Mineral reaction kinetics constrain the length scale of rock matrix diffusion

R A Wogelius,B Yardley,A Van Veelen,L P Field,R Metcalfe,G Carpenter,S Norris,T Lowe,A E Milodowski

doi:10.1038/s41598-020-65113-x

Abstract

Mass transport by aqueous fluids is a dynamic process in shallow crustal systems, redistributing nutrients as well as contaminants. Rock matrix diffusion into fractures (void space) within crystalline rock has been postulated to play an important role in the transient storage of solutes. The reacted volume of host rock involved, however, will be controlled by fluid-rock reactions. Here we present the results of a study which focusses on defining the length scale over which rock matrix diffusion operates within crystalline rock over timescales that are relevant to safety assessment of radioactive and other long-lived wastes. Through detailed chemical and structural analysis of natural specimens sampled at depth from an active system (Toki Granite, Japan), we show that, contrary to commonly proposed models, the length scale of rock matrix diffusion may be extremely small, on the order of centimetres, even over timescales of millions of years. This implies that in many cases the importance of rock matrix diffusion will be minimal. Additional analyses of a contrasting crystalline rock system (Carnmenellis Granite, UK) corroborate these results.

Highlights

Mass transport by aqueous fluids is a dynamic process in shallow crustal systems, redistributing nutrients as well as contaminants
Models of mass transport in fractured crystalline lithologies typically assumed that rock matrix diffusion (RMD) into the matrix led to lateral fluid flux, with diffusion perpendicular to the flow path potentially occurring on the meter to tens of meters length scale[1]
The studied specimens came from the MIU-3 borehole which was drilled by the Japan Nuclear Cycle Development Institute (JNC) near the town of Mizunami in Gifu Prefecture, central Honshu[21,22,23,24,25,26,27]

Summary

Introduction

Mass transport by aqueous fluids is a dynamic process in shallow crustal systems, redistributing nutrients as well as contaminants. Through detailed chemical and structural analysis of natural specimens sampled at depth from an active system (Toki Granite, Japan), we show that, contrary to commonly proposed models, the length scale of rock matrix diffusion may be extremely small, on the order of centimetres, even over timescales of millions of years. This implies that in many cases the importance of rock matrix diffusion will be minimal. Models typically assume that RMD will act to include a large volume of the lithology

Methods

Results

Conclusion