Extreme rare earth element (REE) enrichment during weathering recorded in saprolites from South Carolina

Haldar, U and Chakrabarti, R and Rudnick, RL (2025) Extreme rare earth element (REE) enrichment during weathering recorded in saprolites from South Carolina. In: Chemical Geology, 673 .

	PDF Che_Geo_Vol_Feb_2025.pdf - Published Version Restricted to Registered users only Download (4MB) | Request a copy
	Microsoft Word Che_Geo_Vol_673_Sup_2024.docx - Published Supplemental Material Restricted to Registered users only Download (731kB) | Request a copy

Abstract

Rare earth elements (REE) and other trace element concentrations as well as 143Nd/144Nd and 87Sr/86Sr in saprolites developed on the Cayce metadiabase dike, South Carolina, document extreme REE mobilization during chemical weathering and enrichment of REE on clays. Saprolites display a bimodal distribution in their total REE concentrations (ΣREE) and light-REE (LREE)/heavy-REE (HREE) ratios. Shallower (0�6 m depth), Group I, saprolites have high ΣREE (up to 2633 ppm) with enrichment of LREE > MREE > HREE. Group II saprolites, at >6 m depth, have lower ΣREE (45�67 ppm) and relatively flat LREE/HREE, similar to the unweathered metadiabase. Group I saprolites are more weathered (chemical index of alteration (CIA) values of 87�95), than the Group II saprolites (CIA = 46�88, with most <55). Mass balance calculations using 143Nd/144Nd rules out significant input of dust to the weathering profile, which is consistent with the lack of depth-dependent variation in εNd. Weathering and REE enrichment occurred through a three-stage process. Stage 1 involved regional weathering during which saprolites developed on both the metadiabase dike and Liberty Hill granite country rock. During this stage, breakdown of LREE-rich accessory minerals (e.g., titanite) in the granite released REE and radiogenic Sr to weathering fluids that penetrated the metadiabase where these elements were adsorbed onto clays, consistent with their Nd and Sr isotopic compositions. The kaolinite/smectite (K/S) ratio in Group II saprolites negatively correlates with Sm/Nd and positively with Y/Ho and Rb/Sr ratios indicating preferential adsorption of lighter REE (e.g., Nd), Y, and Rb by kaolinites; no trends are seen in Group I saprolites, suggesting that these samples were overprinted by later events that did not impact the Group II saprolites. Stage 2 involved replacement of smectite veins by siderite in the entire saprolite at high pH and under reducing conditions such as found in swamps, which did not affect the adsorbed REE in the clays. In stage 3, siderite dissolution under acidic and oxidized conditions at the shallowest depths (upper 2 m) led to the formation of Fe3+-smectite and LREE mobilization; this REE-bearing fluid percolated downwards where the REE were adsorbed onto clays to develop a REE-enriched zone locally in the upper 6 m (within the Group I saprolites). Carbonates may have also acted as depositional ligands and induced REE precipitation. This study shows that REE can be extremely mobile during chemical weathering under specific conditions and may be deposited onto secondary clay minerals like kaolinites, which absorb REE, particularly LREE, leading to local enrichments. © 2024 Elsevier B.V.

Item Type:	Journal Article
Publication:	Chemical Geology
Publisher:	Elsevier B.V.
Additional Information:	The copyright for this article belongs to publishers.
Keywords:	Actinides; Carbon; Clay alteration; Concrete dams; Geochronology; Hydrogen; Inert gases; Kaolin; Levees; Manganese nodules; Mica; Neodymium alloys; Nitrogen; Oxygen; Phosphorus; Quartz; Rubidium alloys; Selenium; Strontium; Strontium alloys; Strontium compounds; Sulfur, Accessory mineral weathering; Accessory minerals; Mineral weathering; Mobilisation; Nd-Sr isotopes; Rare earth element mobilization and enrichment; Rare-earths; Saprolite; Smectites; South Carolina, Kaolinite
Department/Centre:	Division of Mechanical Sciences > Centre for Earth Sciences
Date Deposited:	30 Dec 2024 05:00
Last Modified:	30 Dec 2024 05:00
URI:	http://eprints.iisc.ac.in/id/eprint/87167

Actions (login required)

View Item