“…Martin, 1993) and allow for origin of the high-pressure TTG. The mafic gneisses, on the other hand, have geochemical characteristics similar to icelandites (Figures 6,7), a volcanic rock of basaltic andesite to andesite composition that is interpreted to have formed by fractionation of basaltic magma at low pressures (Carmichael, 1964;Grove and Kinzler, 1986), or mixing of basaltic and felsic magmas (Carmichael, 1964;Nicholson et al, 1991;Mancini et al, 2015). The average Th/Nb value of the mafic gneisses (0.12) is consistent with their origin in a within-plate setting (Figure 8).…”
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“…Martin, 1993) and allow for origin of the high-pressure TTG. The mafic gneisses, on the other hand, have geochemical characteristics similar to icelandites (Figures 6,7), a volcanic rock of basaltic andesite to andesite composition that is interpreted to have formed by fractionation of basaltic magma at low pressures (Carmichael, 1964;Grove and Kinzler, 1986), or mixing of basaltic and felsic magmas (Carmichael, 1964;Nicholson et al, 1991;Mancini et al, 2015). The average Th/Nb value of the mafic gneisses (0.12) is consistent with their origin in a within-plate setting (Figure 8).…”
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“…The petrogenetic processes responsible for the generation of felsic magmas in ocean island volcanoes built on thin oceanic crust have long been debated. Many authors suggested that felsic magma production at ocean island volcanoes could be a proxy for the initiation of continental crust formation in the Archean (e.g., Gazel et al, 2014;Mancini et al, 2015).…”
Section: Implications For the Generation Of Felsic Magmas On Ocean Ismentioning
confidence: 99%
“…Many ocean islands such as Iceland, Socorro, and the archipelagos of Hawaii, the Galapagos and the Canaries have been studied in detail (e.g. Sparks & Sigurdsson, 1987;Geist et al, 1988;Ablay et al, 1998;Geist et al, 1995;Bohrson et al, 1996;Harpp & White, 2001;Koppers & Staudigel, 2005;Carracedo et al, 2007;Carley et al, 2011;Mancini et al, 2015), but some ocean island volcanoes remain relatively poorly understood (e.g. the Azores, St Helena), and yet still pose a significant hazard to populations often living proximal to volcanic vents.…”
Ocean island volcanoes erupt a wide range of magmatic compositions via a diverse range of eruptive styles. Understanding where and how these melts evolve is thus an essential component in the anticipation of future volcanic activity. Here we examine the role of crustal structure and magmatic flux in controlling the location, evolution and ultimately composition of melts at Ascension Island. Located in the South Atlantic, Ascension Island is an ocean island volcano that has produced a continuum of eruptive compositions from basalt to rhyolite in its 1 Myr subaerial eruptive history. Volcanic rocks broadly follow a silica-undersaturated subalkaline evolutionary trend, and new data presented here show a continuous compositional trend from basalt through trachyte to rhyolite. Detailed petrographic observations are combined with in situ geochemical analyses of crystals and glass, and new whole-rock major and trace element data from mafic and felsic pyroclastic and effusive deposits that span the entire range in eruptive ages and compositions found on Ascension Island. These data show that extensive fractional crystallization is the main driver for the production of felsic melts for Ascension Island, a volcano built on thin, young, oceanic crust. Strong spatial variations in the compositions of erupted magmas reveal the role of a heterogeneous lower crust; differing degrees of interaction with a zone of plutonic rocks are responsible for the range in mafic lava compositions, and for the formation of the central and eastern felsic complexes. A central core of nested, small-scale plutonic, or mush-like, bodies inhibits the ascent of mafic magmas, allowing sequential fractional crystallization within the lower crust, and generating felsic magmas in the core of the island. There is no evidence for magma mixing preserved in any of the studied eruptions, suggesting that magma storage regions are transient, and material is not recycled between eruptions.
“… Figure 1 Compositional data of natural plagioclase obtained by EPMA. Grey symbols correspond to barren calc-alkaline, hollow symbols to hotspot-related, and red and green symbols to porphyry copper-related samples [FTC = Fish Canyon Tuff 42 ; LIP = Lipari 43 ; ADA = Southern Adamello 44 ; KPT = Kos Plateau Tuff 45 ; SRP = Central Snake River Plain 46 ; ACCWT and LHSR = Walcott Tuff and Lidy Rhyolite from Heise 47 ; HEID = Heidarspordur 48 ; BA = Bajo de la Alumbrera 49 , 50 ; BH = Batu Hijau 51 , 52 ]. The left error bars corresponds to the average 2 σ uncertainty, the right error bars to the maximum 2 σ uncertainty (measurements on standards excluded).…”
Measuring water contents of magmas is fundamental to resolving a number of geological questions, such as the mechanisms of silicic magma evolution, the triggering of volcanic eruptions, and the formation of porphyry copper deposits. This study focuses on the correlation between apparent deviations from stoichiometry of plagioclase crystals and high water concentration in the magmatic melt from which they grew. We considered this relationship as a potential geo-hygrometer (water activity indicator). To test and potentially calibrate this new technique, a range of natural and experimental plagioclase crystals were analysed, with particular care taken to identify and avoid analytical bias and artefacts. In contrast to recently published material, we found no systematic aluminium excess in plagioclase, irrespective of the water concentration of the silicate melt it crystallised from. This suggests that aluminium excess in plagioclase cannot serve as a geo-hygrometer. The high likelihood of misinterpreting analytical artefacts (due to alkali migration and imprecise standardisation) as small deviations from stoichiometry, also requires its application as a mineral exploration tool to be treated with caution.
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