Abstract We conducted hydrothermal experiments at 300°C and at pressure varying from 2.2 to 3.4 kbar to study the effect of fluid salinity on the coupling between molecular hydrogen (H 2 ) formation and olivine serpentinization, where peridotite and olivine with 25–50 μm of starting grain sizes were reacted with pure H 2 O and saline solutions (0.5, 1.5, and 3.3 M NaCl). Serpentine, the main hydrous mineral in most experiments, was quantified according to calibration curves based on Fourier‐transformed infrared spectroscopy and X‐ray diffraction analyses. Compared to pure H 2 O, saline solutions promote the hydrothermal alteration of olivine and peridotite. For experiments with peridotite and pure H 2 O, 67% of reaction extent was achieved after 14 days, which increased to 89% in experiments with medium‐salinity solutions (1.5 M NaCl) over the same period. Medium‐ and high‐salinity solutions inhibit H 2 formation during serpentinization, which is associated with the serpentinization of pyroxene especially clinopyroxene. The redox conditions were constrained according to the equilibrium H 2,aq = H 2,g , and very reducing conditions were achieved during the serpentinization of olivine and peridotite. This study is the first to show iowaite formation directly from peridotite serpentinization, indicating alkaline solutions. Thermodynamic calculations suggest that the hydrolysis of NaCl (NaCl + H 2 O = HCl + NaOH) may yield alkaline solutions, due to higher dissociation constants of NaOH compared to HCl. This study suggests that chlorine greatly influences the serpentinization of olivine and peridotite in natural geological settings. It also indicates that iowaite formation may not require oxidizing conditions as previously thought.
Evaporites of Middle Miocene Badenian stage occur widely in basins from the Carpathian Mountain region (Poland, Slovakia and Ukraine), but their source and formation process are still debatable. A detailed boron isotope study in combination with previous chlorine isotope and chemistry data of the salt samples from three localities (Wieliczka mine, Trans-Carpathian Basin, East Slovakian Basin) reveal ranges of δ11B (−4.5 to +35.7‰) and δ37Cl (−0.2 to +0.8‰). Modelling calculation indicates that both Rayleigh fractionation and incorporation of fluid inclusion solutes cannot cause such a large shift on δ11B values in halite. Instead, the B and Cl isotope data imply multiple brine sources, including evaporite brine, dissolved diapiric halite and basin brine in addition to the predominant seawater source during the formation of these evaporites. At Wieliczka salt mine, a positive δ11B excursion (from −5‰ to + 30‰) matches the negative δ37Cl variation (from +0.5 ± 0.1‰ to -0.2‰) stratigraphically upwards, which indicate both terrestrial boron and non-marine chloride made significant contributions to the composition of the basin brine during the early development of the basin. In the upper column, the δ11B values are within the marine range, but show influence by the sorption of boron onto clay, whereas the δ37Cl values at +0.5‰ still indicate the presence of non-marine chloride, possibly from recrystallized, diapiric halite. In the Trans-Carpathian Basin, constant δ37Cl (+0.3‰) and δ11B (+15‰) in the middle of the profile are consistent with a dominant marine source, whereas the lower δ11B (+2.2‰) and higher δ37Cl (+0.8‰) in the lower column suggest terrestrial fresh water flowed into the basin during the formation of basal halite. Halites in the upper part of the profile show near 0‰ of δ37Cl, suggesting incorporation of Cl from a mixture of expelled basin brines. In the Slovakian Basin, the δ11B values (+18.1 to +19.1‰) at the base of the profile lie within the marine range, but high δ37Cl values (+0.7 to +0.8‰) require a non-marine chloride source. In the upper part of the profile, boron isotope data indicate a change from marine (+12.2 to +23.4‰) to non-marine (+5.4 to +6.1‰) derivation of B, but the sources of Cl remain marine (+0.0 to +0.5‰). Overall, both B and Cl isotopes show coupled variation in the Middle Miocene Badenian evaporites and suggest multiple sources of B and Cl.
Ringwoodite is an important mineral in the mantle transition zone, and its cationic disorder can profoundly affect its physicochemical properties, but there is currently much controversy about this disorder. In this study, we investigate the cation disorder states of pure Mg2SiO4-ringwoodite and defective ringwoodite under mantle transition zone conditions through DFT calculations and thermodynamic models. Two stable endmembers are seen, one with normal ringwoodite structure and the other with inverted structure (its Si atoms and half of its Mg atoms have swapped sites). Our results indicate that pure ringwoodite does not invert (swap Mg and Si cations) under normal mantle temperatures but the introduction of a Si-excess, Mg-deficient defect induces a swap at normal mantle temperatures and this swap is likely induced by a wide range of defects including water. Thus, in the presence of such a defect or similar defects the olivine phase transition sequence may then go from olivine to wadsleyite to inverse ringwoodite, and then normal ringwoodite. We calculate the seismic properties of normal and inverse ringwoodite and find significantly slower wave speeds in inverted ringwoodite. Due to this difference the presence of inverse ringwoodite may provide a potential explanation for the discontinuous interface of seismic waves at the depth of ∼560 km.
Carbonate-bearing fluids widely exist in different geological settings, and play important roles in transporting some elements such as the rare earth elements. They may be trapped as large or small fluid inclusions (with the size down to <1 μm sometimes), and record critical physical-chemical signals for the formations of their host minerals. Spectroscopic methods like Raman spectroscopy and infrared spectroscopy have been proposed as effective methods to quantify the carbonate concentrations of these fluid inclusions. Although they have some great technical advantages over the conventional microthermometry method, there are still some technical difficulties to overcome before they can be routinely used to solve relevant geological problems. The typical limitations include their interlaboratory difference and poor performance on micro fluid inclusions. This study prepared standard ion-distilled water and K2CO3 aqueous solutions at different molarities (from 0.5 to 5.5 mol/L), measured densities, collected Raman and infrared spectra, and explored correlations between the K2CO3 molarity and the spectroscopic features at ambient P-T conditions. The result confirms that the Raman O–H stretching mode can be used as an internal standard to determine the carbonate concentrations despite some significant differences among the correlations, established in different laboratories, between the relative Raman intensity of the C–O symmetric stretching mode and that of the O–H stretching mode. It further reveals that the interlaboratory difference can be readily removed by performing one high-quality calibration experiment, provided that later quantifying analyses are conducted using the same Raman spectrometer with the same analytical conditions. Our infrared absorption data were collected from thin fluid films (thickness less than ~2 μm) formed by pressing the prepared solutions in a Microcompression Cell with two diamond-II plates. The data show that both the O–H stretching mode and the O–H bending mode can be used as internal standards to determine the carbonate concentrations. Since the IR signals of the C–O antisymmetric stretching vibration of the CO32− ion, and the O–H stretching and bending vibrations from our thin films are very strong, their relative IR absorbance intensity, if well calibrated, can be used to investigate the micron-sized carbonate-bearing aqueous fluid inclusions. This study establishes the first calibration of this kind, which may have some applications. Additionally, our spectroscopic data suggest that as the K2CO3 concentration increases the aqueous solution forms more large water molecule clusters via more intense hydrogen-bonding. This process may significantly alter the physical and chemical behavior of the fluids.
A rare massive yellowish-green serpentinized dunite, covering a minimum area up to ~50 m2, has been found in Ji’an County, Jilin Province, Northeast China. It contains primary olivine and secondary serpentine (antigorite) and brucite. Other primary minerals like orthopyroxene, clinopyroxene, and aluminum-rich phase (such as garnet, spinel, and plagioclase), frequently appearing in ultramafic rocks, have not been identified. The olivine is essentially pure forsterite, with an Mg# (100 × Mg/(Mg + Fe)) of ~99.6–99.7. Due to these distinct features, we especially name the protolith of this dunite as jianite (集安岩). The forsterite grains range up to ~2 mm, show clear equilibrium textures such as nearly straight grain boundaries and ~120° dihedral angles at their triple junctions, and display no intragranular or intergranular composition variations. They are extensively ruptured and hydrated (i.e., serpentinized), with the fractures (and the grain boundaries as well) filled by fine-grained antigorite (ideally Mg6(Si4O10)(OH)8) and brucite (ideally Mg(OH)2). These secondary phases are also extremely poor in Fe, indicating a good chemical equilibrium with the forsterite. The serpentinization reaction may have proceeded as forsterite + fluid = antigorite + brucite at temperatures of ~425(25) °C and at relatively low but undetermined pressures. The fluid was likely a B-rich, but Si-poor dilute aqueous fluid, as implied by the trace element characteristics and water-related infrared features of the forsterites in equilibrium. The petrogenesis of the jianite is presently unclear.
Three batches of Mg2SiO4-ringwoodites (Mg-Rw) with different water contents (CH2O = ~1019(238), 5500(229) and 16,307(1219) ppm) were synthesized by using conventional high-P experimental techniques. Thirteen thin sections with different thicknesses (~14–113 μm) were prepared from them and examined for water-related IR peaks using unpolarized infrared spectra at ambient P-T conditions, leading to the observation of 15 IR peaks at ~3682, 3407, 3348, 3278, 3100, 2849, 2660, 2556, 2448, 1352, 1347, 1307, 1282, 1194 and 1186 cm−1. These IR peaks suggest multiple types of hydrogen defects in hydrous Mg-Rw. We have attributed the IR peaks at ~3680, 3650–3000 and 3000–2000 cm−1, respectively, to the hydrogen defects [VSi(OH)4], [VMg(OH)2MgSiSiMg] and [VMg(OH)2]. Combining these IR features with the chemical characteristics of hydrous Rw, we have revealed that the hydrogen defects [VMg(OH)2MgSiSiMg] are dominant in hydrous Rw at high P-T conditions, and the defects [VSi(OH)4] and [VMg(OH)2] play negligible roles. Extensive IR measurements were performed on seven thin sections annealed for several times at T of 200–600 °C and quickly quenched to room T. They display many significant variations, including an absorption enhancement of the peak at ~3680 cm−1, two new peaks occurring at ~3510 and 3461 cm−1, remarkable intensifications of the peaks at ~3405 and 3345 cm−1 and significant absorption reductions of the peaks at ~2500 cm−1. These phenomena imply significant hydrogen migration among different crystallographic sites and rearrangement of the O-H dipoles in hydrous Mg-Rw at high T. From the IR spectra obtained for hydrous Rw both unannealed and annealed at high T, we further infer that substantial amounts of cation disorder should be present in hydrous Rw at the P-T conditions of the mantle transition zone, as required by the formation of the hydrogen defects [VMg(OH)2MgSiSiMg]. The Mg-Si disorder may have very large effects on the physical and chemical properties of Rw, as exampled by its disproportional effects on the unit-cell volume and thermal expansivity.
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