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.
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.
High‐temperature volcanic gas is widely considered to originate from ascending, mantle‐derived magma. In volcanic arc systems, crustal inputs to magmatic gases mainly occur via subducted sediments in the mantle source region. Our data from Merapi volcano, Indonesia imply, however, that during the April‐October 2006 eruption significant quantities of CO2 were added from shallow crustal sources. We show that prior to the 2006 events, summit fumarole gas δ13C(CO2) is virtually constant (δ13C1994–2005 = −4.1 ± 0.3‰), but during the 2006 eruption and after the shallow Yogyakarta earthquake of late May, 2006 (M6.4; hypocentres at 10–15 km depth), carbon isotope ratios increased to −2.4 ± 0.2‰. This rise in δ13C is consistent with considerable addition of crustal CO2and coincided with an increase in eruptive intensity by a factor of ∼3 to 5. We postulate that this shallow crustal volatile input supplemented the mantle‐derived volatile flux at Merapi, intensifying and sustaining the 2006 eruption. Late‐stage volatile additions from crustal contamination may thus provide a trigger for explosive eruptions independently of conventional magmatic processes.
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