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Magnesium (Mg) is a major rock-forming element and biological nutrient element, and is thus an important link between continents, oceans and the Earth' s interior. Marine carbonates are a major reservoir of Mg and an important component of the global Mg cycle. Using Mg isotopes to trace the deposition-diagenesis process of marine carbonate formation is the prerequisite for reconstructing the deep-time seawater Mg isotopic composition (δ26MgSW) and quantifying the Earth' s historical marine Mg cycle. Studies using this approach have made considerable progress in the past two decades; for example: (1) investigation of Mg isotope fractionation during the formation of different carbonate minerals has been enhanced; (2) numerical models of Mg isotopic behavior have been established, providing constraints on the dolomitization process; and (3) the use of Mg isotopes to trace the early diagenetic fluid system has been explored. These have provided a theoretical base for using marine carbonate data to reconstruct δ26MgSW. The deposition-diagenesis process and its effects on Mg isotopic composition should be considered when selecting information from marine carbonate archives to reconstruct the marine Mg cycle. Critical evaluation combined with geochemi-cal models are necessary when choosing an archive for reconstructing deep-time <526MgSW. [ABSTRACT FROM AUTHOR]

Studies of the sedimentary facies and sequence stratigraphy have indicated that the main depositional environments of the Lower Triassic Jialingjiang Formation in the eastern Sichuan Basin were a restricted platforms and evaporite platforms. Two third-order sequences (SQ1, SQ2) and five fourth-order sequences (SSQ1-SSQ5) were recognized. Sediments in the Jialingjiang strata evolved from a restricted platform to an evaporite platform, then to restricted platform again, and finally to an evaporite platform. The main hydrocarbon reservoirs and producing layers appear in the SSQ1-SSQ2 sequence. Sequences SSQ1-SSQ5 are further subdivided into transgressive systems tracts (TST) and highstand systems tracts (HST). Taking these sequences as mapping units, lithofacies paleogeographic maps were prepared for each of the fourth-order tracts. Evidence from the study of the distribution of favorable reservoirs indicates that the restricted platform-to-platform shoal in the SSQ1 and SSQ2 sequences are key areas for future exploration. [ABSTRACT FROM AUTHOR]

Dolostone-evaporite paragenesis systems (DEPS) have been widely found in Precambrian to Quaternary strata worldwide. However' despite its wide spatio-temporal distribution' the formation mechanism and controlling factors of DEPS remain poorly studied. By reference to copious literature from China and elsewhere combined with previous knowledge of DEPS in China' this study comprehensively summarizes the development characteristics' combination patterns and formation mechanism of DEPS. Firstly' its spatio-temporal distribution is clarified based on published studies: laterally' DEPS is known to be widely distributed in 51 regions globally' mainly in the northern hemisphere (mostly in Asia' followed by Europe and North America). Comparatively' it is found less often in Africa and is only scattered in South America and Oceania. Stratigraphically' although DEPS is found in many Precambrian to Quaternary strata' it appears mostly in the Permian. The combination patterns of evaporite and dolostone in DEPS include the interbedding of dolostone and evaporite' with thick dolostone overlying thick evaporite' thick evaporite overlying thick dolostone' thick dolostone mixed with thin evaporite' and thick evaporite mixed with thin dolostone. The formation process' distribution and main controlling factors of symbiotic differences as well as the particularity and universality of the paragenetic system development are discussed. Secondly' based on macro- to microscale characterization of mineral types in DEPS' six types of evaporites and five types of dolostones have been found and their genesis is discussed. Finally' the properties' characteristics' sources and evolution paths of diagenetic fluids in DEPS are summarized. DEPS studies provide new concepts for solving the "dolostone (or dolomite?) " question' and promote the understanding of the important basic geological problems associated with DEPS. Such studies also provide theoretical guidance for the exploration of oil and gas resources in the paragenetic system. [ABSTRACT FROM AUTHOR]

The Lower Permian Fengcheng Formation in the Mahu Sag of the Junggar Basin was developed in a volcanic-induced alkaline saline lacustrine environment, with deposition of thick-bedded sodium carbonates and high-quality source rocks, as well as the borosilicate reedmergnerite, which is a rare mineral globally but is very rich in the Fengcheng Formation. Based on petrography, fluid inclusion and boron isotope studies, the aim of this study was to discover the origin of the reedmergnerite which, in the Fengcheng Formation, often replaces carbonate minerals, including shortite, eitelite, northupite, calcite, trona, and even barytocalcite. The homogenization temperature of the primary inclusions in the reedmergnerite ranged from 100 to 116°C. Previous synthetic experimental studies have found that the δ11B values of reedmergnerite are 0.33-2.13‰. It is proposed here that, due to the limitations of temperature and pressure in surface or shallow burial processes, the source of boron in reedmergnerite was deep hydrothermal fluid in the original sedimentary environment, mainly in deep diagenetic environments (>3000 m). Therefore, the enrichment of reedmergnerite in the Fengcheng Formation is interpreted as the result of intense magmatic hydrothermal activity and a long diagenetic history. [ABSTRACT FROM AUTHOR]

Siliceous-sulphate rock coatings were observed at Zhenzhu Spring, an acid sulphate hot spring in the Tengchong volcanic field, China. These rock coatings are mainly formed of gypsum and amorphous silica. Some alum-(K), voltaite, α-quartz and muscovite were also found. Four different laminae are developed in the rock coatings: gypsum layer, tight siliceous layer, tabular siliceous layer and siliceous debris layer. The gypsum layer is located at the top of the rock coatings, while other siliceous layers appear below the gypsum layer. Geochemical modelling of the fluids was performed to identify the mechanisms responsible for the formation of gypsum and amorphous silica. The results indicated that the occurrence of gypsum is related to the acid-fog deposition and amorphous silica mainly originates from spring water. Fog deposition provided the rock coatings with abundant SO42− and Ca, and the subsequent complete evaporation of the condensed fluids produced gypsum. Seasonal climate change (especially variation in rainfall) determines the fluctuations of capillary action and dissolution. Rainfall events in the wet season led to periods of non-precipitating gypsum and promoted the capillary rise of the spring water. Slightly diluted capillary water (a small amount of rainwater) covered the rock coatings, formed a tight siliceous layer on the rock-coating surface and/or filled the pores among the gypsum crystals forming many tabular siliceous aggregates. Heavy rainfall (high dilution), however, resulted in non-precipitating amorphous silica and accelerated the gypsum dissolution, leaving tabular pores around tabular siliceous aggregates and forming a tabular siliceous layer. [ABSTRACT FROM AUTHOR]

Karst rocks from the Huanglong Formation exposed at the margin of the Eastern Sichuan Basin can be divided into four types: slightly corroded, moderately corroded porous, intensely corroded brecciated and intensely corroded and replaced secondary calcic karstic rocks. The carbon, oxygen and strontium isotope compositions of the various karst rocks are analyzed systematically and compared to rocks without karst corrosion. The results indicate that (1) the Huanglong Formation in the eastern Sichuan Basin was a restricted bay supplied and controlled by freshwater in which mud-micrite and mud-dolomicrite exhibit low δ13C and δ18O values and high 87Sr/86Sr ratios; (2) all types of karstic rocks in the paleokarst reservoirs of the Huanglong Formation in the research area are affected by atmospheric freshwater with the δ13C and δ18O values and 87Sr/86Sr ratios in the original formation approaching those of atmospheric freshwater, which reflects ancient hydrological conditions, fluid properties, isotopic source and the fractionation effect; (3) the intensely corroded and replaced secondary limestone is affected by a variety of diagenetic fluids, often reflected by δ13C and δ18O values, while the 87Sr/86Sr ratios exhibit the strong degree of the corrosion; (4) after comparing the 87Sr/86Sr ratios of each type of karst rock, the diagenetic fluids are determined to be mainly atmospheric freshwater, and depending on the strength of corrosion, and the low 87Sr/86Sr ratio fluids in the layer will participate in the karst process. The carbon, oxygen, and strontium isotopes of different karstic reservoirs can provide meaningful geochemical information for forecasting and evaluating the development and distribution rules of the Huanglong Formation at the margin of the eastern Sichuan Basin in time and space. [ABSTRACT FROM AUTHOR]