Carbonate Assimilation at Merapi Volcano, Java, Indonesia: Insights from Crystal Isotope Stratigraphy

Abstract

Recent basaltic andesite lavas from Merapi volcano contain abundant, complexly zoned, plagioclase phenocrysts, analysed here for their petrographic textures, major element composition and Sr isotope composition. Anorthite (An) content in individual crystals can vary by as much as 55 mol% (An 40^95 ) across internal resorption surfaces with a negative correlation between high An mol% (470), MgO wt% and FeO wt%. In situ Sr isotope analyses of zoned plagioclase phenocrysts show that the 87 Sr/ 86 Sr ratios of individual zones range from 0Á70568 to 0Á70627.The upper end of this range is notably more radiogenic than the host basaltic andesite whole-rocks (50Á70574). Crystal zones with the highest An content have the highest 87 Sr/ 86 Sr values, requiring a source or melt with elevated radiogenic Sr, rich in Ca and with lower Mg and Fe. Recent Merapi eruptive rocks contain abundant xenoliths, including metamorphosed volcanoclastic sediment and carbonate country rock (calc-silicate skarns) analysed here for petrographic textures, mineralogy, major element composition and Sr isotope composition. The xenoliths contain extremely calcic plagioclase (up to An 100 ) and have whole-rock 87 Sr/ 86 Sr ratios of 0Á70584 to 0Á70786.The presence of these xenoliths and their mineralogy and geochemistry, coupled with the 87 Sr/ 86 Sr ratios observed in different zones of individual phenocrysts, indicate that magma^crust interaction at Merapi is potentially more significant than previously thought, as numerous crystal cores in the phenocrysts appear to be inherited from a metamorphosed sedimentary crustal source. This has potentially significant consequences for geochemical mass-balance calculations, volatile saturation and flux and eruptive behaviour at Merapi and similar island arc volcanic systems elsewhere.

Magma plumbing beneath Anak Krakatau volcano, Indonesia: evidence for multiple magma storage regions

“…The large compositional variation of plagioclase ), the sieve-like textures and complex zoning patterns are features commonly observed in basaltic and andesitic, subduction-related volcanic rocks, and there is a widely held view that they reflect crystallisation in a highly dynamic magma system (e.g. Tepley et al 1999;Troll et al 2004;Chadwick et al 2007). Sieve-like textures may develop as a result of crystallographic-controlled dissolution of crystals in a system where the particular plagioclase composition has become unstable.…”

Thermal weakening of the carbonate basement under Mt. Etna volcano (Italy): Implications for volcano instability

“…Lentini, 1982;Grasso and Lentini, 1982;Pedley and Grasso, 1992), Mt. Vesuvius (Bruno et al, 1998;Iacono-Marziano et al, 2009), the Colli Albani volcanic district (Chiodini and Frondini, 2001;Iacono-Marziano et al, 2007;Freda et al, 2008;Gaeta et al, 2009;Mollo et al, 2010a) and the Campi Flegrei volcanic district (D'Antonio, 2011) in Italy, Popocatépetl volcano (Goff et al, 2001) and the Colima volcanic complex (Norini et al, 2010), both Mexico, Yellowstone volcanic system, USA (Werner and Brantley, 2003) and Merapi, Indonesia (Chadwick et al, 2007;Deegan et al, 2010;Troll et al, 2012). It becomes clear therefore, that an important element of volcano stability must focus on the understanding of the potential thermally-induced weakening of rock representative of the carbonate successions present under active volcanoes.…”

“…The geochemical signature of magma-carbonate interaction can sometimes be seen within volcanic products, and is known as "carbonate assimilation" (e.g., Wenzel et al, 2002;Barnes et al, 2005;Gaeta et al, 2006;Piochi et al, 2006;Chadwick et al, 2007;Freda et al, 2008;Deegan et al, 2010;Mollo et al, 2010a). Recent petrological studies have documented in detail that magma contamination is marked by the overgrowth of Ca-rich phases (mainly calcic clinopyroxene) on primary minerals (essentially olivine); as a result, the magma becomes progressively depleted in silica and enriched in alkalis.…”

Multi-stage mixing in subduction zones: Application to Merapi volcano (Java island, Sunda arc)

“…A recent study of Merapi samples by Chadwick et al (2005) has provided evidence for crustal influence at the mineral scale. However, in the case of Java, a significant role of the crustal contamination on the wholerock isotopic compositions was previously dismissed (Alves et al, 1999(Alves et al, , 2002Gertisser and Keller, 2003a).…”