Experimental investigation of C-O-H volatile effects at the subcontinental lithosphere-asthenosphere boundary

Abstract

This study presents an experimental simulation of interactions between two distinct lithologies, representing the contact zone between the asthenosphere and subcontinental lithosphere. These compositions were layered in the capsule (sandwich runs) and subjected to pressures and temperatures representative of two subcontinental mantle of 75 and 130 km, corresponding to Phanerozoic lithosphere thicknesses. The temperatures range from 900°C to 1450°C to simulate different metasomatic reactions and fusion processes in normal geothermal environments and anomalous conditions of high potential temperatures. The experiments were performed using a belt-type high-pressure-high-temperature apparatus, using toroidal pressure plates. Compositions were prepared from pure oxides, carbonates, and hydroxides.

The asthenospheric representative layer is a mixture of fertile lherzolite (MPY) enriched with 30% eclogite (GA1) and 0.75 wt.% CO₂. The lithospheric representative layer consists of NHD, a depleted lherzolite metasomatized with 0.8 wt.% H₂O and 0.17 wt.% K₂O. These compositions have been used in previous experimental studies, enabling direct comparison of our results with those from simpler compositional systems.

The results confirm that small amounts of C-O-H volatiles significantly lower the melting point of peridotite. Melting begins at 900°C at 2.5 GPa and at 1050°C at 4.5 GPa. Amphibole stability is observed up to 4.5 GPa, demonstrating the lithosphere's substantial capacity to retain water when interacting with enriched asthenospheric compositions, likely influenced by prior subduction events. Carbon remains dissolved in carbonate minerals at 4.5 GPa up to 1050°C. As the temperature increases, carbon transitions from being dissolved in the melt to the vapor phase. Liquid compositions are basanitic at low melt fractions and evolve to trachyandesitic above 1200°C.

This research focuses on metasomatic reactions involving fluids and melts generated under adiabatic decompression. Insights contribute to understanding magmatic processes at rift systems in supercontinent cycles, where ancient lithospheric plates accumulate volatiles. Additionally, this study advances the understanding of mantle geodynamics by examining water and carbon storage in mineral phases and the dehydration and decarbonation reactions observed experimentally.

Keywords: experimental petrology, mantle metasomatism, volatile geodynamics, high-pressure melts.