Formation of juvenile arc crust at ocean-continent convergent margins is a driving process for continental growth. It initiates with loss of COH-bearing fluids-melts from subducting oceanic plates, is driven by fluid-melt mediated mass transfer into supra-subduction the mantle wedge and causes mantle metasomatism and melting. Magma underplating beneath the arc crust enhances anatexis and differentiation of the continental crust. In this scenario, uncertainties still concern the evolution of fluid-melt/rock interaction with the sub-arc mantle and with the deep arc crust through temperature, space and time, and regard also the following challenging aspects:
(a) the phase relations in slab and mantle wedge rocks in presence of fluids and melts bearing COH species at variable temperature and redox conditions;
(b) the grain scale mechanisms accounting for segregation and migration of fluids/melts;
(c) the melting and differentiation of the arc crust.
The project tackles the above uncertainties through 3 main scientific targets.
Aim: Establishing release of COH-bearing subduction-zone fluids and melts.
Goals: (i) Establishing the depths for fluid and melt release and the fluid/melt compositions based on the analysis of natural rocks, of fluid and melt inclusions and of synthetic systems.
(ii) Defining volatiles and trace element recycling at variable redox conditions.
Aim: Establishing the interaction of slab-derived fluids/melts with the mantle wedge.
Goals: (i) Assessing fluid and melt interaction with the supra-subduction mantle (cold plate interface and hot mantle wedge) at variable depths, environments and redox conditions.
(ii) Investigating the extent and kinetics of metasomatic fluid-melt/rock exchange as a function of temperature and supra-subduction mantle setting.
Aim: Defining the partial melting processes of the deep arc crust as the result of magma underplating.
Goals: (i) Characterizing the P-T-aH2O conditions during anatexis, the chemical (major, trace and volatiles) composition of crustal melts in the different rocks, the melt-related microstructures, and the petrophysical properties of deep crustal rocks with different composition.
(ii) Thermodynamic modeling of the above processes.
MATERIALS: To achieve the above targets, we plan to investigate (1) metasomatized, high and ultrahigh-pressure garnet peridotite (from Alps and Norway) representative of cold subduction interface domains, and (2) the xenolith suite from SW Colombia, representing the entire lithosphere column underneath an arc volcano and including metasomatized garnet-bearing xenoliths from the hot mantle wedge together with deep arc crust xenoliths. Results from natural samples are compared with laboratory experiments on phase relations and melt-rock reaction.
METHODS: Optical and Scanning Electron Microscopy; Microprobe mineral analysis and Bulk Rock analysis of major and trace element compositions; Rock texture and EBSD analysis; Classic thermobarometry coupled with Raman Spectroscopy for the analysis thermobarometry of mineral host-inclusion pairs: Coupled EBSD and TEM-EELS (Electron Energy Loss Spectroscopy) for Fe3+ distribution in inclusions and their hosts the redox state of rocks and inclusions; ICP-Ms laser ablation analysis for in-situ trace element distribution in minerals and SIMS spectrometry for isotope analysis.
Coordinator
University of
Genova
Responsible - RU
University of
Milano
Responsible - RU
University of
Padova
Responsible - RU
University of
Milano-Bicocca
Responsible - RU
University of
Pavia