REY‐Th‐U Dynamics in the Critical Zone: Combined Influence of Reactive Bedrock Accessory Minerals, Authigenic Phases, and Hydrological Sorting (Mule Hole Watershed, South India)

Abstract

The sources of REY‐Th‐U and long‐term mass balance were assessed in the gneissic tropical forested Critical Zone Observatory of Mule Hole, India. The study relies on the characterization of the solid compartments (bedrock, soils, streambed and suspended sediments), on batch leaching experiments of the parent gneiss and on extractions of cation‐exchangeable and iron‐related pools of selected soil samples. The REY‐Th‐U primary reservoir is controlled by monazite, xenotime, thorite, allanite, bastnesite, titanite, apatite, and, to a lesser extent, by zircon. This accessory mineral assemblage was profoundly modified by episodes of metamorphism and hydrothermal activity. Allanite, bastnesite, titanite, and apatite are prone to break down at incipient weathering stage while monazite, xenotime and zircon are resistant minerals. On a long‐term basis, REY are roughly inert in the immature saprolite and depleted in the soil cover and are strongly redistributed among secondary phases. LREE and HREY are mainly controlled by poorly crystallized secondary iron oxides and oxyhydroxides, and, to a lesser extent, by secondary phosphates (e.g., rhabdophane). Th, inert in the saprolite, is controlled by well‐crystallized secondary iron oxides and oxyhydroxides and likely by secondary thorianite (ThO2). REY and Th are not easily mobilized during surface processes such as chelation by organic matter and uptake by plants. REY and Th are, by far, exported as suspended sediments associated with iron oxides. The U export by groundwater and suspended sediments dominates over those of streambed sediments and stream.

Controls on the Dynamics of Rare Earth Elements During Subtropical Hillslope Processes and Formation of Regolith-Hosted Deposits

“…The role of the primary REE mineralogy, REE complexation, and external REE input on REE fractionation during weathering has been discussed extensively in previous studies (Banfield and Eggleton, 1989;Braun et al, 1993;Johannesson and Lyons, 1994;Harlavan and Erel, 2002;Braun et al, 2018). Here, we examine the relative importance of these factors to the behavior of the REE in the different zones of the regolith in the study area.…”

“…The behavior of Ce is usually different from that of the other REEs during weathering because Ce occurs dominantly as Ce 4+ and commonly precipitates as cerianite [CeO2] whereas the other REEs occur in the 3+ state and are leached by the weathering solutions (Braun et al, 1990;Braun et al, 1998;Duzgoren-Aydin and Aydin, 2009;Babechuk et al, 2014;Berger et al, 2014;Janots et al, 2015;Su et al, 2017;Braun et al, 2018). The ratio of Ce 4+ /Ce 3+ will vary with the Eh, which, when high, will lead to very high Ce 4+ /Ce 3+ ratios and positive Ce anomalies such as that at the depth of ~0.5 m (Fig.…”

“…As a result of these studies, it has been established that the mobility of REEs during weathering is controlled by pH, redox conditions, their aqueous complexation, the primary REE mineralogy, the precipitation of secondary REE minerals and the adsorption of the REEs by clay minerals, Fe-Mn oxyhydroxides, and organic substances in soils (e.g. Cantrell and Byrne, 1987;Banfield and Eggleton, 1989;Braun et al, 1990;Koeppenkastrop and De Carlo, 1992;Braun et al, 1998;Aubert et al, 2001;Pourret et al, 2007;Laveuf and Cornu, 2009;Yusoff et al, 2013;Su et al, 2017;Braun et al, 2018;Lara et al, 2018;Li et al, 2019).…”

Potash fertilizer promotes incipient salinization in groundwater irrigated semi-arid agriculture

“…To assess the variability in weathering rates between the two catchments, we used Europium, a Rare Earth Element which is more concentrated in plagioclase compared to the other REE. Its relative enrichment in groundwater with respect to its neighbouring REEs, Sm and Gd, induces a positive Europium anomaly (further notes as Eu*) and can be used as a proxy of relative plagioclase weathering in the vadose zone 54,55 . We compared the positive Eu-anomaly in the groundwater of both pristine and cultivated systems, assuming that a similar average and variability of the positive Eu-anomaly in groundwater for both sites would indicate a similar Na production by plagioclase weathering.…”

“…where all the concentrations are in μeq/L. The Europium anomaly (Eu*) is a proxy for plagioclase weathering 54,55 . It is calculated by normalization of its concentration by those of his neighbours (Sm & Gd) and to the average gneissic bedrock 23 .…”