Formation of shallow-water glaucony in weakly oxygenated Precambrian ocean: An example from the Mesoproterozoic Tieling Formation in North China

摘要

Authigenic glaucony precipitation in Phanerozoic oceans takes place mostly in middle shelf to upper slope environments with low depositional rate or sediment starvation. Precambrian glaucony, however, is more common in stratigraphic successions deposited from shallower-water environments with high and variable depositional rates. This phenomenon has long been noticed in literature, but the controlling factors of shallow-water glaucony precipitation in Precambrian oceans have not been adequately investigated. To better understand the glauconitization processes in Precambrian oceans, we have conducted an integrated study of the glaucony in stromatolitic carbonates of the Mesoproterozoic Tieling Formation (ca. 1437 Ma) in North China, using sedimentological, mineralogical, and geochemical data obtained from field observations, petrography, XRD, SEM, quantitative EDS and ICP-MS analyses. Macro-and microscopic observations show that the Tieling glaucony fills voids of varying sizes and shapes, and records different maturation stages of glauconitization. Geochemical analyses show that the Tieling glaucony has high K2O (avg. > 8%) but low and variable total Fe2O3 (TFe2O3) contents (1.92-13.65 wt%). The TFe2O3 contents increase with maturation of glaucony. Titration results show that the Tieling glaucony contains both Fe3+ and Fe2+ ions, but has Fe2+/Fe3+ ratios much higher than that of the Phanerozoic glaucony. REE results of glaucony-hosting carbonates show weak negative and positive Ce anomalies with average Ce! Ce* ratio close to 1.0, suggesting carbonate precipitation near the redoxcline of Fe-Mn oxides. All these features suggest that the Tieling glaucony was precipitated in seawater around the Fe-redoxcline, where both Fe2+ and Fe3+ were available throughout the glaucony maturation stages. High Fe2+/Fe3+ ratios in the depositional environments led to Fe2+ occupation at octahedral sites of glaucony and negative charges on octahedrons, which resulted in high K content (to balance the negative charges on octahedrons) and low TFe2O3 (limited by dioctahedral structure). The formation of the Tieling glaucony and other similar Precambrian glauconies is likely controlled by low oxygen concentration in seawater and a shallow redoxcline that controls the availability of Fe and K cations during initial precipitation and maturation of glaucony. The shift of authigenic glaucony precipitation from shallow water in the Precambrian to deep water in the Phanerozoic may record the deepening of ocean chemocline in response to increased ocean oxygenation. (C) 2017 Elsevier B.V. All rights reserved.