Melt Focusing Along Permeability Barriers at Subduction Zones and the Location of Volcanic Arcs. (4th December 2020)

• Record Type: Journal Article
• Authors: Ha, Goeun; Montési, Laurent G. J.; Zhu, Wenlu
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 21:Number 12(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-12-04
• Subjects: mantle wedge -- melt focusing -- numerical modeling -- permeability barrier -- subduction zone -- volcanic arc; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2020GC009253
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: Fluids released from dehydration reactions occurring in subducting slabs trigger partial melting in the mantle wedge. The resulting magma rises through the overlying mantle wedge and lithosphere and forms arc volcanoes at subduction zones. In general, the location of the volcanic arc is narrowly defined even though the melting region in the mantle wedge can be broad. We propose here that a thermally controlled low permeability barrier at the base of lithosphere is able to focus melts to the place where the volcanic arc is actually observed. As the melt ascends, the permeability of the mantle rock decreases as a result of melt crystallization. A low permeability barrier may form in the cooler lithosphere and can trap ascending melt and redirect it laterally according to the slope of the permeability barrier, so that the ascending melt is focused at the apex of the permeability barrier. We model the location and shape of isotherms that approximate the permeability barriers in the mantle wedge based in two‐dimensional numerical subduction models that follow the specific geometry of various subduction zones. In 28 of 31 globally distributed test regions, the arc locations estimated from our model show good agreement with the actual arc locations. The modeling results indicate that volcanic arcs can be explained as the surface projection of the apex of the permeability barrier, regardless of the distribution of melt deeper in the mantle wedge. Plain Language Summary: Arc volcanoes are the source of numerous natural hazards as well as many of the gases that form our atmosphere. Despite their importance, it is not yet clear what controls their exact location within the broader geodynamical context of subduction zones. Magma erupted at arc volcanoes may be generated over a broad region in the mantle where water released from the subducting slab can enter. At the surface, though, the arc itself is quite narrow, implying that there is a mechanism collecting and focusing deeply generated magma. We suggest that magmas are guided by a low permeability barrier that forms near the base of the lithosphere where rising magma crystallizes and clogs the passageways through which it is traveling. We show that the location where magma is expected to pool along the permeability barrier matches the actual volcanic arc location at 28 of 31 globally distributed subduction zones. Therefore, it is the temperature structure of the upper plate in subduction zone and the resulting shape of the permeability barrier, not the details of where the slab releases water or the trajectory of fluids rising through the mantle wedge, that controls where volcanic arcs are located. Key Points: Location of permeability barrier at subduction zones is predicted using the numerically calculated thermal structure in the mantle wedge Melt flow along the permeability barrier focuses melt produced over a wide region onto a more specific location In 28 of 31 locations, the location of the arc is successfully predicted by the apices of the isotherms forming the permeability barrier