Magma–Carbonate Interaction Processes and Associated CO2 Release at Merapi Volcano, Indonesia: Insights from Experimental Petrology

Abstract

There is considerable evidence for ongoing, late-stage interaction between the magmatic system at Merapi volcano, Indonesia, and local crustal carbonate (limestone). Calc-silicate xenoliths within Merapi basaltic-andesite eruptives display textures indicative of intense interaction between magma and crustal carbonate, and Merapi feldspar phenocrysts frequently contain individual crustally contaminated cores and zones. In order to resolve the interaction processes between magma and limestone in detail we have performed a series of time-variable de-carbonation experiments in silicate melt, at magmatic pressure and temperature, using a Merapi basaltic-andesite and local Javanese limestone as starting materials. We have used in-situ analytical methods to determine the elemental and strontium isotope composition of the experimental products and to trace the textural, chemical, and isotopic evolution of carbonate assimilation. The major processes of magmacarbonate interaction identified are: i) rapid decomposition and degassing of carbonate, ii) generation of a Ca-enriched, highly radiogenic strontium contaminant melt, distinct from the starting material composition, iii) intense CO 2 vesiculation, particularly within the contaminated zones, iv) physical mingling between the contaminated and unaffected melt domains, and v) chemical mixing between melts. The experiments reproduce many of the features of magmacarbonate interaction observed in the natural Merapi xenoliths and feldspar phenocrysts. The Carich, high 87 Sr/ 86 Sr contaminant melt produced in the experiments is considered as a pre-cursor to the Ca-rich (often "hyper-calcic") phases found in the xenoliths and the contaminated zones in Merapi feldspars. The xenoliths also exhibit micro-vesicular textures which can be linked to the CO 2 liberation process seen in the experiments. This study, therefore, provides well-constrained petrological insights into the problem of crustal interaction at Merapi and points toward the substantial impact of such interaction on the volatile budget of the volcano.

Pre- and syn-eruptive degassing and crystallisation processes of the 2010 and 2006 eruptions of Merapi volcano, Indonesia

“…Previous workers (e.g. Chadwick et al 2007;Deegan et al 2010;Troll et al 2012Troll et al , 2013Borisova et al 2013) have proposed that CO 2 liberation via crustal carbonate assimilation has the potential to sustain and intensify eruptions at Merapi. Monitoring data of the 2010 eruption indicate large CO 2 emissions prior to the 2010 eruption (Surono et al 2012), and previous melt inclusions have been interpreted to reflect CO 2 fluxing at Merapi (Nadeau et al 2013).…”

“…Clocchiatti et al 1982;Camus et al 2000;Gertisser and Keller 2003;Chadwick et al 2007;Deegan et al 2010;Troll et al 2012Troll et al , 2013. Magma-carbonate interaction liberates CO 2 through decarbonation reactions of crustal carbonates to the diopside and wollastonite assemblages observed in the xenoliths, adding to the magmatic volatile budget with the potential to sustain and intensify eruptions at Merapi (Deegan et al 2010;Troll et al 2012Troll et al , 2013.…”

“…In addition, crustal carbonate assimilation and the resulting CO 2 liberation has also been invoked to play a significant role in governing the explosivity of eruptions (Chadwick et al 2007;Deegan et al 2010;Troll et al 2012Troll et al , 2013Borisova et al 2013). Secondary ion mass spectrometry (SIMS) volatile data (CO 2 , H 2 O, F, Cl, S) of Merapi clinopyroxene and amphibole-hosted silicate melt inclusions in lava and scoria from unknown eruptive origins have recently been published by Nadeau et al (2013).…”

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.…”

Investigating the deepest part of a volcano plumbing system: Evidence for an active magma path below the western flank of Piton de la Fournaise (La Réunion Island)

“…Among released volcanic gases, carbon dioxide (CO 2 ) is early by an uprising magma, already at mantle depth and represents the most important contribution to diffuse degassing, whereas water (H 2 O), the most abundant magmatic volatile species, is released at shallower depths, and undergoes condensation both along its pathways toward the surface, and in the soil (Granieri et al, 2010;Di Muro et al, 2016 ). Besides magmatic degassing, soil CO 2 degassing at the surface may also be related to several other sources such as (1) mantle, (2) subducted crustal rocks and sediments, (3) carbonate rocks and, (4) biogenic components Deegan et al, 2010;Troll et al, 2012;Burton et al, 2013;Dionis et al, 2015). Carbon isotopic composition is a useful tool to discriminate between these different sources of CO 2 emissions and to reveal potential mixing among gases deriving from different sources.…”