Quantifying Water Diffusivity and Metamorphic Reaction Rates Within Mountain Belts, and Their Implications for the Rheology of Cratons. (30th October 2021)

• Record Type: Journal Article
• Authors: Whyte, A. J.; Weller, O. M.; Copley, A. C.; St‐Onge, M. R.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 22:Number 11(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-10-30
• Subjects: metamorphism -- tectonics -- reaction rates -- fluid diffusion -- rheology; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2021GC009988
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: The distribution of rheologically strong cratons, and their weakening by metamorphic hydration reactions, is of fundamental importance for understanding first‐order strength contrasts within the crust and the resulting controls on the tectonic evolution of the continents. In this study, the Douglas Harbor structural window within the Paleoproterozoic Trans‐Hudson orogen of Canada is used to study the hydration of the footwall Archean Superior craton basement by water released from the overlying Paleoproterozoic Cape Smith thrust‐fold belt. Phase equilibria modeling is applied to quantify the Archean and Paleoproterozoic metamorphic conditions, and to determine the effect of hydration on basement mineralogy. The amount of structurally bound water within the basement is calculated and shown to decrease as a function of distance below the basal décollement of the thrust‐fold belt. Applying a reactive fluid transport model to these results, the rate coefficient for fluid‐rock reaction is constrained to be 10 − 19 m o l − 1 m 3 s − 1, and the diffusivity of water through the grain boundary network to be 10 − 9 m 2 s − 1 at the ambient metamorphic conditions of 570 ° C and 7.5 kbar. This newly documented rate of water diffusion is three orders of magnitude slower than thermal diffusion, implying that hydration by diffusion may be the rate‐limiting factor in the weakening of cratons, and therefore plays an important role in their geological persistence. This conclusion is consistent with field observations that Paleoproterozoic strain in the Douglas Harbor structural window is restricted to hydrated portions of the Archean Superior craton basement. Plain Language Summary: The ancient cores of continents are highly durable and have persisted for billions of years, in part due to the "dry" mineral assemblages they contain. The distribution of ancient continental cores is important in controlling how mountain ranges form and evolve, and where earthquakes occur. Therefore, it is important to understand how continental cores can be altered by geological processes. In this study, we analyze how continental cores can be weakened through the addition of water during mountain‐building and the formation of "wet" minerals. We do this by analyzing a 2 billion‐year old mountain belt in Arctic Canada, where the transition from "wet" to "dry" rocks is now exposed at the surface due to uplift and erosion from ∼ 20 km depth. By analyzing samples from across the region, and calculating their water contents, we are able to model the distribution of water and place constraints on the rate of fluid movement, and of fluid‐rock reaction. Our results show that the rates are very slow, which contributes to the geological persistence of the dry continental cores, and influences the behavior of the mountain belts that are formed when continents collide. Key Points: The Trans‐Hudson orogen in Arctic Canada is used to investigate the hydration and weakening of cratonic basement during mountain‐building The fluid‐rock reaction rate coefficient is 10 − 19 m o l − 1 m 3 s − 1 and the diffusivity of water in the grain boundary network is 10 − 9 m 2 s − 1 The rate of water diffusion is slower than thermal diffusion, and rate‐limiting for the weakening of cratons in orogenic settings