MULTI-EPISODIC RECRYSTALLIZATION AND ISOTOPIC RESETTING OF EARLY-DIAGENETIC DOLOMITES IN NEAR-SURFACE SETTINGS

摘要

The lower Eocene Rus Formation in Qatar reflects carbonate deposition in a semirestricted to fully restricted marine setting on a shallow ramp. Petrographic, mineralogical, and geochemical evidence from three research cores show early diagenesis has extensively altered nearly every petrological attribute of these rocks despite not having been deeply buried. In southern Qatar, the lower Rus (Traina Mbr.) consists of fabric-retentive dolomite intervals that preserve mudstone, wackestone, and packstone textures that are interbedded with depositional gypsum beds. In northern Qatar, the same member is dominated by fabric-destructive planar-e dolomite, and evaporites are absent. In both northern and southern Qatar, the upper Rus (Al Khor Mbr.) is composed of fabric-retentive dolomite intervals as well as limestone intervals rich with Microcodium textures that display evidence of dedolomitization. Geochemical analysis reveals that the limestones have an average delta O-18(cal) of -10.73 parts per thousand VPDB and delta C-13(cal) of -7.84 parts per thousand VPDB, whereas average dolomite delta O-18(dol) is significantly higher (-1.06 parts per thousand VPDB) but delta C-13(dol) values (-3.04 parts per thousand VPDB; range -10 to 0 parts per thousand) overlap with delta C-13(cal) values. Additionally, delta C-13(dol) trends toward normal marine values with depth away from the calcite-dolomite contact in all three cores. Petrographic observations demonstrate that dolomite crystals are commonly included in calcite and partially to completely replaced by calcite in these intervals and suggests that dolomite formed before calcite in the Microcodium-bearing intervals. Furthermore, the dolomites are commonly cemented by gypsum in the Traina Mbr. in southern Qatar, suggesting that dolomitization may have also occurred before, or concurrent with, bedded gypsum formation and indicates that dolomitization occurred early. Early dolomites were subsequently replaced by Microcodium-bearing limestones at and immediately below paleoexposure surfaces, and at greater depths recrystallized in mixed marine-meteoric fluids, producing a negative delta C-13(dol) signature that trends toward more positive values away from the limestone-dolomite contact. Lastly, the dolomites underwent another phase of recrystallization in either marine-dominated fluids or possibly a well-mixed aquifer setting, resulting in a near-0 parts per thousand delta O-18(dol) signature but retaining the negative delta C-13 signature. These findings thus have implications for reconstructing the diagenetic history of carbonate rocks, as they suggest that early diagenesis of carbonates can be extremely complex, resulting in multiple stages of mineral replacement and isotopic exchange in meteoric and shallow marine fluids before significant burial. Furthermore, this study shows that dolomitization of a limestone does not necessarily prevent additional early diagenesis and multiple recrystallization events. Lastly, it emphasizes the importance of incorporating petrographic observations with geochemical data when interpreting the diagenetic history of carbonate rocks.