Chemical Geology最新文献

Novel biosignatures of laminated, microbial, digitate sedimentary structures – stromatolites – from modern geothermal fields of the Taupō Volcanic Zone, New Zealand, and from El Tatio, Chile, provide an opportunity to investigate evidence of extremophile life preserved in siliceous hot spring deposits, or sinters, interpreted as analogs for early life on Earth and possibly Mars. Synchrotron-μXRF, electron microprobe analysis, Raman spectroscopy, and optical microscopy are used in a coordinated approach to identify corroborating textural and chemical (organic, inorganic) evidence of life in these modern, opaline (amorphous) siliceous materials. Fluid mobile elements, such as As and Sr, track the growth history of the digitate structures. Trace element enrichments of Ca, Al, Ga, +/− Fe, Mn, As, Rb, Cs, and Sr, are identified in silicified sheaths of microbial filaments embedded within the sinter. In contrast, silicified diatoms in some sinter samples show no trace element enrichment. Gallium enrichments have also been observed in other 16 ka and Jurassic (150 Ma) microbial palisade sinter textures, suggesting the potential for preservation through geologic time, even after recrystallization to quartz. Raman analysis reveals spectra of organics, consistent with pigments for UV protection in cyanobacteria, in silicified sheaths around microbial filaments and are co-located with trace metal enrichments in digitate structures. Due to spectral bands, the location of these molecules (i.e., in the sheaths), and the sampling locations, we ascribe the spectra to scytonemin and carotenoid class molecules. The combined analytical approach outlined here provides a robust means to assess the validity of novel biosignatures, with application to the exploration of Mars, where preservation of opaline silica in >3.6 Ga deposits has the potential to preserve a range of microbial biosignatures.

The primary geological conditions for hydrothermal systems can be reconstructed using fluid inclusions, which generally requires that fluid inclusions trapped a single homogeneous phase and experienced no mass and volume changes after entrapment (Roedder's Rules). However, heterogeneous entrapment and post-entrapment modifications have been frequently identified in fluid inclusion studies, making the recovery of primary fluid properties from fluid inclusion data challenging or impossible. In this study, we constructed a specific tool, FlincPro, for the calculations of ideal and nonideal Fluid inclusion Properties in the H₂O-NaCl system (ideal here refers to fluid inclusions that adhere to Roedder's Rules). Multiple modules have been developed to quantify the variations of fluid inclusion properties caused by heterogeneous entrapment, volume contraction/expansion, and water loss. The developed software illustrates possibilities in the variation of fluid inclusions in a specific assemblage with the input of a certain starting point.

We found that compositions of the vapor endmember fluid are more significantly affected by heterogeneous entrapment than the liquid endmember, as the latter may have several orders of magnitude higher salinity and density. Homogenization pressure and temperature of halite-bearing fluid inclusions trapped on the halite liquidus will increase sharply with the addition of halite, but the homogenization behavior will not be changed. Volume contraction/expansion are effective ways to change the homogenization behaviors, especially for high salinity fluid inclusions. Water loss is not likely to cause the homogenization behavior change from total homogenization by vapor disappearance to total homogenization by halite dissolution for a halite-bearing fluid inclusion without additional volume contraction. Low-salinity and low-density fluid inclusions with total homogenization temperature around the critical temperature of water are least affected by post-entrapment modifications. Therefore, the interpretation of fluid inclusions formation depends on more parameters, e.g., petrographic observations, density and composition variations in fluid inclusion assemblages, and geological information, than only microthermometric data.

Triple oxygen isotope compositions of authigenic minerals are an emerging tool for the reconstruction of fluid temperatures and the fluid isotopic composition. In this study, we analyzed euhedral authigenic quartz crystals from a halite deposit of the Upper Permian Zechstein evaporitic basin for their triple oxygen isotope composition using laser fluorination isotope ratio mass spectrometry. In the triple oxygen isotope space, additional information, such as the triple oxygen isotope composition of the ambient water, provides insights into equilibrium conditions during quartz formation that cannot be identified from δ¹⁸O alone. The triple oxygen isotope composition of authigenic quartz shows, that single oxygen isotope (δ¹⁸O) thermometry is not applicable for samples from evaporitic environments, where the fluids' δ¹⁸O deviates from dynamic oxygen isotope equilibrium. By combining the Craig and Gordon evaporation model with H₂O – SiO₂ triple oxygen isotope equilibrium fractionation, we reconstruct the formation temperature of authigenic quartz and the triple oxygen isotope composition of the marine-derived saline brine pore fluid in equilibrium with the quartz. We obtain temperatures of 90 to 130 °C that is interpreted as crystallization temperatures of microcrystalline quartz in pore fluid of halite during burial diagenesis. The triple oxygen isotope composition of authigenic quartz from evaporitic environments is suited as a geochemical tracer for fluid oxygen isotope compositions and ancient fluid or diagenetic temperatures.

We report isotope dilution thermal ionization mass spectrometry (ID-TIMS) and laser ablation split stream inductively coupled plasma mass spectrometry (LASS) UPb data for a suite of widely available reference apatites: Fish Canyon Tuff, Mount Dromedary, TEMORA 2, and Duluth Complex anorthosite. We apply different common-Pb correction strategies to the UPb data sets: (1) anchoring to a Stacey and Kramers (1975) model Pb composition; (2) unanchored 2-D ²³⁸U/²⁰⁶Pb-²⁰⁷Pb/²⁰⁶Pb isochron regressions; and (3) unanchored 3-D ²³⁸U/²⁰⁶Pb-²⁰⁷Pb/²⁰⁶Pb-²⁰⁴Pb/²⁰⁶Pb isochron regressions. The different common-Pb corrections yield consistent dates within each ID-TIMS and LASS data set, with 3-D regression method producing the highest precision isochrons. FCT apatite produces an ID-TIMS 3-D isochron age of 28.8 ± 3.7 Ma with (²⁰⁷Pb/²⁰⁶Pb)_i = 0.851 ± 0.021. Mount Dromedary apatite yields an ID-TIMS 3-D isochron age of 98.4 ± 0.5 Ma with (²⁰⁷Pb/²⁰⁶Pb)_i = 0.839 ± 0.003. TEMORA 2 apatite has an ID-TIMS 3-D isochron age of 402 ± 7 Ma and (²⁰⁷Pb/²⁰⁶Pb)_i = 0.839 ± 0.008. Duluth Complex anorthosite apatite yields an ID-TIMS 3-D isochron age of 1077 ± 9 Ma with (²⁰⁷Pb/²⁰⁶Pb)_i = 0.849 ± 0.046. The MSWDs associated with isochrons calculated from both the ID-TIMS and LASS data sets are larger than expected for a single age population, revealing complexities that are otherwise not captured by 2-D isochron methods. In the case of FCT apatite, the ID-TIMS data indicate significant heterogeneity in the initial Pb ratio ((²⁰⁷Pb/²⁰⁶Pb)_i = 0.845–0.856), invalidating this sample as a viable reference apatite for high-precision geochronology. Additionally, the common-Pb compositions of TEMORA 2 and Duluth Complex anorthosite apatites calculated using the ID-TIMS data deviate from bulk Earth Pb evolution models beyond 2σ uncertainty. The data emphasize the utility of unanchored age regressions in generating the highest fidelity apatite UPb dates. Further, TEMORA 2 and Duluth Complex apatite ages are both younger than their corresponding zircon UPb ages, highlighting the need to independently verify the ages of prospective reference apatites.

A natural gem quality, inclusion-free, U-poor xenotime has been metasomatically altered at 900 °C and 500–1000 MPa in a series of alkali-bearing fluids including 2 N KOH, 2 N NaOH, Na₂Si₂O₅ + H₂O, and NaF + H₂O to which UO₂ and SiO₂ have been added. All experiments were buffered to CO₂-CO-graphite with the exception of the NaF + H₂O experiment at 900 °C and 500 MPa, which was buffered to magnetite-hematite. With the exception of the Na₂Si₂O₅ + H₂O experiment, the xenotime reacted with the fluid tried in all of the remaining experiments to varying degrees. The xenotime showed the greatest degree of reactivity in the two NaF + H₂O experiments tried. This reactivity takes the form of a coupled dissolution-reprecipitation reaction, which involves enriching reacted areas of the xenotime in U + Si as well as remobilizing the HREE in these areas inherent to the original xenotime. One result was the formation of alternating bands of Y-rich and HREE- + U-rich bands in the NaF + H₂O experiment when the system was oxidized, which are presumed to have formed due a combination of simple chemical zoning coupled with diffusion and coupled dissolution-reprecipitation in the altered areas of the xenotime. A total lack of fluid and mineral inclusions on the nano-scale, as well as no disturbances in the crystal lattice in the reacted areas of the xenotime under HRTEM imaging, indicated that total recrystallization of the reacted areas took place during the course of the NaF + H₂O experiment. The results of these experiments has important implications for the metasomatic resetting of the xenotime geochromometer in the presence of alkali-bearing fluids, which would potentially allow metasomatised xenotime to time and record metasomatic events affecting the rock.

The geochemistry of arc rocks provides a window to probe compositional variability in mantle wedge and slab-derived inputs. However, quantitative estimation of their contribution is a great challenge. Here we report an integrated study for determination of variable contributions to early Paleozoic arc plutons (the Mafan diorites and gabbros), from the northern Hong'an orogen, central China. Zircon grains from the diorites yielded crystallization age of 445 ± 3 Ma, whereas the gabbros have zircon UPb age of 432 ± 3 Ma. Although both the two mafic rocks suites exhibit typical arc-like trace element distribution patterns, they show a series of differences in stable Li–Mo–O and radiogenic Sr–Nd–Hf isotope compositions, as well as major and trace element features. Geochemically, the diorites are medium-K calc-alkaline series with high and positive whole-rock ε_Nd(t) (+3.8 to +4.5) and zircon ε_Hf(t) values (+12.4 to +14.3), and normal mantle-like zircon δ¹⁸O values (5.1 ± 0.2 ‰). Contrarily, the gabbros are tholeiitic and have relatively low whole-rock ε_Nd(t) (+ 2.3 to +3.1) and zircon ε_Hf(t) (+5.0 to +8.5) values with normal mantle-like zircon δ¹⁸O values (5.4 ± 0.3 ‰). The diorites have δ⁹⁸Mo values of −0.15 to −0.03 ‰, while the gabbros have δ⁹⁸Mo values of −0.28 to −0.12 ‰. These values are slightly higher than those of the mid-ocean ridge basalts (MORB) (−0.20 ± 0.01 ‰). Whereas, δ⁷Li values of the studied diorites (2.15–4.49 ‰) and gabbros (2.89–5.33 ‰) are mostly consistent within the range of MORB. In combination with the above results, the primary magma of diorites was inferred to be derived from 1 to 5% partial melting of depleted lithospheric mantle metasomatized by slab-derived fluid with minor sediment-derived melts. While the gabbros were suggested to be generated by 5–10% partial melting of more depleted lithospheric mantle that had undergone perhaps extra 0.3% melt extraction, which had further been metasomatized by more subducted materials. In summary, both the slab-derived components and nature of the mantle wedge sources are quantitatively constrained for the mafic rocks in the Mafan plutons. Our new data suggest that the Mafan plutons exhibited the Mariana arc-affinity and represented products of an intra-oceanic subduction system in the northern Hong'an orogen. Combined with previously published data for the early Paleozoic mafic magmatic rocks along the Qinling-Tongbai-Hong'an orogenic belt, there exists an oceanic arc in the east (Hong'an orogen) and an Andean-type continental arc in the west (Qinling-Tongbai orogen).

The electron gain and loss of natural organic matter (NOM) plays an important role in the biogeochemical cycling of element and contaminant attenuation. The electron-donating capacity (EDC) and electron-accepting capacity (EAC) of NOM determine its electron exchange ability. However, the influences of bonded matrices on EDC and EAC of NOM remain unclear. Here we investigated the variations in EDC and EAC of NOM during its interaction with Fe oxyhydroxides. Compared to original NOM, the presence of Fe oxyhydroxides slightly decreased the EDC of dissolved NOM by 0.58–2.08 mmol e⁻/g C and of adsorbed NOM by 0.33–2.67 mmol e⁻/g C. However, the EAC of dissolved NOM and adsorbed NOM significantly (p < 0.05) increased by 1.92–14.17 and 3.08–36.67 mmol e⁻/g C, respectively. The excessive increase of EAC was mainly attributed to changes in NOM chemical components, particularly increases in oxidative components such as Fe(III) and quinonoid carbonyls, rather than changes in EDC. For dissolved NOM, the heightened EAC was mainly attributed to the complexation of Fe(III) by carboxyl in NOM. For adsorbed NOM, the boosted EAC was predominantly linked to the enrichment of quinonoid carbonyl through the selective molecular fractionation and the oxidative polymerization of polyphenols in NOM. Our finding highlights the previously overlooked phenomenon of asymmetrical changes of EDC and EAC of NOM during its interaction with Fe oxyhydroxides. The increased EAC could potentially affect various biogeochemical processes, such as methane production, anaerobic ammonium oxidation and microbial Fe(III) reduction.

A set of ten diamonds from different sources representing the main types of physical classification (IaA, IaAB, IaB and IIa) has been first explored by FTIR spectroscopy and SIMS calibrated by hydrogen ion implantation. The concentrations of hydrogen in the studied diamonds measured by SIMS range between 9.98 and 47.6 at.ppm or from 1.76 × 10¹⁸ to 8.40 × 10¹⁸ at./cm³. The 3107 cm⁻¹ absorption line has been detected by FTIR spectroscopy (resolution near 1 cm⁻¹) in five studied diamonds (IaB and IaAB). The 3107 cm⁻¹ IR absorption lines due to H defects in these diamonds have been successfully described by the Lorentzian shape and their parameters have been evaluated. Based on the linear regression equation relating intensities of the 3107 cm⁻¹ IR absorption lines in the studied diamonds with their H defect concentrations, the H defect IR absorption at 3107 cm⁻¹ in diamond has been quantified. The absorption calibration constant for diamond's 3107 cm⁻¹ absorption line is 386 ± 64 ppb cm². The corresponding cross-section per one H defect σ₃₁₀₇ (cm²) = (9.34 ± 1.25) × 10⁻¹⁸ /Γ (cm⁻¹), Γ is the full width at half maximum (FWHM) of the 3107 cm⁻¹ absorption line. Based on a widely accepted interpretation that the H defect resulting in the 3107 cm⁻¹ IR absorption peak in diamonds is the N₃VH defect, the values of the obtained calibration constant and cross-section have been assigned to the N₃VH defect in diamond. The equations relating concentration of the N₃VH defect ([N₃VH]) with the 3107 cm⁻¹ IR absorption line integrated intensity (I₃₁₀₇) and intensity (α₃₁₀₇) are as follows: [N₃VH] (ppb) = (386 ± 64) I₃₁₀₇; [N₃VH] (ppm) = (0.607 ± 0.080) α ₃₁₀₇ (cm⁻¹)Γ (cm⁻¹). Hydrogen impurities that do not contribute to the IR absorption at the 3107 cm⁻¹ peak were detected in all studied diamonds.

Thermochemical sulfate reduction (TSR) is an important redox reaction that markedly alters hydrocarbons in deep reservoirs. Although the major achievements that involve reaction mechanism, process, and geochemical characteristics were attained in the last 50 years, a quantitative TSR model that includes major products was not built, which restricts the understanding of the reaction mechanism and the generation process of hydrocarbons in TSR. In this study, two series of thermal simulation experiments of the TSR involving n-octadecane (n-C18) and n-dodecylbenzene (C₁₂B) with MgSO₄ and H₂O in a confined gold-tube system were conducted, and all the major products in different phases, particularly oxygen-containing compounds, were quantitatively analyzed. The results indicated that the high-molecular-weight (HMW) oxygenated compounds in heavy products were predominated by ketones, followed by alcohol and acids, and the low-molecular-weight (LMW) organic acids in water mainly comprised acetic acids, followed by formic acid and propionic acid. Both oxygen-containing compounds prompted the TSR process but in different ways. The accumulation of HMW oxygenated compounds in the non-autocatalytic stages was essential for initiating the catalyzed reaction of TSR. In addition, the oxygenated compounds of hydrocarbons enhance polarity, making hydrocarbons easier to contact with HSO₄⁻ or [MgSO₄] _CIP. The LMW organic acids possibly prompted the TSR process by acidifying the water and thus facilitating the formation of HSO₄⁻ and [MgSO₄] _CIP. The oxidation reaction during TSR possibly converted HMW oxygenated compounds to LMW organic acids and CO₂. The quantitative analysis of products in TSR revealed that TSR destroyed the hydrocarbons more rapidly but in a different way from thermal cracking and that the generation process of hydrocarbon types in TSR was in the following order: heavy hydrocarbon, gaseous hydrocarbon/light hydrocarbon/solid bitumen, and gaseous hydrocarbon/solid bitumen. Compared with oil cracking, the TSR promoted the oil reservoir to rapidly progress to more mature stages. The hydrocarbon precursor and reaction extent affected the generation process and the character of all products in the TSR. These results can provide an improved understanding of the TSR mechanism and process and be useful in the evaluation of petroleum potential for TSR-altered oils.

Typical resistant REE phosphates xenotime and monazite are important REE carriers in various types of rocks but the supergene mobility of REE in these minerals remains controversial. In this study, we hypothesize that microbes drive the natural weathering of these resistant REE phosphates. We conducted room-temperature bio-weathering experiments of xenotime concentrate with a common soil bacterium (Bacillus thuringiensis, Bt) isolated from a regolith-hosted REE deposit. Our results showed that Bt was able to promote the dissolution of xenotime and monazite in the concentrate, and the release of REE was enhanced by up to two orders of magnitude. In the bio-weathering medium buffered at pH = 6, the apparent release rates of total REE were in the range of 10⁻¹³–10⁻¹² mol·m⁻²·s⁻¹, with Y releasing at the fastest rates of ∼10⁻¹³ mol·m⁻²·s⁻¹. Furthermore, the estimated dissolution rate of monazite (∼10⁻⁹ g·m⁻²·s⁻¹) was one order of magnitude higher than that of xenotime (∼10⁻¹⁰ g·m⁻²·s⁻¹) due to a more refractory nature of xenotime determined by its chemical and mineralogical characteristics. On account of the extremely low solubility of REE phosphates, portions of the released REE could be re-precipitated as meta-stable phosphates during mineral dissolution, resulting in the underestimation of the release of REE from primary minerals. Bt could produce various organic acids and acidify the media, promoting the dissolution of resistant phosphates through proton- and ligand-promoted mechanisms. The results of our study suggest that microbes have a high potential to facilitate REE liberation from resistant xenotime and monazite, posing new insight into the biogeochemical cycling of REE on Earth's surface.