geochemistry; subduction; laser-ablation ICP-MS; isotopes; mantle rocks; serpentinites; metasomatism; global element cycling
Bretscher Annette, Hermann Joerg, Pettke Thomas (2018), The influence of oceanic oxidation on serpentinite dehydration during subduction, in Earth and Planetary Science Letters
, 499, 173-184.
Kessel R., Pettke T., Fumagalli P. (2015), Melting of metasomatized peridotite at 4–6 GPa and up to 1200 °C: an experimental approach, in Contrib. Mineral. Petrol.
, 169, 37.
Angiboust S., Pettke T., DeHoog J. C. M., Caron B., Oncken O. (2014), Channelized fluid flow and eclogite-facies metasomatism along the subduction shear zone, in J. Petrol.
, 55, 883-916.
Spandler C., Pettke T., Hermann J. (2014), Experimental study of trace element release during ultrahigh-pressure serpentinite dehydration, in Earth Planet Sci. Lett.
, 391, 296-306.
Scambelluri M., Pettke T., Rampone E., Godard M., Reusser E. (2014), Petrology and trace element budgets of high-pressure peridotites indicate subduction dehydration of pristine serpentinites (Cima di Gagnone, central Alps, Switzerland), in J. Petrol.
, 55, 459-498.
Kendrick M. A., Honda M., Pettke T., Scambelluri M., Phillips D. (2013), Halogen and noble gas systematics of ocean floor and ophiolitic serpentinites: implications for global volatile recycling, in Earth Planet Sci. Lett.
, 365, 86-96.
Aqueous fluids profoundly influence chemical and physical processes in subduction zones. Serpentinite dehydration is considered to represent the major water supply that eventually triggers magmatism at convergent plate margins, and residual rocks may return back to the mantle and thus recycle exogenous chemical components to the deep earth. Surprisingly, quantitative constraints from natural samples on the chemical composition of fluids released at specific serpentinite dehydration reactions at forearc and subarc depths have remained only scarce to date. Which and how much of the soluble elements inventory of arc magmas interpreted to originate from slab dehydration actually originates from subducted serpentinites as opposed to subducted sediments or altered oceanic crust has thus remained largely speculative.This PhD thesis aims at quantifying the chemistry of the brucite and antigorite dehydration reactions, including that of the liberated aqueous fluid phase, based on two prime field localities. Cerro del Almirez, Spain, records the antigorite-out reaction. Erro Tobbio, NW Italy, records the brucite-out reaction. The chosen approach emphasizes in-situ analytical techniques to analyze all reaction educt and product phases, most importantly Laser ablation ICP-MS, EPMA and SIMS, and is complemented by RAMAN identification of serpentine minerals in lower-grade rocks. LA-ICP-MS will be used to analyze bulk, high-P fluid inclusions trapped in eclogitic reaction product minerals. At an advanced stage of the project, SIMS will be employed for the quantification of "anionic" elements, notably Cl, S, F and possibly Br and I, to address the long-standing debate on the importance of halides for trace element mobility in the otherwise aqueous silicate fluids released at forearc to subarc depths.The Cerro del Almirez locality offers the investigation of another fundamental question, namely whether antigorite dehydration was triggered by external fluid influx (likely equilibrated with metasediments of the type cropping out nearby), or whether it represent an isograd and the prograde oli¬vine+orthopyroxene+ chlorite grew "in its own juice". Strontium and Pb isotope ratios of bulk rocks and prograde fluid inclusion fractions (bulk inclusion samples) will be employed to test the "fluid-from-sediment-infiltration" hypothesis, the answer to which has profound consequences for our understanding of what chemical signal may be released upon antigorite dehydration and, thus, what serpentinites may contribute to the slab signal at subarc depth.Chlorite in progressively subducted serpentinites is a relevant water carrier and may be the last hydrous phase to decompose along common subduction P-T gradients. Specific emphasis will be put on the thorough chemical characterization of chlorite for all the localities investigated to obtaining the trace element inventory fixed in chlorite with progressive subduction and to constraining the possible fluid fingerprint of chlorite devolatilization at subarc depth.The requested research will produce top-quality data efficiently and fill gaps in our understanding of the chemical cycle of serpentinite subduction, including refertilization of deep mantle domains. This should allow to much better constraining the impact of serpentinite dehydration in the fluid chemical cycling at subduction zones.