wAbstract
@River waters have been shown to be systematically enriched in
the heavy molybdenum (Mo) isotopes when compared to typical granites
and basalts, which generally possess Mo isotopic compositions
(Β98/95Mo) of around 0ρ. This inconsistency has been
used to argue against weathering of crustal rocks as the cause
for heavy riverine Β98/95Mo signatures. Incongruent
dissolution of primary bedrock, however, may be an important process
by which the anomalous Mo signatures of the river dissolved load
are produced. This study therefore investigates the effect of
igneous crustal rock weathering on the aquatic Β98/95Mo
signal by comparing stream water and bedrock Mo isotope data to
results of bulk rock leach experiments. For this purpose, stream
water and bedrock (orthogneiss, granite, basalt), as well as soil
and vegetation samples were collected in a small catchment in
the French Massif Central. In accordance with the results of earlier
studies on riverine Mo, both streams are isotopically heavier
(Β98/95Mo = 0.5-1.1ρ) than the typical granites and
basalts. The excellent agreement of these data with those of Mo
released during experimental leaching of the basalt bedrock (0.6-1.0ρ)
identifies a predominance of basalt weathering over the stream
water Mo geochemistry, while other processes (i.e. soil formation,
secondary mineral precipitation and adsorption) are subordinate
in this catchment. Given that the basalt bulk rock Β98/95Mo
reflects a value typical for crustal magmatic rocks (ca. 0.1ρ),
Mo isotope fractionation during the incongruent dissolution of
basalt can explain the observed isotopically heavy aquatic Mo
signatures. Laser ablation analyses demonstrate that the volumetrically
minor magmatic sulfides can be highly enriched in Mo and mass
balance calculations identify the sulfide melt inclusions as the
principal Mo source for the leach solutions. These data suggest
that the magmatic sulfides posses a distinctly heavier Β98/95Mo
signature than the coexisting silicate melt. In this case, Mo
would behave like Fe by showing a detectable isotope fractionation
at magmatic temperatures. Incongruent crustal bedrock weathering
may thus cause a preferential release of heavy Mo isotopes. This
effect, however, is highly dependent on the primary bedrock mineralogy.
Consequently, the average continental runoff may have been significantly
affected by incongruent weathering during periods when the Earth
system was exceptionally far from steady state, e.g., large glaciations
with enhanced physical weathering or large subaerial basalt eruptions
such as the Deccan and the Siberian plateau.x
1. Introduction
2. Study site and sampling
@2.1. Geological setting
@2.2. Sampling
@2.3. Rock sample descriptions
3. Analytical methods
@3.1. Leach experiments
@3.2. Preparation of water, rock, soil and vegetation samples
@3.3. Chemical purification and isotope analysis of Mo and Sr
@3.4. Major anion and cation analyses of stream waters and leach
solutions
@3.5. Laser ablation ICP-MS analyses of bedrock minerals
4. Results
@4.1. Natural samples
@@4.1.1. Stream water and bedrock geochemistry
@4.2. Single grain and matrix element concentrations determined
by LA-ICP-MS
@4.3. Soil, vegetation and suspended load
@4.4. Laboratory leach experiments
@@4.4.1. Basalt leach solutions and residues
@@4.4.2. Orthogneiss and granite leach solutions
5. Discussion
@5.1. Natural samples: bedrock and stream waters
@5.2. Experimental primary mineral dissolution
@@5.2.1. Incongruent bedrock weathering
@@5.2.2. Mo released from basalts: host phases and mass balance
@@5.2.3. The role of Mo adsorption to Fe-Ti oxide surfaces during
experimental dissolution
@5.3. Implications for the natural environment
@5.4. Consequences for the global Mo isotope budget
6. Conclusions
Acknowledgements
References