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

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 CO2 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 87Sr/86Sr 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 CO2 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

Strontium isotope systematics of experimentally produced melts: understanding magma-carbonate interaction at Merapi volcano, Indonesia

There is considerable evidence for ongoing, late-stage interaction between the magmatic system at Merapi volcano, Indonesia, and local crustal carbonate. In order to resolve the interaction processes in detail, we have performed a series of time-variable carbonate dissolution experiments in silicate melt using Merapi basaltic-andesite and local limestone as starting materials, at magmatic pressure and temperature. Major element profiling of the experimental products has identified strongly contrasting compositional domains of glass: a Ca-enriched zone containing up to 36 wt% CaO, and an unaffected, Ca-normal zone containing 8 to 10 wt% CaO. To investigate the systematics of strontium isotopes and trace elements (TE) during carbonate assimilation, we have used micro-sampling and high-precision analytical techniques to measure 87Sr/86Sr ratios and TE concentrations over the magma-carbonate and intra-melt interfaces in two of our experimental products. The isotope variation between the different glass compositions is distinct, with 87Sr/86Sr ranging from 0.705641 in the Ca-normal glass to 0.706532 in the Ca-enriched glass. The upper end of this range is considerably more radiogenic than the range reported for Merapi whole rock volcanic products (0.70501 to 0.70583, Gertisser & Keller, 2003 J Pet, 44, 457-489). Our data hence support a model of assimilation of crustal carbonate with highly radiogenic 87Sr/86Sr (0.708799) at Merapi volcano. Given that the starting materials used in the experiments have markedly distinct 87Sr/86Sr values we here present new and detailed insights about the behaviour of Sr isotopes during carbonate assimilation, with a focus on the processes that operate across the carbonate-melt interface and the intra-melt transitions. Strontium is a reliable tracer of magma-crust interaction and so we anticipate that our results will significantly help to quantify our comprehension of magma-carbonate interaction processes occurring at Merapi volcano

Experimental simulation of magma–carbonate interaction beneath Mt. Vesuvius, Italy

We simulated the process of magma–carbonate interaction beneath Mt. Vesuvius in short duration piston-cylinder experiments under controlled magmatic conditions (from 0 to 300 s at 0.5 GPa and 1,200 °C), using a Vesuvius shoshonite composition and upper crustal limestone and dolostone as starting materials. Backscattered electron images and chemical analysis (major and trace elements and Sr isotopes) of sequential experimental products allow us to identify the textural and chemical evolution of carbonated products during the assimilation process. We demonstrate that melt–carbonate interaction can be extremely fast (minutes), and results in dynamic contamination of the host melt with respect to Ca, Mg and 87Sr/86Sr, coupled with intense CO2 vesiculation at the melt–carbonate interface. Binary mixing between carbonate and uncontaminated melt cannot explain the geochemical variations of the experimental charges in full and convection and diffusion likely also operated in the charges. Physical mixing and mingling driven by exsolving volatiles seems to be a key process to promote melt homogenisation. Our results reinforce hypotheses that magma–carbonate interaction is a relevant and ongoing process at Mt. Vesuvius and one that may operate not only on a geological, but on a human timescale

Strontium isotope systematics of experimentally produced melts: understanding magma-carbonate interaction at Merapi volcano, Indonesia

There is considerable evidence for ongoing, late-stage interaction between the magmatic system at Merapi volcano, Indonesia, and local crustal carbonate. In order to resolve the interaction processes in detail, we have performed a series of time-variable carbonate dissolution experiments in silicate melt using Merapi basaltic-andesite and local limestone as starting materials, at magmatic pressure and temperature. Major element profiling of the experimental products has identified strongly contrasting compositional domains of glass: a Ca-enriched zone containing up to 36 wt% CaO, and an unaffected, Ca-normal zone containing 8 to 10 wt% CaO. To investigate the systematics of strontium isotopes and trace elements (TE) during carbonate assimilation, we have used micro-sampling and high-precision analytical techniques to measure 87Sr/86Sr ratios and TE concentrations over the magma-carbonate and intra-melt interfaces in two of our experimental products. The isotope variation between the different glass compositions is distinct, with 87Sr/86Sr ranging from 0.705641 in the Ca-normal glass to 0.706532 in the Ca-enriched glass. The upper end of this range is considerably more radiogenic than the range reported for Merapi whole rock volcanic products (0.70501 to 0.70583, Gertisser & Keller, 2003 J Pet, 44, 457-489). Our data hence support a model of assimilation of crustal carbonate with highly radiogenic 87Sr/86Sr (0.708799) at Merapi volcano. Given that the starting materials used in the experiments have markedly distinct 87Sr/86Sr values we here present new and detailed insights about the behaviour of Sr isotopes during carbonate assimilation, with a focus on the processes that operate across the carbonate-melt interface and the intra-melt transitions. Strontium is a reliable tracer of magma-crust interaction and so we anticipate that our results will significantly help to quantify our comprehension of magma-carbonate interaction processes occurring at Merapi volcano.PublishedDavos, Switzerland2.3. TTC - Laboratori di chimica e fisica delle rocceope

Dykes and structures of the NE rift of Tenerife, Canary Islands: a record of stabilisation and destabilisation of ocean island rift zones

International audienceMany oceanic island rift zones are associated with lateral sector collapses, and several models have been proposed to explain this link. The North-East Rift Zone (NERZ) of Tenerife Island, Spain offers an opportunity to explore this relationship, as three successive collapses are located on both sides of the rift. We have carried out a systematic and detailed mapping campaign on the rift zone, including analysis of about 400 dykes. We recorded dyke morphology, thickness, composition, internal textural features and orientation to provide a catalogue of the characteristics of rift zone dykes. Dykes were intruded along the rift, but also radiate from several nodes along the rift and form en échelon sets along the walls of collapse scars. A striking characteristic of the dykes along the collapse scars is that they dip away from rift or embayment axes and are oblique to the collapse walls. This dyke pattern is consistent with the lateral spreading of the sectors long before the collapse events. The slump sides would create the necessary strike-slip movement to promote en échelon dyke patterns. The spreading flank would probably involve a basal decollement. Lateral flank spreading could have been generated by the intense intrusive activity along the rift but sectorial spreading in turn focused intrusive activity and allowed the development of deep intra-volcanic intrusive complexes. With continued magma supply, spreading caused temporary stabilisation of the rift by reducing slopes and relaxing stress. However, as magmatic intrusion persisted, a critical point was reached, beyond which further intrusion led to large-scale flank failure and sector collapse. During the early stages of growth, the rift could have been influenced by regional stress/strain fields and by pre-existing oceanic structures, but its later and mature development probably depended largely on the local volcanic and magmatic stress/strain fields that are effectively controlled by the rift zone growth, the intrusive complex development, the flank creep, the speed of flank deformation and the associated changes in topography. Using different approaches, a similar rift evolution has been proposed in volcanic oceanic islands elsewhere, showing that this model likely reflects a general and widespread process. This study, however, shows that the idea that dykes orient simply parallel to the rift or to the collapse scar walls is too simple; instead, a dynamic interplay between external factors (e.g. collapse, erosion) and internal forces (e.g. intrusions) is envisaged. This model thus provides a geological framework to understand the evolution of the NERZ and may help to predict developments in similar oceanic volcanoes elsewhere