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Travertine mounds are common spring‐constructed landforms with highly variable geometries, but factors controlling their development have not yet been fully understood. To investigate processes linked with their lateral and vertical growth, this study investigated travertine mounds at Heinitang (China) and compared them with typical mounds around the world. Calcium concentration variation modelling and height evaluation of travertine mounds were also performed. The lateral extent of travertine mounds was found to be mainly controlled by Ca2+ and HCO3‐ concentrations and water discharge: relatively small metre‐scale mounds, like those from Heinitang, are largely the products of Ca2+‐deficiency (<150 mg L−1, commonly <100 mg L−1) or HCO3‐‐deficiency (<500 mg L−1) springs with low discharges (<0.1 L s−1), whereas springs with higher Ca2+ and HCO3‐ concentrations and discharges give rise to the formation of wider mounds. Additional factors affecting mound lateral development include infiltration, evaporation, micro‐organisms, water cooling, CO2 degassing and flow paths. Height of travertine mounds cannot be simply considered to represent the piezometric surface, because the vertical growth of travertine mounds is also influenced by the water table change triggered by tectonic pulses and climate fluctuations, and the closing of conduits induced by self‐sealing. The extinction of travertine mounds at Heinitang is, for example, probably caused by the water table lowering triggered by earthquakes or climate drying after 4 kyr bp. These findings offer new inputs for the better understanding of factors controlling travertine mound development and might assist in the reconstruction of fluid properties, hydrodynamics, tectonics and climate fluctuations in fossil geothermal systems with travertine mounds. [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]

Abstract Reshuitang is an important active hydrothermal area in the Tengchong geothermal field and is characterized by many hot springs and their associated deposits. Detailed mineralogical, crystal morphological, and stable isotopic investigations of the smooth slope fossil travertines at Reshuitang and hydrochemical studies of two active springs around the fossil travertines were conducted to study their genesis. The results show that 1) the modern springs discharge waters with high temperature (>70 °C), near-neutral pH (6.3–8.34), and high saturation indices of calcite and aragonite; 2) the fossil travertines contain calcite and small amounts of Mn-Fe oxides, halite, thenardite, and non‑carbonate debris; 3) the calcite include ray crystals, dendrite crystals, platy crystals, and spike crystals; and 4) δ13C and δ18O of the fossil travertines at Reshuitang range from −1.59‰ to −0.87‰ and 6.48‰ to 7.80‰, respectively. According to comparison between the travertines at Reshuitang and other travertines in Tengchong, all these travertines are thermogene travertines, with a large contribution of mantle degassing to their parent CO 2 , but are formed at different temperatures. The fossil travertines at Reshuitang, similar to those at Shuzhishi, are formed at high temperatures (near 100 °C), which results in weak microbial activities during the formation of the fossil travertines. Most of the calcite crystals at Reshuitang were non-equilibrium products precipitated directly from the spring waters during the deposition process, while the spike crystals originated from diagenetic dissolution of large calcite crystals. Comparison between the fossil travertines at Reshuitang and other smooth slope travertines from other areas shows these smooth slope travertines have different lithofacies and fabrics. In smooth slope environments, temperature may control the distribution and development of microbes and the intensity of CO 2 degassing, and consequently cause the precipitation of contrasting fabrics and lithofacies. [ABSTRACT FROM AUTHOR]

Travertine-depositing hot springs can generate various carbonate minerals with or without the participation of microorganisms. They thus serve as good natural laboratories to study abiotic and biotic factors controlling the precipitation of different carbonate minerals and CaCO 3 polymorphism. Through tens of years' investigations on travertines, considerable advances about carbonate mineral precipitation have been made. Here, we summarized general hydrological features of travertine systems, and driving forces, microenvironments, and mechanisms of the precipitation of different carbonate minerals in travertine-depositing hot springs by integrating present advances and conducting hydrochemical simulations. Travertine-depositing hot springs can be divided into near-neutral pH hot springs and hyperalkaline hot springs and contain four types of microenvironments: hypogean solid-water interface, epigean solid-water interface, air-water interface, and subaerial exposure surface. Both abiotic (passive CO 2 degassing, atmospheric CO 2 uptake, evaporation, and fluid mixing) and biotic (metabolism, organism-related crystal nucleation, and trapping and binding) processes may drive carbonate formation. The specific driving forces, however, depend on bulk water hydrochemistry and microenvironments. Calcite and aragonite are the most common minerals in travertines and the calcite-aragonite polymorphism might be under decisive influences from bulk water chemistry, especially [Mg2+]/[Ca2+], in some conditions. However, calcium carbonate precipitation in hot springs is controlled by solute transport in microenvironments, and any local change induced by hydrodynamics, microorganisms, and extracellular polymeric substances may significantly modify CaCO 3 precipitation and polymorphism. Such integrated control from bulk water composition and microenvironments also affects the formation of other carbonate minerals, but their exact roles remain unclear. Overall, despite the fruitful recent findings, further investigations, especially those focusing on microenvironments, are still imperative to better understand carbonate precipitation in hot springs. These microenvironment-scale studies might also provide insights on carbonate precipitation in other environments. [ABSTRACT FROM AUTHOR]

Although fossil fluvial travertines have been described, factors controlling their formation are poorly understood because of the paucity of their modern counterparts. To disclose processes affecting their deposition, a modern fluvial travertine system at Bagni San Filippo (Siena, central Italy) was studied here and compared with other cases. The studied travertines occur in a valley with an ephemeral stream, but continuous water influx from hot springs on the hillside makes part of the stream become perennial. Four sub-environments were recognized: slopes, waterfalls, pools, and channels. The first two are mainly composed of laminated crystalline crust-boundstone. In contrast, pools are mainly covered by lime mud and some post-flood travertine-encrusted breccia, while channels commonly display abundant travertine-encrusted breccia. Many erosional features and products were also found and are largely the result of episodic erosion triggered by heavy rainfalls and accompanied floods. The predominant erosional processes might include abrasion and plucking. Based on environment distribution, stream bed morphology, and erosional distinctions, the fluvial system was distinguished into slope-pool-waterfall and channel-pool-waterfall subsystems. Such system differentiation is attributed to the original stream bed difference: wide beds promoted the development of slopes, while narrow beds encouraged travertine erosion and subsequent gravel accumulation during flood events, favoring the formation of channels. The comparison shows that fluvial travertine deposition largely occurs within existing rivers. The relative contribution of spring water to original river water controls the deposit composition (the higher contribution ratios, the more abundant carbonate/travertine facies). Furthermore, erosion might be common and unavoidable because the reported fluvial travertines were all formed in non-arid regions. These findings suggest that fluvial travertine deposition is influenced by topography, hot spring contribution, original river bed geometry, and fluvial erosion, and might aid in the interpretation of ancient fluvial travertine systems. Additionally, downstream fluvial travertine systems might show similar characteristics to fluvial tufas, but their formation is clearly hot spring influenced, indicating the importance of analyzing facies distribution and δ13C-δ18O signatures in fluvial carbonate studies. [ABSTRACT FROM AUTHOR]