Major Projects
3. Physics and chemistry of magma chamber processes
(Chadwick, O'Driscoll and Troll)
a) Feldspar zoning in alkali-feldspar of peralkaline ignimbrites on Gran Canaria
A critical issue in large silicic magma systems concerns the processes within the magma chamber and their ultimate driving forces. Due to the general problem of interpreting whole rock and groundmass chemical data derived from eruptive deposits (that have often suffered post-emplacement modification), phenocrysts may be the only reliable indicators that help us to understand the petrogenetic processes that magmas underwent prior to eruption.
Sequence of felsic peralkaline ignimbite deposits and lava flows of Miocene age on Gran Casnaria, Canary Island
We have evidence for large-scale convective magma mixing within a peralkaline felsic magma body on Gran Canaria. There, several discrete layers of magma have mutually exchanged phenocrysts which have been redistributed within the chamber with an overall tendency for crystals to be transported from the chamber top towards lower stratigraphic levels of the reservoir. This is best preserved in the feldspar phenocrysts of the involved end-member magmas which reveal several types of compositional zoning. Two oscillatory zoning types seen in the feldspars show evidence for magma mixing and convective crystal exchange in the form of partial to severe dissolution/resorption rims and distinct zones of drastically different composition. A 'step-cycle' model has been developed involving growth and transport of crystals into another magma batch followed by their return to the original host magma. The model is consistent with both stable and radiogenic isotope data.
A manuscript summarising these results has recently been published in Journal of Petrology (Troll and Schmincke, 2002), while two further manuscripts on these samples are currently in preparation.
Compositional spectrum of end-member magmas involved in ignimbrite "A" eruption on Gran Canaria is reflected in the range of different coloured fiamme (collapsed pumices) set in a fine grained ash matrix. Dark fiamme is trachyte, white and crème coloured fiamme a rhyolite. Ash matrix is a physical mixture of these components. The end-member magmas have experienced pre-eruptive magma mixing and crystal exchange
b) REE-minerals and their impact on the trace element budget of evolved magmatic liquids
Crystallisation of minerals from a magmatic liquid is known to partition elements between the solid and the liquid phase. For the major rock-forming minerals this process is fairly well contrained, however, for accessory minerals, and particularly for REE-bearing minerals, data are largely lacking. In a pilot project, we studied the accessory mineral chevkinite plus the coexisting titanite and the rhyolite glass in samples from Gran Canaria, using the Synchrotron-XRF-microprobe (SYXRF) at the Deutsches Elektronensynchrotron (DESY) in Hamburg, Germany.
This method allows determination of accurate elemental concentrations in the minerals and the glass for REE and trace elements, and partition coefficients (Kd) for these elements can be calculated. For example, a Kd (Ce) of ca. 850 was calculated for chevkinite and glass, suggesting that the crystallisation of only 0.05 wt% of this mineral from a melt with initially 300 ppm Ce would reduce the Ce concentration in the melt by about 100 ppm. This implies that chevkinite, if present, has a major impact on the LREE budget of evolved magmas. An article that summarises these findings has recently been published in Contributions to Mineralogy and Petrology (Troll et al. 2003).
c) In-situ isotope analysis of mineral components in volcanic rocks
To reconstruct the magmatic evolution of the explosive Merapi volcano, volcanoes of the BTIP, and andesite volcanoes in New Zealand, and to evaluate the mechanisms that trigger violent eruptions, we employ textural and geochemical analysis of the lavas and their inclusions. This is to establish a record of processes and events during magma storage that correspond to eruption triggers and to constrain mass fluxes in active subduction zone systems. The objectives of the project are:
::Isotopic and trace element analyses of individual components in the volcanic rocks to track the history of changing conditions through time during magma storage, and to evaluate the time-scales over which processes such as crystallisation, contamination and magma mixing may occur.
:: Integration of these data with textural information from the rocks, allowing us to identify "events" in the history of magma storage, which may correspond to eruption triggers.
:: Experiments and thermodynamic calculations to constrain the conditions (pressure/depth, temperature, volatile content) of magma storage, which control the style of eruption.
:: Modeling of results to provide useful generalisations that can be used to evaluate magma chamber processes and mass fluxes at Merapi and similar volcanoes elsewhere.
The project has recently attracted funding from the European Commission (EC) and from Science Foundation Ireland, and has become associated to the European ERUPT programme co-ordinated by Prof. J. Davidson in Durham, UK (see also list of funding). J. Chadwick has just started to work on the project as part of her PhD course, involving microdrilling and TIMS analysis in Durham, UK and in-situ trace element and Pb isotope analysis at the Danish Lithosphere Centre in Copenhagen, Denmark. Two firther postgraduates will commence work on the project in October 2004.
Zoned plagioclase crystal in dacite from Mt Pelee, Martinique. Note complex zoning and internal resorption surfaces within the crystals. This zoning pattern can be read by the petrologist like tree rings by the botanist
 |
 |
 |
 |
||
 |
 |