Sequential leaching of silicified Archaean carbonates: A Rb-Sr, Sm-Nd and Pb-Pb isotopic contribution to their tectonic-thermal history (Kaapvaal Craton, South Africa)
Introduction
Carbonates represent a minor lithofacies of early Precambrian greenstone belts, but they are of interest as they are able to record tectonic-thermal steps, as well as changes of the atmospheric and oceanic environments during deposition and further recrystallization episodes (e.g., Schidlowski et al., 1975, Veizer et al., 1989, Shields and Veizer, 2002). Because of their sensitivity to multiple alteration processes, these rocks may also provide useful information about the successive post-depositional metamorphic, diagenetic and hydrothermal activities to which they were potentially subjected (e.g., Wang et al., 2018). For instance, Jahn and Cuvellier (1994) published a summary of the isotopic behavior of carbonated Archaean rocks with useful identification aspects of successive depositional and diagenetic/metamorphic events during their evolution.
Historic information was also obtained by removing the soluble components of whole rocks in geochronological studies of Archaean carbonates with various reagents and by dating the obtained aliquots separately. In fact, since the initial Pb-Pb dating of carbonate whole rocks by Doe (1970) and later by Moorbath et al. (1987), many investigations were published to substantiate and enlarge the application of leaching techniques combined with various isotopic methods. Such applications to various sedimentary to meta-sedimentary carbonates, especially of old to very old ones that were lacking fossils of stratigraphic use, were of determining use for evaluating the potentials of new dating methods (e.g., Schwarcz and Latham, 1989, Jahn et al., 1990, Bischoff and Fitzpatrick, 1991, Luo and Ku, 1991, Erel et al., 1997, Babinski et al., 1999, Fölling et al., 2000, Melezhik et al., 2001, Bolhar et al., 2002, Kaurova et al., 2010, Bellefroid et al., 2018). These references reveal the interest of these rocks and methods for many geochronologists during decades. However, as often, such published studies did not always release unequivocal stratigraphic “ages” for various reasons, one being due to specific methodological aspects depending often on the characteristics of each isotopic method, as well as of the minerals on which they were applied. In order to increase potentially the information about the meaning of such results, especially on whole rocks that may host minerals of successive origins and/or differential evolutions, a sequential leaching procedure has been used here to evaluate further the soluble components of a few carbonates from early Archaean Barberton Greenstone Belt of South Africa. Initially, the purpose was the diversification of successively dissolved constituents by using reagents with a progressive impact on the carbonate-type minerals of the rocks. The goal was an estimation of what meant the Rb-Sr, Sm-Nd and Pb-Pb data of systems known for their variable signatures relative to successive tectonic-thermal events. In turn, a complementary goal of the study was an integrated understanding of the global evolution of these ancient rocks by a combination of various isotopic age data.
Section snippets
The geological and geochronological context
Located near the southeastern edge of the Kaapvaal craton in South Africa, the Barberton Greenstone Belt consists of deformed and variably metamorphosed sedimentary and volcanic rocks of the Swaziland Supergroup (e.g., South African Committee for Stratigraphy, 1980; Fig. 1). These supra-crustal units are in structural or intrusive contact with surrounding coeval or younger granitoid plutons (Anhaeusser, 1973, Anhaeusser and Robb, 1981). The lower 8 to 10 km thick succession of this Swaziland
The analytical procedure
Petrographic and stratigraphic descriptions, as well as isotopic data of the selected untreated whole rocks were already published (Toulkeridis et al., 1998). Rb-Sr, Sm-Nd and Pb-Pb dated, these rocks provide isochrons with isotopic ages of 2,632 ± 99 Ma, 2,612 ± 84 Ma and 2,743 ± 36 Ma, respectively. Significantly younger than the estimated depositional age of ∼3,260 ± 20 Ma, these ages were considered to reflect a late combined silicification and sericitization at an apparent temperature of
The results
Leaching with H2O provided the lowest dissolution amounts with releases between 0.12% and 0.42% of the initial whole-rock weights (Table 1). Conversely, the most efficient leaching was that with HCl that removed about 31% of the initial weight, followed by HAc that dissolved about 24%, both of the FTG1 sample. Leaching with HNO3 dissolved only very small amounts of the left-over rock powders, with soluble separates of 0.5 to 1.2% of the sample weights after the already removal of the “ss”
Discussion
The collective Rb-Sr data of the H2O residues organize into an alignment that can be assimilated to an isochron with an age of 2,799 ± 42 Ma that fits well with the 2,632 ± 99 Ma for the previously published isochron of the untreated whole rocks (Toulkeridis et al., 1998). The very gentle action of H2O explains this proximity as a surface cleaning of the constitutive minerals. Despite large analytical errors due to the limited spread of the Sm/Nd ratio, the corresponding ages for the HAc and
Conclusions
Based on a sequential leaching of silicified Archaean carbonates from Kaapvaal Craton, the chemical data and Rb-Sr, Sm-Nd and Pb-Pb isotopic ages show that the successive dilute HAc, HCl and HNO3 acids interacted with soluble minerals of variable chemical compositions. Application of the isotopic dating methods on the untreated, leached and residual whole rocks also contributed to the reconstruction of the complex post-depositional evolution of these Archaean rock-forming minerals. Already
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Sadly, our co-author and friend Alfred passed away on May the 22nd, 2019. He contributed to this study long ago and we wanted to thank him, memorize his help and somehow honor his continuous sharing and ever-lasting interest for our work in isotope geochemistry. We would also like to take this opportunity to express thanks to many individuals who contributed to this long-term episode. Among those are P. Karcher, J. Samuel, B. Kiefel and D. Tisserand (CGS-CNRS/ULP, Strasbourg), E. Mackiolzyk
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