![]() ![]() Conversely, calcite ooids are mainly precipitated during periods of lower seawater Mg/Ca ratios. The present understanding is that high-Mg calcitic (>4 mol.% MgCO 3 Stanley and Hardie, 1998) and aragonitic ooids form when Mg/Ca molar ratios in seawater are above a threshold limit of 2 (Stanley and Hardie, 1998 1999 Stanley, 2006 Ries, 2010). Previous workers have used the mineralogy of ooids as a proxy for secular variations in ancient seawater chemistry and, in particular, the reconstruction of what was referred to as oscillating trends in calcite versus aragonite seas has largely been based on ooidal cortex mineralogies (Sandberg, 1983 Wilkinson et al., 1984 1985 Opdyke and Wilkinson, 1990 Tucker and Bathurst, 1990a MacKenzie and Morse, 1992 Cantrell, 2006). Even collophanitic ooids made solely of fine-grained phosphatic material have been reported (Barr, 2007). Previous works describe low-Mg to high-Mg calcitic, aragonitic, dolomitic and chamositic (Van Houten and Purucker, 1984) ooids, as well as gypsum (gypsolites Tekin et al., 2008) and halite ooids (Radwanski and Birkenmayjer, 1977 Kimberley, 1983). The mineralogy of oolites and ooids is extremely diverse. Oolites, that is carbonate rocks formed predominantly by ooidal facies, form on marine carbonate platforms and ramps and make up a significant part of the world's carbonate hydrocarbon reservoirs (Flügel, 2004). Ancient ooids are valuable palaeoenvironmental proxies for water energy, temperature, salinity, seawater geochemistry and bathymetry (after Sandberg, 1975 1983 Plee et al., 2008). Although there is no commonly accepted elucidation for ooid genesis, processes such as accretion of fine particles around a nucleus while agitating on a soft substrate (Mei and Gao, 2012), abiotic precipitation from an ambient supersaturated water around a nucleus (Duguid et al., 2010) and organomineralization of a surface biofilm (Diaz et al., 2014 2017 Li et al., 2017 Batchelor et al., 2018) have been diversely put forward. Ooids, mainly calcareous, spherical to subspherical coated grains consisting of one to several nearly concentric cortices encrusting a nucleus, are ubiquitous features of (sub)tropical oceans throughout Earth's history (Flügel, 2004 Tucker, 2011). The studied bimineralic ooids represent a change in the global carbonate factory during the Middle Triassic in response to secular variations in the chemistry of palaeoseawater. The main control on precipitation of bimineralic ooids was the Mg/Ca ratio of seawater, with lesser influences from seawater temperature, salinity and ρCO 2 which promoted the degree of carbonate saturation, leading to higher precipitation rates for both aragonite and high-Mg calcite. ![]() The cementation history of the grain flow hosting coated grains is well documented by the paragenetic sequence established in half-moon ooids which predicts that calcite precipitated under burial (shallow to deep) conditions. The stratigraphic occurrence of the investigated bimineralic ooids conforms to the global distribution of carbonate ooids through the Middle Triassic period. At the same time, recrystallization of original high-Mg calcite layers to micrite occurred. Zonal, micritic to microsparitic ooids formed through intracortical dissolution of primary aragonite lamellae which were later replaced by (micro)sparry calcite. The bi-partite fabric in half-moon ooids resulted from early pre-compaction rapid dissolution of both aragonitic cortices and the more-soluble components of the high-Mg calcite core in the primary ooid, as well as the collapse of less-dissolved remnants to the bottom of the oomouldic cavity which was later filled with calcite cement. This study reports on the occurrence of formerly bimineralic, ancient, calcitic coated grains, including half-moon ooids and micritic to microsparitic ooids in the Middle Anisian (Middle Triassic) grain flow from Hydra Island (Greece), and discusses their importance for palaeoseawater geochemical interpretations. ![]()
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