『Abstract
Strontium-, Nd-, and rare-earth-element-isotope data are presented
from rock, weathered rock (arene) and saprolite, sediment and
soil, shallow and deep groundwater (e.g. mineral-water springs),
and surface waters in the Margeride massif, located in the French
Massif Central. Granitoid rock and gneiss are the main lithologies
encountered in the Margeride, which corresponds to a large and
5-km-deep laccolith. Compared to bedrock, the Sr isotopes in arene,
regolith, sediment and soil strongly diverge with a linear increase
in the 87Sr/86Sr and Rb/Sr ratios. Neodymium
isotopes fluctuate least between bedrock and the weathering products.
In order to characterise the theoretical Sr isotopic signature
IRf(Sr) of water interacting with granite, a dissolution model
was applied, based on the hypothesis that most of the Sr comes
from the dissolution of plagioclase, K-feldspar and biotite. Similar
to the Sr model, an approach was developed for modelling the theoretical
Nd isotopic signature IRf(Nd) of water interacting with a granite,
assuming that most Nd originates from dissolution of the same
minerals as those that yield Sr, plus apatite. The IRf(Sr) ratio
of water after equilibration with the Sr derived from minerals
was calculated for the Margeride granite and compared to values
measured in surface- and groundwaters. Comparison of the results
shows agreement between the calculated IRf(Sr) and the observed
87Sr/86Sr ratios. When calculating the IRf(Nd)
ratio of water after equilibration with the Nd derived from minerals
of the Margeride granite, the results indicated good agreement
with surface-water values, whereas mineralised waters analysed
within the Margeride hydrosystem could not be directly linked
to weathering of the granite alone. Because the recharge area
of deep groundwater is located on the Margeride massif, very deep
circulation involving interaction with other rocks (e.g. shales)
at depths of ＞5 km must be considered.』
1. Introduction
2. Site description, hydrogeological context
3. Water, bedrock, sediment and soil sampling, and analytical
methods
4. Results
4.1. Mineralogical compositions
4.2. Major-element and REE compositions in whole rock, mineral
separates, sediment and soil
4.3. REE patterns in whole rock, separate minerals, sediment
and soil
4.4. Sr and Nd isotopes in separate minerals, sediment and soil
4.5. REE, Nd and Sr isotopes in ground and river waters
5. Discussion
5.1. Strontium sources in groundwater
5.2. The weathering model for Sr and Nd
5.3. The atmospheric-input correction
5.4. Implications for water-rock interactions and water circulation
6. Conclusions
Acknowledgements
References