Barium isotopic fractionation in latosol developed from strongly weathered basalt

Geochemical soil dynamics on a bimodal post-collisional intrusive complex

The importance of environmental quality for global social and ecological development, including soil degradation, cannot be overstated. Trace elements dispersed in the environment due to anthropogenic or geogenic activities can result in ecotoxicological impacts, negatively influencing environmental quality. The reference values for soil quality concerning trace elements are primarily based on geological, geomorphological, and pedological patterns. However, intrinsic geological factors may diverge some concentration levels from established norms. Therefore, conducting comprehensive surveys of environmental quality reference values becomes imperative, incorporating geological, geomorphological, and pedological patterns. A deeper understanding of the distribution of these elements is also required. Multivariate analysis proves crucial in compartmentalizing the most relevant factors, particularly in regions marked by bimodal magmatism arising from post-collisional distensional processes, such as the Santa Angélica intrusive suite in southeast Brazil. This study collected soil samples from pastures and natural grasslands with minimal anthropogenic intervention at two depths. These samples underwent various chemical and physical analyses. Statistical techniques such as correlation analysis, principal component analysis, hierarchical clustering, and geostatistics were utilized to interpret the data. The analysis revealed a correlation between the clay fraction and trace elements, demonstrating that clustering is an effective methodology for ascertaining landscape distribution patterns of these components. When compared to quality reference values, it was observed that most soil content levels exceeded both global and local standards. This study suggests that the presence of barium (Ba) in the soil might be due to the isomorphic replacement of feldspathic minerals in acidic and intermediate rocks, whereas molybdenum (Mo) seems to be associated with soils in the domain of porphyritic allanite granite. However, additional research is warranted to determine the concentration factor of Mo in this scenario accurately.

Enrichment and speciation of chromium during basalt weathering: Insights from variably weathered profiles in the Leizhou Peninsula, South China

Basalt-derived soils are widespread worldwide. Such soils contain high levels of heavy metals like chromium (Cr), which is a serious environmental concern. However, little is known regarding the enrichment and speciation of Cr during the basalt weathering process. Therefore, two basalt-derived soil profiles (Nitisol and Ferralsol) in the Leizhou Peninsula, south tropical China, were investigated to explore the redistribution and transformation of Cr during basalt weathering. All profiles could be divided into three layers: rocks, saprolites, and soils. The Nitisol and Ferralsol profiles exhibited strong (kaolinization) and extreme (laterization) degrees of weathering, respectively. Results showed that Cr concentrations in the saprolites (234 to 315 mg·kg^-1) were higher than those in basalt rocks (139 to 159 mg·kg^-1), indicating that Cr was enriched with the continuous loss of Si and other mobile macro-elements. While high levels of Cr were also enriched in the soils (178 to 430 mg·kg^-1) accompanied with Fe. However, in the upper soils of the Ferralsol profile, the acidity and organic matter could promote the leaching of Cr. Geochemical fractions and EPMA mapping showed that chromite and olivine were the main Cr-bearing minerals in basalt, but Fe-oxides (e.g., goethite and hematite) contained the highest portion of Cr in weathered saprolites and soils. The availability of Cr in the soil was extremely low due to the high stability of Cr bound to Fe-oxides. However, the decreasing contents of Cr bound to Fe-oxides in the upper soils of the Ferralsol profile indicated that Cr could also be released during Fe leaching. In conclusion, the weathering of basalt can lead to the enrichment of Cr in Fe-(hydro)oxides, which are the main controlling minerals for Cr mobility in basalt-derived soils. Further research is needed to evaluate the effect of Fe-(hydro)oxide formation and dissolution on the release of soil Cr.

Barium isotope evidence for the role of magmatic fluids in the origin of Himalayan leucogranites

As an important post-collisional magmatic product in the orogenic belt, the Himalayan leucogranites are the critical host rocks for a number of rare-metal mineralization (such as Li, Be, Cs, Rb, Nb, Ta, and Sn). However, there is still a lack of good understanding on the formation and evolution of the leucogranites. Particularly, the role of the magmatic fluids in transporting and enriching the rare elements is not clear. Here we measure Ba isotope compositions for leucogranites from the Kampa Dome of the Himalayan belt to understand the fluid activity and behavior of fluid-mobile elements during leucogranite formation. Our results show that the δ^138/134Ba of leucogranites range from -1.32‰ to +0.12‰, much lower than the literature values for S-type granites and various sedimentary materials, suggesting that the Ba isotope compositions of the leucogranites does not reflect the sedimentary source signatures. Instead, their low δ^138/134Ba is accompanied by non-charge-and-radius-controlled (CHARAC) twin-element (such as Nb/Ta) behaviors, clearly showing the involvement of magmatic fluids during magma evolution. Experimental studies suggest that the low δ^138/134Ba of the magmatic fluids most likely results from exsolution from a large deep magma reservoir. Such fluids not only modified Ba isotope compositions of the leucogranites, but also transported many fluid-mobile metal elements which may help form the rare metal ore deposits. Therefore, Ba isotope data provide new insights into formation and evolution of magmatic fluids and exploration of the rare-metal mineralization.

Leaching Methods for Ba Isotope Studies of Carbonates

Barium (Ba) stable isotopes in carbonate rock have great potential to provide valuable information on environmental change and the biogeochemical cycles of oceans in the past. Ba in carbonate rock can exist in various phases, such as adsorbable and silicate-bound Ba. However, only the carbonate-bound phase is considered to record the Ba isotopic compositions of ambient seaweater. Here, we designed a two-step leaching experiment to obtain the carbonate-bound Ba in two typical carbonate rocks: limestone and cap dolostone. The results showed that after leaching by 1 mol L^-1 ammonium acetate, the carbonate-bound Ba extracted by mixed solution of 1.5 mol L^-1 acetic acid and 1 mol L^-1 ammonium acetate in each studied sample have indistinguishable isotope ratios in leaching time conditions between 12 and 72 h. More importantly, the carbonate-bound Ba isotope ratios were quite different from those of the residue (up to 10 times of analytic uncertainty, 2SD ≤ ±0.04‰) after leaching in three out of four leaching experiments, indicating that noncarbonated fraction could overprint a primary seawater signal. Our sequential leaching techniques could improve targeting of carbonate-bound Ba isotope signatures in various carbonate rocks to trace the Ba cycling in the oceans.

Barium isotopic fractionation during strong weathering of basalt in a tropical climate

Barium (Ba) is an element which is toxic to humans, plants, and animals. Deciphering the geochemical behavior of Ba in soils is fundamental for assessing the potentials risks posed by Ba. Ba isotopes are a potentially robust tracer of Ba in soils. In this study, the controlling factors of Ba isotopic fractionation in a latosol profile were investigated through sequential-extraction experiments. Furthermore, dissolution experiments were conducted to understand Ba isotopic fractionation during the dissolution of basalts. The sequential-extraction experiments revealed δ^137/134Ba ratios in various fractions that were remarkably heterogeneous: -0.28‰ to -0.15‰ in the exchangeable fraction; -0.32‰ to -0.16‰ in reducible Fe-Mn (oxyhydr)oxides; and 0.06‰ to 0.46‰ in residues. This indicates that light Ba isotopes are preferentially adsorbed on secondary minerals and associated with Fe-Mn (oxyhydr)oxides. Both processes play important roles in storing Ba originally released from minerals. Results of the sequential-extraction and dissolution experiments revealed that light Ba isotopes favored fluids during the dissolution of silicate minerals, while heavy Ba isotopes were prone to being released from interlayers of micaceous layers. Collectively, the dissolution of minerals, adsorption on secondary minerals, and formation of easily reducible Fe-Mn (oxyhydr)oxides govern Ba isotope fractionation and Ba transport in soils.