in the Pietre Cotte lava flow highlight a complex pre-eruptive magmatic history, including crystal mush remobilisation. Petrographic observations and mineral, bulk rock and glass geochemistry suggest that multiple mingling events occurred during the evolution of the Pietre Cotte magmatic system, evidenced by the recognition of the following components: (1) a remobilised predominantly mafic crystal mush, evident as macrocrysts (crystals >500 μm), which form glomerocrysts within enclaves, (2) a microlitic (<100 μm) trachytic enclave groundmass with microcrysts (100–500 μm), and (3) a rhyolitic glass, which hosts both the enclaves and the glomerocrysts. The macrocrystic mafic assemblage includes clinopyroxene (En38-47Wo45–50; Mg# 0.72–0.89), olivine (Fo49–66) and magnetite (Usp7–26), with plagioclase (An40–63Ab5–50) and rare alkali feldspar (Or41–57) also present. Enclaves are comprised of a groundmass of plagioclase (An43–47) and alkali feldspar (Or33–57) microlites, with clinopyroxene microcrysts (En39-42Wo47–51; Mg# 0.75–0.81) and trachyte groundmass glass. The rhyolitic host is characterised by glass, spherulites, microlites and enclave-derived macrocrysts. Compositionally and texturally distinct magmas are attributed to storage and interactions of distinct magma batches and their cumulates at various temperatures and depths beneath the Fossa Cone. Compositions vary from basaltic-shoshonitic, through latitic-trachytic and rhyolitic magmas. The macrocrystic glomerocryst assemblage shows resorbed, chemically-zoned and cumulate textures; the glomerocrysts are attributed to a shoshonitic parent and remobilisation from a crystal mush. Macrocrysts formed at a pressure of 825 ± 80 MPa and temperatures of 789–1117 °C at around the Moho (~23–28 km). Pressure and temperature calculations of the shoshonitic mineral assemblage give average crystallisation conditions of 710 ± 80 MPa (above the Moho) and 1128 ± 25 °C, respectively. The trachytic magma crystallised at ~640 ± 75 MPa and 1000–1130 °C. The average liquidus of the rhyolitic magma has been calculated at 970 ± 7 °C, at depths of <5 km (<60 MPa). New textural observations and intensive variable calculations permit the development of a new pressure and temperature-constrained model of the magmatic evolution of the Pietre Cotte system prior to eruption, with useful insights into the interactions of different magmatic components prior to and during the rapid onset of eruptions linked to magma mingling/mixing.

Magma mingling, Magmatic cumulates, Pietre Cotte, Rhyolite, Trachyte, Vulcano
0377-0273
1-23
Bullock, Liam A.
c6ffb9b0-0a54-4ab2-9edb-f97280e6ce2d
Gertisser, Ralf
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O'Driscoll, Brian
463f2f0c-856a-416a-8a8d-ac451edaac91
Harland, Sophie
133a620c-6cea-49a5-b8cf-32cad9a3f64e
Bullock, Liam A.
c6ffb9b0-0a54-4ab2-9edb-f97280e6ce2d
Gertisser, Ralf
8b4ea978-2461-4a65-87c8-352c62e52c70
O'Driscoll, Brian
463f2f0c-856a-416a-8a8d-ac451edaac91
Harland, Sophie
133a620c-6cea-49a5-b8cf-32cad9a3f64e

Bullock, Liam A., Gertisser, Ralf, O'Driscoll, Brian and Harland, Sophie (2019) Magmatic evolution and textural development of the 1739 CE Pietre Cotte lava flow, Vulcano, Italy. Journal of Volcanology and Geothermal Research, 372, 1-23. (doi:10.1016/j.jvolgeores.2019.01.017).

Record type: Article

Abstract

Textural evidence from occurrences of mingled magmas in lava flows often yields insights into chemical and thermal disequilibrium between multiple magma batches at depth. An understanding of these interactions is key as they can occur on short timescales and may act as eruption triggers, particularly important in very active volcanic settings. This paper focuses on the Pietre Cotte lava flow (Vulcano, Aeolian Islands, Italy), a short (<1 km in length), texturally-heterogeneous rhyolitic extrusion on the northern slope of the active Fossa Cone. The occurrence of (i) multiple magma compositions, (ii) distinct magmatic cumulates (as glomerocrysts) and (iii) mineral resorption textures within glomerocrysts and isolated feldspar phenocrysts in the Pietre Cotte lava flow highlight a complex pre-eruptive magmatic history, including crystal mush remobilisation. Petrographic observations and mineral, bulk rock and glass geochemistry suggest that multiple mingling events occurred during the evolution of the Pietre Cotte magmatic system, evidenced by the recognition of the following components: (1) a remobilised predominantly mafic crystal mush, evident as macrocrysts (crystals >500 μm), which form glomerocrysts within enclaves, (2) a microlitic (<100 μm) trachytic enclave groundmass with microcrysts (100–500 μm), and (3) a rhyolitic glass, which hosts both the enclaves and the glomerocrysts. The macrocrystic mafic assemblage includes clinopyroxene (En38-47Wo45–50; Mg# 0.72–0.89), olivine (Fo49–66) and magnetite (Usp7–26), with plagioclase (An40–63Ab5–50) and rare alkali feldspar (Or41–57) also present. Enclaves are comprised of a groundmass of plagioclase (An43–47) and alkali feldspar (Or33–57) microlites, with clinopyroxene microcrysts (En39-42Wo47–51; Mg# 0.75–0.81) and trachyte groundmass glass. The rhyolitic host is characterised by glass, spherulites, microlites and enclave-derived macrocrysts. Compositionally and texturally distinct magmas are attributed to storage and interactions of distinct magma batches and their cumulates at various temperatures and depths beneath the Fossa Cone. Compositions vary from basaltic-shoshonitic, through latitic-trachytic and rhyolitic magmas. The macrocrystic glomerocryst assemblage shows resorbed, chemically-zoned and cumulate textures; the glomerocrysts are attributed to a shoshonitic parent and remobilisation from a crystal mush. Macrocrysts formed at a pressure of 825 ± 80 MPa and temperatures of 789–1117 °C at around the Moho (~23–28 km). Pressure and temperature calculations of the shoshonitic mineral assemblage give average crystallisation conditions of 710 ± 80 MPa (above the Moho) and 1128 ± 25 °C, respectively. The trachytic magma crystallised at ~640 ± 75 MPa and 1000–1130 °C. The average liquidus of the rhyolitic magma has been calculated at 970 ± 7 °C, at depths of <5 km (<60 MPa). New textural observations and intensive variable calculations permit the development of a new pressure and temperature-constrained model of the magmatic evolution of the Pietre Cotte system prior to eruption, with useful insights into the interactions of different magmatic components prior to and during the rapid onset of eruptions linked to magma mingling/mixing.

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MS LAB PC 2018 Accepted version - Accepted Manuscript
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More information

Accepted/In Press date: 21 January 2019
e-pub ahead of print date: 23 January 2019
Published date: 15 February 2019
Keywords: Magma mingling, Magmatic cumulates, Pietre Cotte, Rhyolite, Trachyte, Vulcano

Identifiers

Local EPrints ID: 427502
URI: https://eprints.soton.ac.uk/id/eprint/427502
ISSN: 0377-0273
PURE UUID: 1e0d5335-14ed-4219-8132-190ba42cb4da

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Date deposited: 22 Jan 2019 17:30
Last modified: 18 Jul 2019 16:46

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