Outgassing of Mantle Volatiles in Compressional Tectonic Regime Away From Volcanism: The Role of Continental Delamination

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2019-04, Vol.20 (4), p.2007-2020
• Main Authors: Caracausi, Antonio, Sulli, Attilio
• Format: Article
• Language: English
• Subjects: Advective flow; Advective transport; Basins; Carbon dioxide; Continental crust; Cutting; Degassing; delamination; Depth; Earth; Earth mantle; Fault lines; Fluids; Geochemistry; Geology; Geophysical data; Geophysics; Heat; Helium; Isotopes; Lava; Magma; mantle volatiles; mantle wedge; output CO2; Plate tectonics; Subduction; Systematics; Tectonics; Transport; Volcanic activity; Volcanism
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2018GC008046

In this study we discuss the occurrence of mantle‐derived heat and volatiles (i.e., helium and CO2) feeding hydrothermal systems in a seismically active margin between two convergent plates (African and European) without any signals of volcanism. The helium (He) isotopes clearly indicate a mantle‐derived component in the outgassing volatiles. The estimated mantle‐derived He fluxes are up to two to three orders of magnitude greater than those in a stable continental area. Such high He fluxes cannot be provided by a long‐lasting diffusion, thereby implying a more efficient transport (i.e., advective transport through faults). He data coupled to heat‐He relationship suggest the occurrence of active degassing of magmatic intrusions in this area of continental collisional. Geophysical data indicate the presence of a hot mantle wedge below the outgassing of mantle volatiles and a system of faults cutting the continental crust down to the hot mantle wedge. Here we discuss the hot mantle wedge and possible associated magmatic intrusions as the source of the mantle‐derived volatiles outgassing in the region. We also assessed the output of mantle‐derived CO2 from the investigated hydrothermal basins. The possible occurrence of magma at depth as well as the geometry of the thick‐skinned deformed wedge unambiguously indicates delamination processes that are related to continental subduction. Hence, we show that delamination processes can really produce magma at depth without evidences of volcanism at the surface. Finally, we have also provided the fault systems that work as a network of pathways and actively sustain the advective transfer of the mantle fluids toward the surface. Plain Language Summary Volatiles from the Earth's mantle escape into the atmosphere mainly in volcanic districts and in submarine regions where new magma reaches the oceanic bottom. A lesser extent occurs in continental regions undergoing active tectonics. How mantle volatiles degas in continental regions and where evidences of volcanic activity are lacking remain a key challenge, given that in absence of magma the mantle would not lose its volatiles. Here we use the He‐heat systematics and recognize the presence of mantle volatiles in a continental region where there is no evidence of volcanism on the surface. This is a rare case of active outgassing of fluids coming from the mantle in a region that is characterized by continental collision. These geochemical evidences support the occurrence