Early diagenetic evolution of shelly phosphorites: REE signatures traced by LA-ICP-MS mapping

Sedimentary phosphorites are the primary sources of nitrogen-phosphorus-potassium fertilisers, and they have recently been highlighted as a potential economic source of rare earth elements (REE). The growing need for clean technologies strongly influences the demand for REE, and in Europe, most deposits have not been investigated in detail since the 1970-1980s.

Lower-Ordovician shelly phosphorites in Estonia are among Europe's most extensive phosphate rock reserves, with a tonnage of approximately three billion tons. The ore consists of sandstone rich in phosphatic brachiopod fragments deposited in a shallow marine peritidal environment of the Baltic Paleobasin. Mineralisation is carried out carbonate fluorapatite (CFA), an apatite with a highly diverse chemical composition [Ca_10-a-bNa_aMg_b(PO₄)_6-x(CO₃)_x-y-z(CO₃⋅F)_x-y-z(SO₄)_zF₂]. The shells themselves are complex objects, with apatite originating from the crystallisation of organic tissues and the precipitation of secondary phosphate during sediment burial. The partitioning and uptake of the individual REEs in them depend on many factors, including input from marine sources, the oxygenation state of the sedimentary column, and the precursors carriers phases of REEs that may have different affinities for each rare earth.

In the REMHub project, investigations were conducted on three deposits: Toolse, Aseri, and Ülgase; representing a dataset of 630 ablations up to date. The LA-ICP-MS imaging technique developed by Drost (2018), addressed elemental distribution as raster maps, allowing identification and discrimination by integrating semi-quantitative data through elements' stepwise distribution. Diagenetic stages and compositions were evaluated using the following pathfinders as pooling channels. Sr, U, and Ce. 

On average, apatites present homogeneous REE patterns, MREE-enriched up to 15-folds compared to PAAS, with Y-Ce anomalies indicative of early-digenetic overprinting.  However, the degree of overprint varied. In Ülgase, authigenic concretions and shells presented depleted REE signals, close to coastal signature. However, concretions showed a lower enrichment (∑REE 400-800ppm) compared to shells (REE 1500-3000 ppm). In Toolse, shells presented intermediate recrystallised textures, with Sr-U-depleted stages allowing the tracing of pristine signals, and U-rich stages presenting marked Gd-U and La anomalies. The average REE grade is 1966ppm. In Aseri, U-sorting reveals a second, alteration-driven enrichment in which the fragment edges present a ΣREE up to 12 754ppm (120 folds).

Overall, investigations demonstrated a progressive evolution of REE signals during early diagenesis, highly influenced by redox cycles in shallow sediments, authigenic recrystallisation, organic matter decomposition within the shells, and possibly late distal alteration fluids.