CO2 and other volatiles in subduction zones: reducing the deficit by crustal-scale sequestration

Craig Manning

UCLA

Studies of volatile budgets in convergent margins show that more volatiles are subducted than are returned to the surface at volcanic arcs. The deficit, D, expressed as the ratio of annual mass degassed to mass subducted, ranges from <0.1 for sulfur, through ~0.2-0.3, for H2O, B, N, Cl, to ~0.6 for CO2. D<1 holds for all volatiles even after accounting for mid-ocean ridges, ocean-island basalts, and diffuse degassing on volcano flanks. This deficit has been used to calculate long-term volatile enrichment of the mantle or mediation of Earth’s volatile budget by extraterrestrial supply. However, such calculations ignore potential loss to the deep, subvolcanic crust. Hydrologic modeling and experimental data on the solubility of volatile- bearing minerals suggest that an actively metamorphosing suprasubduction-zone crustal column can accommodate sufficient subduction-derived volatiles to erase the “deficit.” Volatiles are liberated from the slab and transported to the mantle wedge by slab devolatilization. Experimental results on calcite, anhydrite and fluorite solubility in H2O-NaCl fluids demonstrate high solubilities in slab-derived fluids at high P [1-4], leading to volatile enrichment of the source region for arc magmas. Significant volatile loss from arc magmas may begin upon ponding at the base of the crust, or upon deep saturation with a multicomponent fluid phase. In tectonically active regions, the crustal column is permeable to fluid at all depths [5]. For example, crustal permeability during metamorphism is sufficient to transport not only fluids produced within the crust, but also the entire deficit in subducted H2O (~7x1014 g/yr [6]), assuming delivery by degassing of magma ponded at the base of the crust. Other volatiles may be carried by this fluid or lost to minerals precipitated along its path. The solubility of calcite, anhydrite and fluorite in H2O±NaCl decrease significantly as fluid moves upward in the crust, leading to precipitation and volatile sequestration in the crust. For example, calcite solubility in H2O increases by nearly 104 times from 1-20 kbar at 600 °C [1], yielding a strong potential for CO2 sequestration in the crust as CaCO3. This simple model is supported by isotopic evidence (O, C, Sr, He) for a volumetrically small but real involvement of mantle-equilibrated fluids in many metamorphic systems. Mediation of the terrestrial volatile budget by a crustal-scale fluid system in arc crust reduces the apparent deficit in volatiles returned from the mantle in subduction zones.

[1] Caciagli N.C. and Manning C.E. (2003) Contrib. Mineral. Petrol. 146, 275-285
[2] Newton R.C. and Manning C.E. (2002) Am. Mineral. 87, 1401-1409
[3] Newton R.C. and Manning C.E. (2005) J. Petrol., 46, 701-716
[4]Tropper P. and Manning C.E. (2007) Chem. Geol., 242, 299-306
[5] Manning, C.E. and Ingebritsen, S.E. (1999) Rev. Geophys. 37, 127-150
[6] Ingebritsen S.E. and Manning C.E. (2002) Proc. Nat. Acad. Sci. 99, 9113-9116