Elemental and isotopic geochemistry of carbonate rocks from the Pila Spi Formation (Middle–Late Eocene), Kurdistan Region, Northern Iraq: implication for depositional environment

Abstract

Major, trace, rare earth elements (REE), and stable isotopic compositions in carbonate rocks of Pila Spi Formation were measured to constrain the source of rare earth elements and depositional environment. The low ΣREE content (average = 6.53 ppm, n = 14) indicates that the carbonates seawater-like REE patterns with (1) LREE depletion (average (Nd/Yb)n = 0.81), (2) HREE enrichment, (3) negative Ce anomaly (average = 0.91), and (4) superchondritic Y/Ho ratio (average = 30.07); slightly lower average value of Y/Ho ratio than the typical seawater value (~ 44–74) suggests modification of the seawater by input of fresh water. The (Nd/Yb)n and (Dy/Yb)n ratios of the Pila Spi carbonates have similarity with that of the Arabian sea carbonate. The low concentrations of U (0.35–1.0 ppm) suggest the deposition under oxygen-rich environment. This study indicates that the carbonates still preserve their original seawater-like REE pattern, provided that the contamination with terrigenous materials was small, and they serve as a seawater proxy. The Pila Spi carbonates have δ13C values ranged from − 6.17 to 0.76‰ PDB, and the δ18O values range between − 3.85 and 0.46‰ PDB. The positive correlation between δ13C and δ18O (r = 0.949, n = 14) values indicates the alteration of original isotopic compositions due to diagenesis. Finally, it is conceived that the Pila Spi carbonate was deposited in the lagoon environment with seawater invading and mixing of the continental (terrigenous) materials to the basin is viable.