Oceanic slab melting and mantle metasomatism

Area of Science:

• Geology
• Tectonics
• Petrology

Background:

• Plate tectonics drives oceanic crust subduction, leading to dehydration or melting.
• Hydrous fluids and melts from subducting slabs migrate into the mantle wedge, triggering arc magma generation and continental crust formation.
• Modern slab melting is primarily observed in young (< 50 Ma) subducted plates.

Purpose of the Study:

• To investigate the geochemical characteristics of modern slab melts.
• To compare modern slab melts with Archean Trondhjemite-Tonalite-Granodiorite (TTG) rocks.
• To infer differences in petrogenetic pathways and tectonic models between modern and Archean crust formation.

Main Methods:

• Analysis of geochemical evidence in exposed slab melts.
• Comparison of melt compositions (silicic, sodic, trondhjemitic) with mantle peridotite interaction products.
• Evaluation of temperature and water conditions during melt generation.

Main Results:

• Modern slab melts are silicic, sodic (trondhjemitic), and form at low temperatures (< 1000°C) under water-excess conditions.
• Interaction with mantle peridotite alters slab melts, making them less silicic (dacitic to andesitic) and enriching them in Mg, Ni, and Cr.
• Geochemical signatures indicate significant mantle material ingestion in modern slab melts, contrasting with Archean TTG rocks.

Conclusions:

• Modern slab melts exhibit distinct geochemical properties compared to Archean TTG rocks.
• The petrogenetic pathways for modern and Archean crustal magmas likely differ significantly.
• This suggests that Archean crust production may have involved tectonic models distinct from present-day subduction processes.