『Abstract
To understand the factors that control rare earth elements (REE)
release and fractionation during shale weathering, we investigate
the REE contents in solid (bedrock, regolith, stream sediments),
and natural waters (stream, and pore waters) from a first-order
catchment developed entirely on gray shales in central Pennsylvania,
USA. Up to 65% of the REE (relative to parent bedrock) is depleted
from the weathering profiles in the acidic and organic-rich soils
due to chemical leaching. In addition, newly formed fine particles
were also lost along with the down-slope movement of soil waters.
Weathering profiles on the south-facing slope show less depletion
of REE than those on the north-facing slope (33% vs. 45% on average).
We hypothesize that the difference degrees of REE depletion on
the two transects reflect a history of different chemical weathering
rates and possible different surface erosion rates, controlled
by contrasting slope aspect-induced microclimate conditions. In
addition, weathering profiles, natural waters and sequential extractions
all show a preferential removal of Middle REE (up to 22% more)
relative to Light REE and Heavy REE during shale weathering, due
to preferential release of MREE from rhabdophane.Furthermore,
the long-term phosphate mineral dissolution rates (e.g., rhabdophane)
were estimated at 10-15 to 10-14 mol m-2
s-1 under field conditions, based on REE depletion
profiles.
Strong positive Ce anomalies (average [Ce/Ce*]N value: 1.79) observed in the regolith, stream
sediments, and regolith extractions point to the fractionation
and preferential precipitation of Ce as compared to other REE,
due to the generally oxidizing conditions during release, transport,
and redistribution of REE in the surface and subsurface environments.
Positive Eu anomalies (average [Eu/Eu*]N
value: 1.30) observed in the natural waters of the catchment are
attributed to weathering of plagioclases in the shale bedrock.
This study highlights the use of REE as natural tracers for low-temperature
geochemical processes.
Keywords: Rare earth elements; Regolith; Pore water; Chemical
weathering; Phosphate mineral dissolution; Slope aspect』
1. Introduction
2. Geological setting
3. Samples, experimental protocol and analytical methods
4. Results
4.1. REE in parent materials and regolith
4.2. REE in stream sediments
4.3. REE in pore water and stream water
4.4. REE in sequential extractions
4.5. Evaluation of REE mobility: mass transfer coefficient τ
5. Discussion
5.1. Mobility of REE during shale weathering and regolith
formation
5.2. Effects of slope aspect on shale weathering
5.3. REE fractionation patterns: enhancement of MREE mobility
5.4. Phosphate mineral dissolution rates estimated from REE depletion
profiles
5.5. Ce and Eu anomalies in soils and oxides/hydroxides
6. Summary and conclusions
Acknowledgments
References