Publication Date:
2022-06-24
Description:
We investigate the chemical budget of subduction zones at sub‐solidus conditions using a thermodynamic‐numerical simulation in which all major rock components are treated as soluble and potentially mobile in aqueous fluids. This new strategy significantly improves the accuracy of predicted fluid‐rock equilibrium compositions in open petrological systems. We show that all slabs release volatiles and nonvolatiles to the mantle wedge, contributing to its refertilization. But some mobile constituents, such as alkali and alumina, may be trapped along layer boundaries or traverse without interaction depending on chemical contrasts between adjacent lithologies. The accumulation of igneous alumina and silica in the limestones of the central‐eastern Pacific slabs drives their decarbonation and is marked by metasomatic garnet growth. Those slabs are also predicted to lose much of their alkalis before sub‐arc depth. Even when they are produced in the altered mafic and ultramafic layers, fluids reach the slab/mantle wedge interface with distinct compositional signatures that are typical of the sedimentary cover. We distinguished supply and transport limited regimes of element subduction by testing the sensitivity of our mass balance to changes in slab hydration state (HS). Transport limited slabs sensitive to HS include notably a hotspot of carbon release to the mantle wedge (e.g., Costa Rica). Finally, we show that the quantitative budgets do depend on the geometry of fluid flows, and on assuming that slabs are mechanically continuous structures, which is questionable. Taken together, these insights will help better constrain the long‐term chemical evolution of the shallow planetary interior, and the thermomechanical behavior of the subduction interface.
Description:
Plain Language Summary:
Subduction zones return chemical elements from the surface to the deep Earth. But quantifying this transfer has been challenging. Here, we present a model where all major elements are partly mobile in the fluid phase, enabling us to compile a chemical budget for subduction zones in which only fluids mediate mass transport. We identify transport and supply regimes of element subduction as a function of lithospheric hydration state, rock compositions, and slab temperature. We show that the transport of many rock‐forming elements such as SiO2, CaO, and Al2O3, within and out of the slab, modifies rock composition and contributes to the efficiency of slab decarbonation. Our model of subduction fluid and rock compositions has important implications to understand the role of slab‐derived metasomatic fluid in modifying the composition of the mantle wedge over time, the mechanical properties of deeply subducted rocks, and it will inform future investigations for the high‐pressure petrology of rocky planets in general.
Description:
Key Points.
Inter dependence of element transfers in subduction zones.
Thermodynamics of intra‐slab metasomatism of major elements.
Transport‐ and supply limited regimes of carbonate subduction.
Description:
Alexander von Humboldt‐Stiftung (Humboldt‐Stiftung)
http://dx.doi.org/10.13039/100005156
Description:
Branco Weiss Fellowship—Society in Science
Description:
Swiss National Science Foundation
Description:
https://osf.io/y84d2/
Keywords:
ddc:551
Language:
English
Type:
doc-type:article
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