Ca-Mg Carbonates Research Articles

The composition of fluids stored in the central Mexican lithospheric mantle: Inferences from noble gases and CO2 in mantle xenoliths

We present the first isotopic (noble gases and CO2) characterization of fluid inclusions coupled to Raman microspectroscopy analyses in mantle xenoliths from Central Mexico, a geodynamically complex area where the Basin and Range extension was superimposed on the Farallon subduction (terminated at 28 Ma). To characterize the isotopic signature of the Central Mexican lithospheric mantle, we focus on fluid inclusions entrapped in mantle xenoliths found in deposits of the Joya Honda maar (JH), a Quaternary monogenetic volcano belonging to the Ventura Espiritu Santo Volcanic Field (VESVF) in the state of San Luis Potosí (central Mexico). Thirteen ultramafic plagioclase-free xenoliths were selected, all exhibiting a paragenesis Ol > Opx > Cpx > > Sp, and being classified as spinel-lherzolites and harzburgites. All xenoliths bring textural evidence of interstitial glass veins bearing dendritic trails of secondary melt and fluid inclusions (composed of silicate glass ± CO2 ± Mg-Ca carbonates ± pyrite). These are related to pervasive mantle metasomatism driven by carbonate-rich silicate melt. The Ar and Ne systematics reflect mixing between MORB-like upper mantle and atmospheric fluids, the latter interpreted as reflecting a recycled air component possibly inherited from the Farallon plate subduction. The 3He/4He ratios vary between 7.13 and 7.68 Ra, within the MORB range (7–9 Ra), and the 4He/40Ar* ratios (0.4–3.11) are similarly close to the expected range of the fertile mantle (1–5). Taken together, these pieces of evidence suggest that (i) either the mantle He budget was scarcely modified by the Farallon plate subduction, and/or (ii) that any (large) crustal contribution was masked by a later metasomatism/refertilization episode, possibly during the subsequent Basin and Range extension. A silicate melt-driven metasomatism/refertilization (revealed by the association between glass veins and fluid inclusions) is consistent with calculated helium residence time for the Mexican lithospheric mantle (20 to 60 Ma) that overlaps the timing of the above geodynamic events. We propose that, after the refertilization event (e.g., over the last ~20 Ma), the lithospheric mantle has evolved in a steady-state, becoming slightly more radiogenic. We also estimated 3He fluxes (0.027–0.080 mol/g), 4He production rates (340–1000 mol/yr), and mantle CO2 fluxes (3.93 × 107 mol/yr to 1.18 × 108 mol/yr) using the helium isotopic values measured in JH mantle xenoliths. Finally, the JH xenoliths exhibit CO2/3He ratios comparable to those of the upper mantle (from 3.38 × 108 to 3.82 × 109) but more positive δ13C values (between −1.0 and − 2.7‰), supporting the involvement of a crustal carbonate component. We propose that the metasomatic silicate melts recycled a crustal carbonate component, inherited by the Farallon plate subduction.

Dissolved silica-catalyzed disordered dolomite precipitation

AbstractThere is a great abundance of sedimentary dolomite in the Proterozoic and Lower Paleozoic, but examples of primary dolomite are scarce in the Cenozoic. This discrepancy suggests a poorly understood but dramatic shift in the geochemical system that inhibited dolomite formation. Previous research on microbial-mediated dolomite formation demonstrated that microbial activity could promote disordered dolomite precipitation through the catalytic role of polysaccharides. However, the microbial-mediated model cannot explain some of the Precambrian dolomite for which there is no evidence of microbial origin. Here, we present an abiotic mechanism with dissolved silica catalyzed dolomite precipitation that provides new insight into this long-lasting “dolomite problem.” In this study, we demonstrate that the presence of 1–2 mM of aqueous Si(OH)4 in high Mg:Ca ratio solutions at room temperature will promote disordered dolomite precipitation (with up to 48.7 mol% MgCO3) and inhibit aragonite formation. Dissolved silica in solution also promotes Mg incorporation into the Ca-Mg carbonates. Dissolved silica possesses low-dipole moment and dielectric constant similar to hydrogen sulfide, dioxane, polysaccharide, and exopolymeric substances (EPS), which are catalysts in previously established room-temperature dolomite synthesis. The molecules with low-dipole moment adsorbed on the dolomite surface can lower the dehydration energy barrier of a surface Mg2+-water complex and promote dolomite nucleation and growth. This study provides a new model for abiotic sedimentary dolomite formation, which is likely to be responsible for the significant amount of primary dolomite in Earth history.

Precipitation of low-temperature disordered dolomite induced by extracellular polymeric substances of methanogenic ArchaeaMethanosarcina barkeri: Implications for sedimentary dolomite formation

AbstractA correlation between methanogenesis and dolomite formation has been reported; however, the mechanism underlying this association is not fully understood. In this study, we conducted forced carbonate precipitation experiments at room temperature in calcite-seeded Ca/Mg carbonate solutions containing either purified non-living biomass or bound extracellular polymeric substances (EPS) of the methanogen Methanosarcina barkeri. Purified non-living biomass and bound EPS was used so as to avoid the possible influence of the complex components of the growing microbial culture on carbonate crystallization. Our results demonstrated that non-living biomass of M. Barkeri can enhance the Mg incorporation into calcitic structure and induce the crystallization of disordered dolomite. In the presence of ~113 mg L–1 of non-living biomass, disordered dolomite with ~41 and 45 mol% of MgCO3 was precipitated in solutions with initial Mg:Ca ratios of 5:1 and 8:1, respectively. A systematic increase in the MgCO3 contents of the precipitated Ca-Mg carbonates was also observed with the increased non-living biomass concentration. Bound EPS was shown to be the component of non-living biomass that catalyzed the precipitation of disordered dolomite. At only ~25 mg L–1 of bound EPS, disordered dolomite with ~47 and 48 mol% of MgCO3 was precipitated in solutions with initial Mg:Ca ratios of 5:1 and 8:1, respectively. We propose that adsorption of bound EPS to growing carbonate surfaces through hydrogen bonding is the key to catalyzing disordered dolomite crystallization, and that this mechanism is also applicable to natural EPS-induced dolomite formation. This study provides significant insight into the formation mechanism of microbial-induced dolomite with high δ13C values.

Experimental constraints on nonskeletal CaCO3 precipitation from Proterozoic seawater

Abstract Precambrian carbonates record secular variations in the style of CaCO3 nucleation and growth, yet the geochemical conditions recorded by some enigmatic textures remain poorly quantified. Here, we performed CaCO3 nucleation experiments in synthetic seawater in order to constrain the mineralization pathways of synsedimentary calcite microspar cement, a prolific component of Proterozoic carbonates. We found that dissolved PO4 above ∼12 μmol/L (µM) inhibits the nucleation of aragonite and calcite and permits the formation of an amorphous Ca-Mg carbonate (ACMC) precursor once CaCO3 supersaturation (Ωcal) is ≥ 45. Depending on seawater Mg/Ca, ACMC then rapidly recrystallizes to monohydrocalcite and/or calcite. This precipitation mechanism is consistent with sedimentological, petrographic, and geochemical characteristics of Proterozoic synsedimentary calcite microspar cement, and it suggests that kinetic interactions among common seawater ions may open nontraditional CaCO3 mineralization pathways and sustain high CaCO3 supersaturation.

Microstructure and mineral composition of Holocene stromatolites from Lagoa Vermelha, a hypersaline lagoon in Brazil: Insights into laminae genesis

ABSTRACT Stromatolites are domes, columns, or nearly flat crusts of laminated sedimentary rocks, usually consisting of Ca-Mg carbonates. Stromatolites result from lithification of microbial mats, which are benthic microbial ecosystems where microorganisms arrange themselves in layers according to their physiology. Despite a century of research, the hypothesis of stromatolite genesis by lithification of microbial mats remains controversial, and a convincing explanation for how stromatolites arise from microbial mats is still lacking. In this work, we analyze in detail a stromatolite from Lagoa Vermelha, a coastal hypersaline lagoon in Rio de Janeiro State, Brazil. The stromatolite presents a laminated core and thrombolitic regions at the periphery. Both thrombolitic and laminated facies consist of fine-grained authigenic minerals with minor contributions of bioclasts and quartz grains. X-ray diffraction shows aragonite, high-magnesium calcite (HMC) containing about 17% MgCO3, a very-high-Mg calcite (VHMC) containing 29–46% MgCO3, and small amounts of quartz and pyrite. Scanning electron microscopy of polished samples coupled to energy-dispersive X-ray analysis (EDS) showed that each lamina was composed of 1–4 distinct mineral phases embedded within each other, indicating sequential steps of precipitation of Ca-Mg carbonates under distinct biogeochemical conditions. The coexistence of different phases in a single lamina suggests that several processes contribute to mineral deposition as the incipient stromatolite laminae are left behind by microorganisms from the lower layers of the microbial mat when they grow and/or move upwards.

Contemporary inorganic carbon fluxes from rapidly changing glacierized watersheds of the Himalaya

Transport of Dissolved Inorganic Carbon (DIC) by glacier fed Himalayan streams links the atmospheric carbon dioxide (CO2) concentrations, glacial weathering, and the global carbon cycle. However, the importance of weathering-derived DIC fluxes through glacial streams of the Himalaya with concomitant short- and long-term variability in carbon cycle has rarely been examined. This study provides new constraints on the present day CO2 drawdown by chemical weathering and glacial streams DIC fluxes in the Himalaya. Consequently, we reassessed meltwater ionic composition of five glacierized basins in the central and the western Himalaya between the ablation seasons of 1989 to 2017. We demonstrate that the drawdown of CO2 through silicate weathering is ~2.9 × 105 mol km−2 yr−1 (~17%) for Na bearing minerals, ~3.8 × 105 mol km−2 yr−1 (~22%) for K bearing minerals, and 0.5 × 105 mol km−2 yr−1 (~3%) for Ca bearing minerals. Similarly, CO2 drawdown through carbonate weathering accounts for ~9 × 105 mol km−2 yr−1 (~58%) for Ca and Mg associated minerals (Ca-Mg carbonate). The contribution of DIC flux from the Himalayan glacierized basins to the ocean suggests a total ~0.02 Gg C km−2 yr−1 (1 Gg = 109 g), of which, ~0.009 Gg C km−2 yr−1 is released through silicate weathering (Na, K, and Ca bearing minerals) and ~0.011 Gg C km−2 yr−1 through carbonate (Ca-Mg bearing minerals) weathering. We observed that the present day atmospheric CO2 drawdown and its associated DIC fluxes in retreating Himalayan glacierized basins is dominantly controlled by carbonate weathering. Our findings indicate the importance of carbonate weathering from glacial terrains in carbon sink studies for global carbon models in elucidating climate change effects.

Authigenic formation of Ca–Mg carbonates in the shallow alkaline Lake Neusiedl, Austria

Abstract. Despite advances regarding the microbial and organic-molecular impact on nucleation, the formation of dolomite in sedimentary environments is still incompletely understood. Since 1960, apparent dolomite formation has been reported from mud sediments of the shallow, oligohaline and alkaline Lake Neusiedl, Austria. To trace potential dolomite formation or diagenetic alteration processes in its deposits, lake water samples and sediment cores were analyzed with respect to sediment composition, hydrochemistry and bacterial community composition. Sediments comprise 20 cm of homogenous mud with 60 wt % carbonate, which overlies dark-laminated consolidated mud containing 50 wt % carbonate and plant debris. Hydrochemical measurements reveal a shift from oxic lake water with pH 9.0 to anoxic sediment pore water with pH 7.5. A decrease in SO42- with a concomitant increase in ΣH2S and NH4+ from 0 to 15 cm core depth indicates anaerobic heterotrophic decomposition, including sulfate reduction. The bacterial community composition reflects the zonation indicated by the pore water chemistry, with a distinct increase in fermentative taxa below 15 cm core depth. The water column is highly supersaturated with respect to (disordered) dolomite and calcite, whereas saturation indices of both minerals rapidly approach zero in the sediment. Notably, the relative proportions of different authigenic carbonate phases and their stoichiometric compositions remain constant with increasing core depth. Hence, evidence for Ca–Mg carbonate formation or ripening to dolomite is lacking within the sediment of Lake Neusiedl. As a consequence, precipitation of high-magnesium calcite (HMC) and protodolomite does not occur in association with anoxic sediment and sulfate-reducing conditions. Instead, analytical data for Lake Neusiedl suggest that authigenic HMC and protodolomite precipitate from the supersaturated, well-mixed aerobic water column. This observation supports an alternative concept to dolomite formation in anoxic sediments, comprising Ca–Mg carbonate precipitation in the water column under aerobic and alkaline conditions.

Catalytic effect of microbially-derived carboxylic acids on the precipitation of Mg-calcite and disordered dolomite: Implications for sedimentary dolomite formation

The genesis of dolomite is a contentious issue partly due to the difficulty in its synthesis at ambient temperature. Certain types of microorganisms have been demonstrated to be effective in promoting the precipitation of disordered dolomite, an important precursor of sedimentary ordered dolomite. In contrast to a growing body of research on the catalytic role of microbial exopolymers in the crystallization of disordered dolomite, the role of other microbial exudates (e.g., carboxylic acids) remains unknown. To fill in this knowledge gap, precipitation experiments, mimicking the carbonation process within microbial mats, were conducted in saline solutions containing 0–30 mM succinic acid or citric acid, which are commonly produced by microbes. The starting salinities of experiment solutions were set to 35‰ and 70‰, in order to evaluate the effect of solution salinity on dolomite formation. Our results showed that both succinic acid and citric acid enhanced the incorporation of Mg2+ into growing Ca-Mg carbonates. Solution salinity also played a positive role in enhancing Mg signature in Ca-Mg carbonates. Disordered dolomite with 40.92 mol% MgCO3 was detected under the conditions of 30 mM succinic acid and 70‰ salinity, whereas in other reactors with succinic acid, Mg-calcites formed. Citric acid was more effective in loading Mg2+ into Ca-Mg carbonates compared with succinic acid, as evidenced by the predominant occurrence of disordered dolomite with MgCO3 content ranging from 40.91 mol% to 46.75 mol% in most conditions tested. The results of this study have implications for the formation mechanism of sedimentary dolomite.

The impact of Mg2+ ions on equilibration of Mg-Ca carbonates in groundwater and brines

At temperatures below 50 °C, the log10 (aMg2+/aCa2+) values in groundwater and brines, irrespective of their origin – either carbonaceous or siliceous rocks/sediments – cover the range between −1.5 and +1.0. Calculations of thermodynamic equilibria between the minerals calcite, aragonite, dolomite and huntite suggest a spread of log10 (aMg2+/aCa2+) between minus infinity and +2.3. Log10(aMg2+/aCa2+) in solution of dissolving ordered dolomite at 25 °C fits the thermodynamical equilibrium between disordered dolomite and calcite and nearly corresponds to that of pure calcite with a dolomitic surface layer due to exchange of Ca2+ against Mg2+ in Mg2+-containing solutions. This observation suggests that the solubility of Mg-Ca carbonates is controlled by the composition of their monomolecular surface layers in equilibrium with the ambient aqueous phase. Incongruently dissolving minerals such as dolomite attain equilibrium between individual surface compositions of different carbonates. The bulk composition of these carbonates hardly if ever equilibrates with the ambient solution due to extremely low ion mobility in the lattice. Because the thermodynamical equilibria are based on the composition of bulk minerals, their estimates of equilibria between carbonates, i.e., log10 (aMg2+/aCa2+) in solution, differ significantly from values established by the chemical composition and structure of the surface layer of carbonates.

Precipitation of protodolomite facilitated by sulfate-reducing bacteria: The role of capsule extracellular polymeric substances

The origin of dolomite has long puzzled geologists. It has recently been documented that sulfate-reducing bacteria (SRB) are capable of catalyzing the formation of protodolomite, a previously proposed precursor of ordered sedimentary dolomite. However, the catalytic mechanism of SRB remains incompletely understood. This experimental study is aimed at probing the effect of capsule extracellular polymeric substances (capsule EPS) from SRB on the crystallization of protodolomite in vivo. The capsule EPS tested herein was isolated from a protodolomite-mediating SRB, Desulfotomaculum ruminis, and added into a solution wherein the degree of oversaturation was close to the growth medium of D. ruminis at stationary phase. The solid products were characterized with X-ray diffraction (XRD), Raman spectroscopy and, scanning and transmission electron microscopy (SEM and TEM). Our results indicated that aragonite emerged in the reactors without capsule EPS, while Ca-Mg carbonates (Mg-calcite and protodolomite) were produced in the systems amended with capsule EPS. The incorporation amount of Mg2+ in Ca-Mg carbonates was enhanced with the increasing concentration of capsule EPS. The predominant occurrence of protodolomite was found in the reactor with 140 mg/L capsule EPS. These resulting protodolomites were spherical in shape, and composed of numerous nano-particles. The catalytic influence of capsule EPS on the precipitation of protodolomite might be attributed to their strong Mg2+ binding capacity, potentially diminishing Mg-hydration, which is a potent inhibitor of protodolomite crystallization. The results of Fourier transformation infrared (FT-IR) spectra showed that Mg2+ was bonded with carboxyl and hydroxyl groups on capsule EPS. This inferred adsorption capacity of capsule EPS was also supported by new calculations of complexation chemistry between Mg-H2O complex and organic compounds present in capsule EPS.

Aerobically incubated bacterial biomass-promoted formation of disordered dolomite and implication for dolomite formation

With the realization that the formation of low-temperature dolomite could involve the microbial activity, a new pathway to understand the origin and mechanism of dolomite is emerging. Although microbially mediated CaMg carbonates and dolomite occur in some aerobic and high-salinity conditions, little information about the exact role of aerobic microbes is available. Herein, a strain of moderately halophilic bacteria, Shewanella piezotolerans WP3, was selected to study the involvement of aerobically incubated bacterial biomass in the CaMg carbonate mineralization. Different biomass components were isolated from the bacterial cultures by a set of separation techniques, and used to mediate CaMg carbonate mineralization under a carbon dioxide diffused system. The experimental results showed that bacterial cells play a dominant role in the formation of disordered dolomite. And the mineralization of the disordered dolomite can be attributed to the bare cells and bound extracellular polymeric substances (bound EPS) isolated from the bacterial cells, most likely due to the promotion of high concentration and density of carboxyl and phosphoryl groups on the bacterial biomass (i.e., bare cells and bound EPS). Hence, it does imply that bacterial biomass, even without the active microbial activities, can also play an important role in the formation of dolomite. Current results can not only extend the insight into the biologically influenced mineralization of CaMg carbonates, but also shed light on the precipitation of disordered dolomite/dolomite in modern settings and geological records.

Geochemistry of carbonate rocks in early Precambrian and phanerozoic metamorphic complexes of East Siberia, north-west of Russia, Pamir

Geochemical study of carbonate rocks of Early-Precambrian and Phanerozoic metamorphic Complexes was carried out and their differences were revealed. Precambrian marbles and calciphyres studied in the Оnot Greenstone Belt, Kitoy and Sharyzhalgay granulitic Complexes Presayan uplift, Yenisei Series of the Angara-Kan Protrusion of the Siberian craton, the Belomorian and Lapland Complexes, North of the Pechenga Structure, Sortavala Suite of the Fennoscandian shield, Wakhan Complex of the Badakhshan Array; Phanerozoic – in Olkhon, Slyudyanka, Svyato nossky Complexes of the Baikal region, Boxon Series and Irkutnu Suite of the Eastern Sayan, Derbinsky Complex and Alchadur Suite of the Prisayan, Judin Suite and Panimba-Rybinsk Zone of the Yenisei Ridge, Muzkol Complex of the Eastern Pamirs. Precambrian carbonate rocks are enriched with Fe, Mn and depleted Sr, Ba in comparison with Phanerozoic rocks at a close low level of REE content. The Archean enrichment of Fe, Mn protoliths of marbles and calcifers is due to the dominance of basic and ultrabasic rocks in the feeding provinces. In the Paleoproterozoic, compared with the Phanerozoic, the proportion of Fe, Mn in carbonate rocks decreased, and Al, K, Ba, Sr have increased due to participation in occurrence their protoliths of the granite-metamorphic layer of the Earth. The distribution of petrogenic and rare elements in marbles and calcifirs is determined by the forms of their location: 1) in the isomorphic series of Ca-Mg carbonates with admixture of Fe, Mn, Ba, Sr, REE; 2) the presence of minerals Na, K, Ba, Sr in marbles; 3) location of thin (in marbles) and large (calcifications) fractions of minerals with iron, Mn Al, Ti, Zr, Cr, V, Ni, S. Performed paleoreconstruction behavior of REE in carbonate rocks in the Archean and Early-Paleoproterozoic dominated intracratonic shallow sea. Open oceans appeared at the turn of 2–1.9 billion years, but widespread development of carbonate rocks were in the Meso-Neoproterozoic and Phanerozoic. The revealed features are the basis of age-related paleoreconstructions of protoliths according to petrogeochemical characteristics of carbonate rocks of Precambrian and Phanerozoic metamorphic complexes.

Comparison of the Effects of Ca-Mg Carbonate Chewable Tablets with Calcium Carbonate Tablets on Phosphate Binding in Patients on Hemodialysis Taking H2-R Blockers

Comparison of the Effects of Ca-Mg Carbonate Chewable Tablets with Calcium Carbonate Tablets on Phosphate Binding in Patients on Hemodialysis Taking H2-R Blockers

Carbonate Beach Sand of Abaiang Atoll, Kiribati: Geochemistry, Biogenic Sources, and Properties

Sandy beaches are critical resources for low-lying Pacific atoll communities, providing protection during storms, and land area for many coastal villages. Information on the nature of atoll beach sediment, its geochemistry and composition, can help to establish priorities to effectively protect the sources of Pacific island beach sediment. To understand sand sources, this study evaluated its physical characteristics including grain-size, geochemistry and composition, from windward and leeward beach profiles around Abaiang Atoll, Kiribati. Beach sand was >99% carbonate, averaging 37% coral fragments, 30% mollusc shells, 12% foraminifera, and 20% calcareous algae. Significant differences were found between reef and lagoonal sites in proportions of coral and mollusc fragments and foraminifera tests, with lagoon beaches having higher mollusc and coral proportions and lower foraminifera relative to reef beaches. This is attributed to high foraminiferal productivity offshore of reef beaches, and taphonomic durability of coral fragments in longshore drift into the lagoon. Mean sediment diameter increased from the upper to lower beaches at all sites, but fine sediment was lacking, attributed to its dissolution by rainfall and groundwater outflow. Geochemical analysis showed a mean of 84% Ca-Mg carbonates, of which 80% was calcium carbonate. There was no significant difference in the mean calcium percentage or calcium carbonate composition of the sediment between lagoon and reef beach sediment sources. Magnesium and magnesium carbonate content were significantly higher at reef sites relative to lagoon sites, attributed to higher proportions of foraminifera. Sediment-producing near shore habitats are critical to village protection through provision of beach sand, and this study shows the need to better conserve and manage coral reefs and habitats such as lagoon seagrass beds, to ensure continued atoll beach sand supply.

Phase relations in the system Na2CO3–CaCO3–MgCO3 at 3 GPa with implications for carbonatite genesis and evolution

The phase relations in the system Na2CO3−CaCO3−MgCO3 have been studied at 3 GPa and 700–1285 °C using a Kawai-type multianvil press. At 700 °C, the system has five intermediate compounds: dolomite, Mg-bearing Na2Ca4(CO3)5 burbankite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and eitelite. As temperature increases to 800 °C, the system is complicated by an appearance of Ca-dolomite and Mg-bearing shortite, while Na2Ca4(CO3)5 disappears. At 850 °C, Na4Ca(CO3)3 decomposes to produce Na carbonate and nyerereite. The latter melts incongruently at 875 ± 25 °C to form Na2Ca3(CO3)4. Incongruent melting of eitelite to magnesite and liquid, occurs at 925 ± 25 °C. Mg-bearing shortite melts incongruently at 950 ± 50 °C, producing Na2Ca3(CO3)4 and liquid. Na2Ca3(CO3)4 disappears at 1000 °C via incongruent melting to calcite and liquid. The liquidus projection of the studied ternary system has seven primary solidification phase regions for magnesite, dolomite-calcite solid solutions, Na2Ca3(CO3)4, Mg-bearing shortite, nyerereite, eitelite, and Na carbonate. The primary solidification regions are separated by five peritectic and three cotectic monovariant lines. The system has six ternary peritectic points and one minimum on the liquidus at 850 °C and 52Na2CO3∙48(Ca0.62Mg0.38)CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which, on cooling, a liquid transforms into three solid phases: shortite, Na carbonate, and eitelite. Since the system has a single eutectic at 3 GPa, there is no thermal barrier preventing continuous liquid fractionation from Na-poor toward Na-rich dolomitic compositions more alkaline than eitelite and nyerereite. Considering the present results and previous data, a range of Na-Ca-Mg double carbonates changes in the following sequence upon pressure and temperature increase: Na2Ca2(CO3)3 (Amm2) shortite, Na2Ca(CO3)2 (P21ca) nyerereite, Na2Mg(CO3)2 (R3¯) eitelite (0.1 GPa) → Na2(Ca0.97–0.98Mg0.02–0.03)4(CO3)5 (P63mc), Na2(Ca≥0.91Mg≤0.09)3(CO3)4 (P1n1), Na2(Ca ≥ 0.81 Mg0≤0.19)(CO3)2 (R3¯) nyerereite, Na2(Ca0.77–0.93Mg0.07–0.23)2(CO3)3 (Amm2) shortite, Na4(Ca0.90–0.98Mg0.02–0.10)(CO3)3 (Ia3d), Na2(Mg≥0.9Ca0≤0.1)(CO3)2 (P21ca) eitelite (3 GPa) → Na2(Ca≥0.87Mg0≤0.13)4(CO3)5 (P63mc), Na2(Ca≥0.89Mg≤0.11)3(CO3)4 (P1n1), Na4(Ca ≥ 0.7 Mg0≤0.3)(CO3)3 (Ia3d), Na2(Mg≥0.92Ca0≤0.08)(CO3)2 (P21ca) eitelite (6 GPa). Using the present results at 3 GPa and previous data at 6 GPa in the Na2CO3−CaCO3−MgCO3 system, we constrained isopleths of the Na2CO3 content in melt coexisting with Ca-Mg carbonates. We found that the cratonic geotherms cross the isopleths so that the carbonatite melt percolating upward via the continental mantle lithosphere should become progressively enriched in Na, evolving from alkali-poor dolomitic composition at depths exceeding 200 km toward sodic dolomitic with the ~52 mol% Na2CO3 at 80–120 km depths.

Secondary arsenic minerals from the Złoty Stok As-Au abandoned mine (SW Poland)

Secondary arsenic minerals (SAM) formed recently in abandoned adits of the former Au-As mine at Zloty Stok (SW Poland) constitute two assemblages. The first consists of two types of scorodite, pitticite, kaňkite, hornesite, picropharmacolite and minor amounts of jarosite and gypsum. Formation of the Fe arsenates took place under acidic conditions (pH ~3–4) as a result of lollingite, arsenopyrite and pyrite oxidation. Hornesite and picropharmacolite crystallized as products of interactions between acidic arsenic-rich pore solutions with Mg-Ca carbonates from rocks that surround the ore mineralisation. The interaction of carbonates with acid pore solutions caused a rapid increase in pH that reached neutral or weakly alkaline values. The chemical compositions of hornesite and picropharmacolite correspond well to their ideal compositions: (Mg 3.17 Ca 0,07 ) Σ3.24 (AsO 4 ) 1.90 × 8H 2 O and Ca 4.31 Mg 0.92 (HAsO 4 ) 1.91 [(AsO 4 )1.99(SO 4 )0.01] Σ2.00 × 11H 2 O, respectively. The second assemblage of SAM comprises exclusively the Mg-enriched erythrite [(Co 1.66 Mg 1.03 Ni 0.28 Ca 0.05 Zn 0.02 ) Σ3.03 (AsO 4 )1.99 × 8H 2 O)] – annabergite [(Ni 1.48 Mg 0.94 Co 0.66 Ca 0.12 Fe 0.01 Zn 0.01 ) Σ3.20 (AsO 4 )1.92 × 8H 2 O] series. These minerals crystallized from slightly acidic (pH ~5–6) to neutral media. Dissolution of SAM and other secondary phases (e.g., schwertmannite) causes the release of arsenate and sulphate ions into mine waters. These ions can be reduced under anaerobic conditions by different strains of bacteria. The product of this process is orpiment

Composition and emplacement of the Benfontein kimberlite sill complex (Kimberley, South Africa): Textural, petrographic and melt inclusion constraints

The Benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. We have undertaken a detailed petrographic and melt inclusion study of the Benfontein Upper, Middle and Lower sills. These sills range in thickness from 0.25 to 5 m. New perovskite and baddeleyite U/Pb dating produced ages of 85.7 ± 4.4 Ma and 86.5 ± 2.6 Ma, respectively, which are consistent with previous age determinations and indicate emplacement coeval with other kimberlites of the Kimberley cluster.The Benfontein sills are characterised by large variations in texture (e.g., layering) and mineral modal abundance between different sill levels and within individual samples. The Lower Sill is characterised by carbonate-rich diapirs, which intrude into oxide-rich layers from underlying carbonate-rich levels. The general paucity of xenogenic mantle material in the Benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading. The low viscosity is likely responsible for non-explosive emplacement of the Benfontein sills, while the rhythmic layering is attributed to multiple magma injections.The Benfontein sills are marked by the excellent preservation of olivine and groundmass mineralogy, which is composed of monticellite, spinel, perovskite, baddeleyite, ilmenite, apatite, calcite, dolomite along with secondary serpentine and glagolevite [NaMg6[Si3AlO10](OH,O)8•H2O]. This is the first time glagolevite is reported in kimberlites. Groundmass spinel exhibits atoll-textures and is composed of a magnesian ulvöspinel – magnetite (MUM) or chromite core, surrounded by occasional pleonaste and a rim of Mg-Al-magnetite. We suggest that pleonaste crystallised as a magmatic phase, but was resorbed back into the residual host melt and/or removed by alteration.Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali-carbonates and other phases. These inclusions probably represent the entrapment of variably differentiated parental kimberlite melts, which became progressively more enriched in carbonate, alkalis, halogens and sulphur during crystal fractionation. Carbonate-rich diapirs from the Lower Sill contain more exotic phase assemblages (e.g., Ba-Fe titanate, barite, ancylite, pyrochlore), which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate-rich magma. It is likely that any alkali or halogen rich minerals crystallising in the groundmass were removed from the groundmass during syn−/post-magmatic alteration, or in the case of Na, remobilised to form secondary glagolevite. The Benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust.

Late Pleistostene-Holocene sedimentation in lakes of central Transbaikalia: Implications for climate and environment changes

We present integrated mineralogical, geochemical, and palynological data for Late Pleistocene-Holocene bottom sediments of Lake Arakhlei located in the Beklemishev tectonic basin in the southern Vitim Plateau (central Transbaikalia). The sediment samples were studied by X-ray diffractometry (XRD), Fourier-transform infrared (FTIR) spectroscopy, laser particle sizing, spore-pollen analysis, radiocarbon (14C AMS) dating, and XRF spectrometry. The cored 128 cm long section of lake sediments consists of two units: One is composed mainly of layered silicates (illite-smectite, illite, chlorite, chlorite-smectite, muscovite, and kaolinite) and organic matter (OM) but no carbonates from 0 to 80 cm and the other contains authigenic Ca-Mg carbonates (up to 30%) of Mg-calcite and excess-Ca dolomite from 80 to 128 cm. The sediments also contain a rare mineral weddellite CaC2O42H2O discovered for the first time in Transbaikalian lakes. The evolution of Lake Arakhlei and its drainage basin comprised four stages, with pollen zones that mark the Late Pleistocene and Holocene climate history of the Beklemishev basin. The reconstructed history of Lake Arakhlei for the past ~15,500 years followed general climatic changes in the Northern Hemisphere. Thus, integrated research, including detailed analysis of mineral components and spore-pollen assemblages in lake sediments, is a workable tool for studying climatic controls of continental sedimentation.

Taphonomy and palaeoecology of the lower Miocene marine vertebrate assemblage of Ullujaya (Chilcatay Formation, East Pisco Basin, southern Peru)

The taphonomy and palaeoecology of the early Miocene (Burdigalian) vertebrate assemblage of Ullujaya (East Pisco Basin, Peru) is here described. Vertebrate remains are concentrated in marine facies (Ct1a association) of the exposed Chilcatay Formation (dated 19–18 Ma) deposited within a 30–40 m deep, semi-enclosed, offshore environment. Coupled with ichnological observations, the size distribution of pyrite framboid relics reveals fluctuation of euxinic and oxic-dysoxic conditions at the seafloor. The assemblage is dominated by toothed cetaceans (kentriodontids, squalodelphinids, physeteroids, and the eurhinodelphinid-like Chilcacetus), together with a large dermochelyid turtle, some bony fish, and diverse elasmobranchs, mostly juveniles of Carcharhinus brachyurus and Cosmopolitodus hastalis. The vertebrate assemblage comprises a coastal community, dominated by mesopredators, representative of a warm-temperate, sheltered embayment connected with riverine and open-ocean environments. Vertebrate skeletons are typically disarticulated and incomplete, and some bone elements display shark bite marks. Microborings are observed at the bone surface. Bones exhibit a good degree of apatite mineralisation and bone cavities are locally filled by Ca-Mg carbonates. Our taphonomic observations suggest prolonged flotation of carcasses during which they were subject to biogenic and physical processes of partial destruction (including scavenging by sharks), before final deposition on a soft compact substrate. Preservation was favoured by the oxygen-deficient bottom conditions that inhibited the action of benthic macro-scavengers.

Open ocean vs. continentally-derived iron cycles along the Neoarchean Campbellrand-Malmani Carbonate platform, South Africa

The deposition of large amounts of mixed-valence Fe minerals in iron formations during the Archean and Paleoproterozoic indicates that the Fe(II)aq (aqueous) content of coeval anoxic seawater was likely several hundred μM, compared to ca. 1 to 20 nM of the modern oxygenated ocean. It has been suggested that oxygen production along shallow marine continental shelves, which probably started several hundred million years before the rise of atmospheric oxygen, effectively oxidized Fe(II)aq from deeper seawater and removed it as Fe(III)ppt (poorly soluble precipitates). However, the reconstruction of the marine Fe cycle during the Archean is still incomplete, partly because of diagenetic redox processes that challenge the interpretation of Fe concentration and isotope signatures of sedimentary archives. In this study, we present new Fe concentrations and isotope compositions of carbonate and mudrock samples from the Neoarchean Campbellrand-Malmani carbonate platform (CMCP) in South Africa. These samples are from the shelf facies of the CMCP and in combination with previously published data of Czaja and others (2012) from carbonates and mudrocks of the slope facies, we show that different depositional settings and conditions resulted in different data distributions. Coupled δ56Fe values (−3.685 to +0.083 ‰) and iron concentrations (861–27672 μg g−1) of pure carbonates deposited during open marine conditions, can be explained by partial Fe(II) oxidation between ferruginous deeper water and oxygenated shallow water, leaving the residual Fe(II)aq pool isotopically light, although Fe(II) oxidation by anoxygenic phototrophy cannot be ruled out. Pure carbonates deposited in a peritidal setting, with less exposure to open ocean water, show a smaller Fe isotope variability with δ56Fe values of −1.207 to −0.204 permil and Fe concentration range from 388 to 5413 μg g−1, respectively. We propose that the Fe systematics of peritidal carbonates were dominated by early diagenetic Fe cycling between carbonates and adjacent mudrocks. Synchrotron based X-ray adsorption spectroscopy reveals a change in Fe speciation, where Fe(II)-bearing ankerite and Fe-sulfide dominate the carbonates in the lower part of the CMCP, whereas carbonates of the upper part of the CMCP mainly contain Fe(III)-(oxyhydr)oxides. The fact that Fe(III) phases are still preserved argues for a higher oxidation state on the shelf of the upper CMCP. This is likely because of a lower content of reductants in those settings, in particular organic carbon, sulfide species, as well as restricted influx of reducing species from the anoxic open ocean due to the formation of a rimmed margin. Nevertheless, more studies of similar carbonate settings are necessary to verify our model. We propose that unfractionated Fe(II)aq in seawater was about two to three times lower on the shelf (30–310 μM) than along the slope (61–928 μM), which implies that Fe(II)aq was removed from the water column closer to the continent, likely by oxidation and precipitation. Overall, the Fe isotope composition and Fe speciation of CMCP sediments support the presence of molecular oxygen in the shallow-marine system and emphasize the utility of Ca-Mg carbonates as proxies for iron cycling in the aqueous environment.