Volatile und Fluide in Subduktionszonen

Theme C: Slab-arc-atmosphere transfer

The four theme C subprojects investigate the transfer of volatiles and fluids from the subducting plate through the mantle wedge and crust into the atmosphere by metamorphic and magmatic processes, the quantification of magmatic and volatile fluxes, the influence of volatiles on volcanic eruptions and resulting hazards, and finally the impact of volcanic volatiles released into the atmosphere on the past and present global climate.

Metamorphic transformations, such as the gabbro-to-eclogite transformation, are one of the major engines for the subduction process and intimately linked to seismic activity. In phases I and II of SFB 574, subproject C1 studied both dehydration and hydration reactions in the subducting plate. It was demonstrated that the fluids involved are able to mobilize significant amounts of trace elements, which may then contribute to the typical slab signature found in arc magmatic rocks. However, the source of these fluids is not entirely clear, and has been preliminarily allocated to the subducted serpentinized mantle which is believed to play a significant role in fluid and element transfer through subduction zones. In phase III, subproject C1 will focus on fluid circulation in deeper (>100 km) zones of the subducting slab including the serpentinized mantle. The use of relatively novel geochemical tracers will be combined with the experimental determination of solid-melt-fluid partition coefficients to quantify contributions from the different parts (sediments, basaltic crust, mantle) of the slab. In particular, the complex role of accessory phases in influencing the trace element budgets will be evaluated with the experimental approach. Theoretical predictions from experiments and geochemical constraints from field-based observations on magma source compositions will provide important constraints for the interpretation of primitive melt inclusions being studied in subproject C2. C1 is also closely linked to subproject B5, which provides important constraints on the availability of elemental species during subduction, and with which a close cooperation for halogen and Cl isotope analyses is envisaged.

Fluids and volatiles released from the subducting plate variably contaminate the overlying mantle wedge with their elemental load and trigger melting, generating arc volcanism at convergent margins. Melts formed in the mantle wedge transfer subducted components, including volatiles, to the crust and into the atmosphere. Subproject C2 investigates variations in the chemistry of primitive subduction-related magmatic rocks in space (along and across the arc) and time to evaluate the cycling of elements through the subduction system, in particular volatile fluxes from the subducting plate to the atmosphere together with subprojects C1 and C4. Olivine-hosted melt inclusions best preserve the compositional diversity and primary volatile contents of primitive arc melts. In Central Chile - a continental endmember subduction system, subproject C2 will evaluate the role of a short melting column (thin mantle wedge) beneath the volcanic arc situated on continental crust of variable thickness in order to constrain the influence of variations in the depth of melting, expected low degrees of partial melting and pyroxenite on the composition, especially volatile budgets, of the Chilean compared to the Central American arc melts. In addition, C2 will determine the contributions of slab and mantle sources to the volatile contents of these melts. Possible compositional changes from the Pliocene to the present will elucidate if the change from erosional to accretionary subduction 1-2 Ma ago had a significant effect on the chemistry, specifically the volatile inventory, of the arc volcanic rocks. As magmas rise through the thick continental crust, they differentiate and, as shown by previous studies, can suffer extensive crustal contamination. The study of the more evolved arc volcanic rocks by subproject C4 aims to determine how magmatic differentiation and crustal contamination modify the magmatic volatile inventory and to quantify the volatile contributions from the different sources. Temporal variations in differentiation processes and volatile inventories at individual volcanic systems will reveal how (un)steady these processes are, which is essential to determine long-term average characteristics of the arc magmatism. Another aim is to identify how these effects vary with crustal tectonic structure as well as with depth and temperature of the asthenospheric mantle wedge, parameters that we have shown to control magma production rates at Central America. Moreover, C4 investigates the more evolved magmas because these tend to produce the most highly explosive eruptions - depending on their volatile contents - that immediately carry their volatile load into the stratosphere and thus have the most direct climatic and global atmospheric effects. Studying the deposits of such large eruptions will also contribute to the assessment of the volcanic hazards in the work areas. The role of external factors in controlling eruptive behavior has become a major subject in research on volcanic hazards and the selected work areas in Chile provide an excellent opportunity for C4 to investigate relationships between tectonic structures and stresses, seismic activity, and volcanic eruption styles, magnitudes and frequencies in collaboration with the A2, A5, A6 and C2 projects and SERNAGEOMIN.

Projects C2 and C4 will closely collaborate in the same work areas, because knowledge of the mafic compositions is essential to understand differentiation to more evolved compositions while identification of the continued differentiation processes helps to understand compositional variations in the mafic rocks. Results from both C2 and C4 studies need to be combined in order to completely determine the volcanic volatile fluxes through the arc and into the atmosphere. A major emphasis of C2 and C4 during phase III will be to put together a global synthesis of volatile fluxes through subduction zones, which will also serve as a basis for modelling the global impact of arc volatiles on the climate in C5.

Volatile fluxes into the atmosphere by volcanic eruptions, initially from Central America and later from central Chile, are used in subproject C5 to model their effects on global climate taking into account atmospheric composition, e.g. the ozone hole. Both regions investigated by the SFB provide source data for different latitudes. C5 will conduct climate modelling over both short (years) and long (100 000 years) time scales and will investigate the effects of volcanic emissions into past, present and projected future atmospheres, such as adsorption of radiation and depletion of stratospheric ozone by halogen compounds. Another objective of C5 is to model the volatile cycling (H2O, CO2, S, halogens) through the entire subduction system using thermodynamic and fluid-dynamic parameters and the empirical results from all other SFB projects partly as input data and partly to verify numerically obtained results. Project C5, together with Z2 using data base techniques, will integrate the results obtained by the individual subprojects in the different work areas, considering also results from other studies (e.g. US MARGINS programme) for other subduction settings, with the ultimate goal of obtaining a consistent model for the volatile turnover in the global subduction system.

Figure C-1: Schematic diagram of a volcano-magma system. The return of magmatic volatiles (devolatilization) to the hydrosphere and atmosphere depends on the interaction between mantle and crustal sources, petrogenetic processes, and type and quantity of volatiles present in the various reservoirs. The transport of volatiles during emission may be by (a) fluids from subducted sediments; (b) fluids originating from altered and subducted oceanic crust; (c) fluid-rich magmas from remelting of sediments, and d) fluid-rich magmas from melting of altered crust. Metamorphic reactions play a major role in transforming the volatile reservoirs. Subproject C1 looks at the pattern of volatile release to the mantle wedge from metamorphic reactions; C2 analyzes the volatile contents of different crustal end-members, and C4 quantifies the magmatic devolatilization through eruptive processes. Subproject C5 integrates all these processes for quantitative modeling of element cycling through subduction zones; and, in particular, modeling of climate effects by arc-volcano volatile discharges and their feedback mechanisms.