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

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 ε_Nd. 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.