TY - JOUR

T1 - Compositional and thermodynamic variability in a stratified magma chamber

T2 - Evidence from the Green Tuff Ignimbrite (Pantelleria, Italy)

AU - Liszewska, Katarzyna

AU - White, John

AU - MacDonald, Raymond

AU - Bagiński, Bogusław

N1 - This is a pre-copy-editing, author-produced PDF of an article accepted for publication in Journal of Petrology following peer review. The definitive publisher-authenticated version K M Liszewska, J C White, R Macdonald, B Bagiński; Compositional and Thermodynamic Variability in a Stratified Magma Chamber: Evidence from the Green Tuff Ignimbrite (Pantelleria, Italy), Journal of Petrology, Volume 59, Issue 12, 1 December 2018, Pages 2245–2272, https://doi.org/10.1093/petrology/egy095 is available online at: https://academic.oup.com/petrology/article/59/12/2245/5146341

PY - 2018/12/1

Y1 - 2018/12/1

N2 - The Green Tuff Ignimbrite, Pantelleria, is compositionally zoned from pantellerite at the base to comenditic trachyte at the top, the variation apparently representing an inverted vertical zonation in the pre-eruptive reservoir. The main phenocryst assemblages are alkali feldspar + olivine + clinopyroxene + ilmenite + apatite in the trachytes and alkali feldspar + aenigmatite + clinopyroxene ± quartz in the rhyolites. Thermodynamic modelling indicates that the temperature range was ∼900–700°C, fO2 FMQ –1·5 to FMQ –0·5 (where FMQ is fayalite–magnetite–quartz buffer) and aSiO2 (relative to quartz saturation) 0·74–1·00. Melt water contents ranged from ∼1 wt % in the trachytes to ∼4 wt % in the pantellerites. Matrix glass analyses in the more evolved rocks are highly variable, showing that compositional layers in the upper parts of the reservoir, formed by fractional crystallization, were mixed during eruption, the proportion of rhyolitic to trachytic melts increasing towards the top of the reservoir. Some areas of glass have low Al2O3 contents (5·16–5·46 wt %) and high FeO* contents (9·66–10·02 wt %), making them the most evolved melts yet reported from Pantelleria. The new glass data reveal how whole-rock analyses do not truly reflect the complete range of melt compositions in the pre-eruptive reservoir. The trachytes contain >40% modal phenocrysts, which with relatively high Ba contents and positive Eu anomalies are considered to have originated in a feldspar-accumulitic layer. Phenocrysts in the trachytes are commonly heavily resorbed, inferred to be a result of heating by influxes of intermediate composition magma, which, however, were not erupted. It is argued that magmas of intermediate composition were present in the Green Tuff reservoir but were efficiently trapped in a crystal-rich layer below the Green Tuff magmas, which was eventually erupted during a resurgent phase as the Montagna Grande Trachyte.

AB - The Green Tuff Ignimbrite, Pantelleria, is compositionally zoned from pantellerite at the base to comenditic trachyte at the top, the variation apparently representing an inverted vertical zonation in the pre-eruptive reservoir. The main phenocryst assemblages are alkali feldspar + olivine + clinopyroxene + ilmenite + apatite in the trachytes and alkali feldspar + aenigmatite + clinopyroxene ± quartz in the rhyolites. Thermodynamic modelling indicates that the temperature range was ∼900–700°C, fO2 FMQ –1·5 to FMQ –0·5 (where FMQ is fayalite–magnetite–quartz buffer) and aSiO2 (relative to quartz saturation) 0·74–1·00. Melt water contents ranged from ∼1 wt % in the trachytes to ∼4 wt % in the pantellerites. Matrix glass analyses in the more evolved rocks are highly variable, showing that compositional layers in the upper parts of the reservoir, formed by fractional crystallization, were mixed during eruption, the proportion of rhyolitic to trachytic melts increasing towards the top of the reservoir. Some areas of glass have low Al2O3 contents (5·16–5·46 wt %) and high FeO* contents (9·66–10·02 wt %), making them the most evolved melts yet reported from Pantelleria. The new glass data reveal how whole-rock analyses do not truly reflect the complete range of melt compositions in the pre-eruptive reservoir. The trachytes contain >40% modal phenocrysts, which with relatively high Ba contents and positive Eu anomalies are considered to have originated in a feldspar-accumulitic layer. Phenocrysts in the trachytes are commonly heavily resorbed, inferred to be a result of heating by influxes of intermediate composition magma, which, however, were not erupted. It is argued that magmas of intermediate composition were present in the Green Tuff reservoir but were efficiently trapped in a crystal-rich layer below the Green Tuff magmas, which was eventually erupted during a resurgent phase as the Montagna Grande Trachyte.

U2 - 10.1093/petrology/egy095

DO - 10.1093/petrology/egy095

M3 - Journal article

VL - 59

SP - 2245

EP - 2272

JO - Journal of Petrology

JF - Journal of Petrology

SN - 0022-3530

IS - 12

ER -