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1. Fluid Mixing and Spatial Geochemical Variability in the Lost City Hydrothermal Field Chimneys

   https://doi.org/10.1029/2023GC011011  

   Aquino, Karmina_A; Früh‐Green, Gretchen_L; Bernasconi, Stefano_M; Rickli, Jörg; Lang, Susan_Q; Lilley, Marvin_D (February 2024, Geochemistry, Geophysics, Geosystems)

   Abstract Carbonate‐brucite chimneys are a characteristic of low‐ to moderate‐temperature, ultramafic‐hosted alkaline hydrothermal systems, such as the Lost City hydrothermal field located on the Atlantis Massif at 30°N near the Mid‐Atlantic Ridge. These chimneys form as a result of mixing between warm, serpentinization‐derived vent fluids and cold seawater. Previous work has documented the evolution in mineralogy and geochemistry associated with the aging of the chimneys as hydrothermal activity wanes. However, little is known about spatial heterogeneities within and among actively venting chimneys. New mineralogical and geochemical data (⁸⁷Sr/⁸⁶Sr and stable C, O, and clumped isotopes) indicate that the brucite and calcite precipitate at elevated temperatures in vent fluid‐dominated domains in the interior of chimneys. Exterior zones dominated by seawater are brucite‐poor and aragonite is the main carbonate mineral. Carbonates record mostly out of equilibrium oxygen and clumped isotope signatures due to rapid precipitation upon vent fluid‐seawater mixing. On the other hand, the carbonates precipitate closer to carbon isotope equilibrium, with dissolved inorganic carbon in seawater as the dominant carbon source and have δ¹³C values within the range of marine carbonates. Our data suggest that calcite is a primary mineral in the active hydrothermal chimneys and does not exclusively form as a replacement of aragonite during later alteration with seawater. Elevated formation temperatures and lower⁸⁷Sr/⁸⁶Sr relative to aragonite in the same sample suggest that calcite may be the first carbonate mineral to precipitate. 

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2. Kiloyear cycles of carbonate and Mg-silicate replacement at Von Damm hydrothermal vent field

   https://doi.org/10.1130/G53228.1  

   Gartman, Amy; Blackburn, Terrence; Frank, Kiana L; Lang, Susan Q; Seewald, Jeffrey S (May 2025, Geology)

   Abstract The Von Damm vent field (VDVF) on the Mid-Cayman Rise in the Caribbean Sea is unique among modern hydrothermal systems in that the chimneys and mounds are almost entirely composed of talc. We analyzed samples collected in 2020 and report that in addition to disordered talc of variable crystallinity, carbonates are a major class of mineral at VDVF. The carbonate minerals include aragonite, calcite, magnesium-rich calcite, and dolomite. Talc and carbonate mineral textures indicate that, rather than replacing volcanic host rock, they precipitate from the mixing of hydrothermal fluids and seawater at the seafloor, occurring in chimneys and surrounding rubble. Alternating precipitation of this mineral assemblage is pervasive, with carbonate minerals typically being succeeded by talc, and with indications that in some cases talc and carbonate minerals replace one another. Stable carbon isotopic data indicate the carbonate minerals originate from the mixing of seawater and hydrothermal fluid, which is supported by U-Th data. Radiocarbon calcite ages and talc 234U-230Th isochron ages indicate mineral ages spanning over thousands to tens of thousands of years. Analyses of these samples illustrate a dynamic system that transitions from carbonate-dominated to Mg-silicate–dominated precipitation over time scales of thousands of years. Our observations raise questions regarding the eventual fate of seafloor precipitates and whether carbonate and silicate minerals in such settings are sequestered and represented in the rock record. 

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3. Strong and persistent mineral-specific lithium isotope fractionation: no discernable kinetic isotope effects during inorganic calcite and aragonite precipitation

   https://doi.org/10.1016/j.gca.2025.03.030  

   Hite, Corinne; Uchikawa, Joji; Misra, Sambuddha; Satya_Chanakya, IV; Chanda, Pratyusha; Zeebe, Richard E (May 2025, Geochimica et Cosmochimica Acta)

   Stable lithium isotopes (δ7Li) of CaCO3 minerals have increasingly been used as a tracer for changes in silicate weathering processes. However, there is limited understanding of the influence of physical and chemical conditions on δ7Li values of CaCO3 minerals during their formation in aqueous solutions. Here, we examined Li isotope fractionation in inorganic calcite and aragonite precipitation experiments with systematic manipulations of solution pH and concentrations of total dissolved inorganic carbon species ([DIC] ≈ [HCO3−] + [CO32−]) and calcium ion (Ca2+). Calcite and aragonite samples had δ7Li values lower than those of dissolved Li in solutions by about 3‰ and 16‰, respectively, indicating preferential uptake of the lighter 6Li isotopes. Aragonite consistently had δ7Li values lower than those of calcite by ∼13‰, likely due to differences in Li coordination and thereby the strength of bonds formed by/with Li within the respective mineral structure. We observed no statistically significant changes in aragonite nor calcite δ7Li values in response to changing solution pH, [DIC], [Ca2+], and CaCO3 precipitation rates, indicating our solution chemistry manipulations imposed little effect on Li isotope fractionation. These findings lead us to argue that the observed Li isotope fractionations in calcite and aragonite with respect to dissolved Li in solutions are dominated by equilibrium isotope effects, and that kinetic effects for δ7Li values in CaCO3 are either non-existent or too small to be expressed under our experimental conditions. 

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4. A review of the nature and origin of limestone microporosity

   https://doi.org/10.1016/j.marpetgeo.2019.03.037  

   Hashim, Mohammed S. Kaczmarek (April 2019, Marine and petroleum geology)

   Limestone microporosity is ubiquitous and extensively developed in most Phanerozoic limestones. From an economic perspective, microporosity is important because it contributes substantially to the carbonate pore system, which can host significant volumes of water and hydrocarbons. Therefore, determining the presence and distribution of limestone micropores is necessary for accurate hydrocarbon estimations, reservoir characterization, and fluid flow simulations. From an academic standpoint, microporosity is important because its genesis is intimately linked with the mineralogical stabilization of metastable sediments, a fundamental process in carbonate diagenesis. Many types of micropores contribute to what has been referred to as microporosity, but the vast majority is hosted among low-magnesium calcite (LMC) microcrystals that are present in limestone matrix and allochems. Geochemical, textural, and mineralogical data from natural settings and laboratory experiments indicate that LMC microcrystals are diagenetic in origin. More specifically, these data support a diagenetic model of mineralogical stabilization that involves dissolution of precursor sediments dominated by aragonite and high-magnesium calcite (HMC) minerals, and precipitation of LMC microcrystal cements. The stabilization process is inferred to take place in the meteoric, marine, and burial diagenetic realms. Although it has not been directly observed, carbon and oxygen isotopes, as well as trace element data suggest that LMC microcrystals form during burial diagenesis in marine-like fluids. Evidence suggests that porosity is not generated during this dissolution-precipitation process, but rather inherited from the precursor sediments. The final arrangement of the micropores in a limestone, however, depends on the precise diagenetic pathway. LMC microcrystals exhibit a range of microcrystalline textures that are classified on the basis of crystal morphology and size. The three main textural classes - granular (framework), fitted (mosaic), and clustered - have been recognized across a wide range of ages, depositional settings, burial depths, and precursor types, and are characterized by distinct petrophysical properties, such as porosity, permeability, and pore-throat size. Observations from modern sediments also support the hypothesis that LMC microcrystals develop from aragonite and HMC dominated lime mud. The origin of lime mud has been extensively studied but still highly debated. Of particular interest to the discussion of microporosity are proposed secular variations in the dominant mineralogy of carbonate sediments through the Phanerozoic. Microporous limestones comprised of LMC microcrystals are equally abundant during times of aragonite seas and calcite seas, which suggests that no special mineral precursor is required. Microporous textures are also observed in deep marine chalks where micropores are hosted between chalk constituents. Unlike shallow marine limestones, deep marine sediments start out as mostly LMC therefore mineralogical stabilization is not a significant process in chalk diagenesis. 

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5. The Transformation of Aragonite to Calcite in the Presence of Magnesium: Implications for Marine Diagenesis

   Hashim, M.; Kaczmarek, S.E. (September 2021, Earth and planetary science letters)

   null (Ed.)

   Magnesium (Mg) in natural waters plays a critical role in governing carbonate mineral formation, dissolution, and diagenesis. Previous laboratory experiments show that Mg can strongly inhibit direct calcite precipitation as well as aragonite to calcite diagenetic transformation. Data from natural settings, however, suggest that diagenetic calcite in most Phanerozoic limestones has formed in the shallow marine burial realm in the presence of ample Mg. Thus, the diagenetic conditions under which aragonite-rich sediments convert to calcite-rich limestones are poorly understood. Here, we present data from laboratory experiments whereby aragonite is converted to calcite at 70◦C in Mg-bearing solutions to investigate the effects of fluid:solid ratio (F:S), which varies greatly across diagenetic environments, on Mg inhibition and incorporation in calcite. Our data show that not only can the transformation of aragonite to calcite occur in solutions with higher [Mg] than previously shown possible in laboratory experiments, but that progressively lower F:S increase the rate at which aragonite stabilizes to calcite. For example, in experiments with an F:S of 0.3 mL/g, which corresponds to sediments in a closed system with 50% porosity, aragonite stabilizes to calcite in solution with [Mg]=30 mM (Mg/Ca=5.14) when an initial high degree of undersaturation with respect to aragonite is used and in a solution with [Mg]=20 (Mg/Ca=5.14) when a low degree of undersaturation is used. In contrast, aragonite does not stabilize to calcite after nearly 3000 h in experiments with an F:S of 100 mL/g, which is more typical of an open system, even in a solution with [Mg]=5 mM (Mg/Ca=5.14) regardless of the degree of undersaturation. Our results also show that the amount of Mg incorporated into calcite products increases linearly with the increase of F:S. Collectively, these observations further point to F:S as an important factor in carbonate diagenesis with broad implications. First, the observations that transformation of aragonite to calcite is inhibited at high [Mg] and F:S imply that calcite precipitation is unlikely to occur in marine diagenetic environments that are in direct hydrologic contact with seawater. This leaves aragonite dissolution as the dominant diagenetic process in these environments, which may represent an underrated source of alkalinity to the open ocean. Second, transformation from aragonite-rich sediments to the calcite-rich limestones that dominate the rock record is likely promoted by a decrease in the F:S and the development of a closed system during progressive burial. 

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