wAbstract
@We used small-angle and ultra-small-angle neutron scattering
(SANS/USANS) to characterize the evolution of nanoscale features
in weathering Rose Hill shale within the Susquehanna/Shale hills
Observatory (SSHO). The SANS/USANS techniques, here referred to
as neutron scattering (NS), characterize porosity comprised of
features ranging from approximately 3 nm to several micrometers
in dimensions. NS was used to investigate shale chips sampled
by gas-powered drilling (gsaprockh) or by hand-augering (gregolithh)
at ridgetop. At about 20 m depth, dissolution is inferred to have
depleted the bedrock of ankerite and all the chips investigated
with NS are from above the ankerite dissolution zone. NS documents
that 5-6 of the total ankerite-free rock volume is comprised
of isolated, intraparticle pores. At 5 m depth, an abrupt increase
in porosity and surface area corresponds with onset of feldspar
dissolution in the saprock and is attributed mainly to peri-glacial
processes from 15000 years ago. At tens of centimeters below the
saprock-regolith interface, the porosity and surface area increase
markedly as chlorite and illite begin to dissolve. These clay
reactions contribute to the transformation of saprock to regolith.
Throughout the regolith, intraparticle pores in chips connect
to form larger interparticle pores and scattering changes from
a mass fractal at depth to a surface fractal near the land surface.
Pore geometry also changes from anisotropic at depth, perhaps
related to pencil cleavage created in the rock by previous tectonic
activity, to isotropic at the uppermost surface as clays weather.
In the most weathered regolith, kaolinite and Fe-oxyhydroxides
precipitate, blocking some connected pores. These precipitates,
coupled with exposure of more quartz by clay weathering, contribute
to the decreased mineral-pore interfacial area in the uppermost
samples.
@These observations are consistent with conversion of bedrock
to saprock to regolith at SSHO due to: (1) transport of reactants
(e.g., water, O2) into primary pores and
fractures created by tectonic events and peri-glacial effects;
(2) mineral-water reactions and particle loss that increase porosity
and the access of water into the rock. From deep to shallow, mineral-water
reactions may change from largely transport-limited where porosoty
was set largely by ancient tectonic activity to kinetic-limited
where porosity is changing due to climate-driven processes.
Keywords: SANS/USANS; regolith; porosity; fractal dimension; clay
minerals; surface areax
Introduction
Neutron scattering
@Study site
@Weathered zone terminology
@Chemical weathering profile at SSHO
Methods
Results
@Scattering curves
@Nano-scale porosity
@Surface area
@Anisotropy
@Ferrous and total iron
@SEM and TEM images
Discussion
@Fractal dimensions and interfacial features
@Three types of pores
@A conceptual model for weathering
@Physical vs. chemical vs. biological weathering
Implications
Concluding remarks
Acknowledgments
References cited