『Abstract
The primary objective of this research was to investigate the
effects of aliphatic and aromatic low molecular weight organic
acids (LMWOAs) on rare earth element and yttrium (REY) release
from the phosphate minerals apatite and monazite. Since prior
studies have shown that redox status can affect REY partitioning
during incongruent dissolution, a secondary objective was to assess
the influence of dissolved O2 concentration.
Increasing LMWOA concentrations from 0 to 10 mM resulted in enhanced
REY release. In general, REY release increased in the order: no
ligand≒salicylate＜phthalate≒oxalate＜citrate. REY-ligand stability
constants were only useful for predicting REY release for oxalate
reacted with apatite and phthalate reacted with monazite. The
role of dissolved oxygen in dissolution of the phosphate minerals
was mixed and inconsistent. Mineral type was observed to significantly
affect REY pattern development. REY release patterns for apatite
range from nearly flat to those exhibiting the lanthanide contraction
effect (radius-dependent fractionation); whereas, monazite REY
release patterns are best described as exhibiting an M-type lanthanide
tetrad effect (radius-independent fractionation). Weathering of
apatite in the presence of aliphatic LMWOAs resulted in development
of the lanthanide contraction effect fractionation pattern, and
the aliphatic LMWOAs further developed MREE and radius-independent
fractionation during monazite dissolution. Geochemical and mineral-specific
REY signatures may, therefore, have utility for distinguishing
the impacts of biota on soil weathering processes on early Earth.
The development of such signatures mat be mitigated, in part,
by accessory mineral composition, the types and concentration
of LMWOAs present, and precipitation of secondary minerals.
Keywords: Apatite; Dissolution; Monazite; Organic acids; Rare
earth elements; Yttrium』
1. Introduction
2. Materials and methods
2.1. Specimen mineral preparation
2.2. Organic acids
2.3. Oxic and anoxic dissolution experiments
2.4. aqueous phase analyses
2.5. Post-reaction solid phase analysis
3. Results and discussion
3.1. REY release from apatite
3.1.1. Effects of ligands
3.1.2. Effects of oxic/anoxic conditions
3.2. REY release from monazite
3.2.1. Effects of ligands
3.2.2. Effects of oxic/anoxic conditions
3.3. Stoichiometry of element release and other potential factors
mitigating REY patterns
3.4. Implications for using REY as organomakers
4. Conclusions
Acknowledgements
References