The deterioration of monument or building stone materials is mostly due to the growth of black crusts that cause blackening and disaggregation of the exposed surface. This study reports on new oxygen ...(δ17O, δ18O and Δ17O) and sulphur (δ33S, δ34S, δ36S, Δ33S and Δ36S) isotopic analyses of black crust sulphates formed on building stones in Sicily (Southern Italy). The measurements are used to identify the possible influence of volcanic emissions on black crust formation. Black crusts were mostly sampled on carbonate stone substrate in different locations subject to various sulphur emission sources (marine, anthropogenic and volcanic). Unlike atmospheric sulphate aerosols that mostly exhibit Δ33S > 0‰, here most of the analysed black crust sulphates show negative Δ33S. This confirms that black crust sulphates do not result from deposition of sulphate aerosols or of rainwater but mostly from the oxidation of dry deposited SO2 onto the stone substrate. The δ34S and δ18O values indicate that most of black crust sulphate originates from anthropogenic activities. Δ17O values are found to be related to the sampling location. The largest 17O-anomalies (up to ~4‰) are measured in black crust from areas highly influenced by volcanic emissions, which demonstrates the strong involvement of ozone in the formation of black crusts in volcanically influenced environments.
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•Black crusts are mostly collected on carbonate building stones from different environments (coastal, rural, urban and volcanic environments).•δ34S and δ18O indicate that black crust sulphur is mostly derived from anthropogenic sources.•Δ33S and Δ36S of black crust sulphate are distinct from those of atmospheric sulphate aerosols.•The largest Δ17O anomalies are measured in black crust sampled in locations downwind of an active volcano (Mt Etna).
A collection of 61 xenocrystic and 12 eclogite xenolith-derived diamonds from the 600Ma Lahtojoki kimberlite in central Finland has been investigated. Calculated pressure and temperature conditions ...for the diamondiferous eclogites are in excess of 5.5GPa and 1300°C, suggesting residence depths greater than 180km, near the base of the Karelian cratonic mantle lithosphere. Geochemically, the eclogite xenoliths have gabbroic compositions showing positive Eu and Sr anomalies, relatively low ΣREE and elevated Al2O3 contents, yet garnets have ambiguous δ18O values of 5.7‰ and 5.9‰. Gabbroic eclogite formation could therefore be linked to either subduction processes during the 1.9Ga Svecofennian orogeny or to cumulate processes during 2.1Ga rift-induced magmatism. Determination of the oxygen fugacity of Lahtojoki eclogite xenoliths from both this work and previous studies suggests that diamond-bearing eclogites may be more reduced (ΔFMQ-3.5) compared to barren eclogites (ΔFMQ-1.7).
While recycled oceanic crust protoliths for the eclogites remain a possibility, the carbon isotopic compositions and nitrogen abundances of the Lahtojoki diamonds indicate mantle-derived volatile sources. All diamonds (i.e., loose and eclogite xenolith-derived) display a restricted range of δ13C values from −7.8‰ to −3.7‰ that overlaps with the carbon isotopic composition of Earth’s mantle. The Lahtojoki diamond δ13C values form a negatively skewed distribution, indicating diamond growth from reduced mantle-derived carbon sources such as methane- (CH4) bearing fluids. Nitrogen contents of the Lahtojoki diamonds range from 40 to 1830atomic ppm with a mean of ∼670atomic ppm; these elevated nitrogen contents combined with the close association to eclogites suggest an eclogitic or crustal volatile source. However, the Karelian craton was periodically intruded by ultramafic alkaline magmas since at least 1.8Ga, noting in particular the occurrence of phlogopite-rich kimberlites and olivine lamproites between 1200 and 700Ma. We argue that this punctuated volatile-rich magmatism simultaneously metasomatised the cratonic mantle lithosphere, forming nitrogen enriched phlogopite-bearing metasomes. We propose that reduced, carbon-bearing and nitrogen-rich fluids were remobilized to form the Lahtojoki diamonds. The diamond-forming event(s) most probably occurred during or shortly prior to the entraining kimberlite magmatism as indicated by the diamond nitrogen aggregation systematics. Involvement of reduced diamond-forming fluids is supported by both the negative skewness of Lahtojoki diamond δ13C values and the more reduced nature of the diamondiferous Lahtojoki eclogites compared with their more oxidized barren counterparts. Our results from the diamondiferous eclogites derived from the deepest parts of the Karelian cratonic mantle root are in support of methane being the stable carbon volatile species at the base of thick continental lithosphere.
We studied diamonds and barren and diamondiferous eclogite xenoliths from the Jericho kimberlite (Northern Slave craton). The majority of the diamonds are non-resorbed octahedral crystals, with ...moderately aggregated N (IaB < 50%,
N
< 300 ppm) and δ
13
C = −5 to −41‰. The diamonds belong to “eclogitic” (90% of the studied samples), “websteritic” (7%) and “peridotitic” (3%) assemblages. The Jericho diamonds differ from the majority of “eclogitic” diamonds worldwide in magnesian compositions of associated minerals and extremely light C isotopic compositions (δ
13
C = −24 to −41‰). We propose that metasomatism triggered by H
2
O fluids may have been involved in the diamond formation. Multiple episodes of the metasomatism and associated melt extraction of various ages are evident in Jericho eclogite xenoliths where primary garnet and clinopyroxene have been recrystallized to more magnesian minerals with higher contents of some incompatible trace elements and to hydrous secondary phases. The model is supported by the general similarity of mineral compositions in diamondiferous eclogites to those in diamond inclusions and to secondary magnesian garnet and clinopyroxene in recrystallized barren eclogites. The ultimate products of the metasomatism could be “websteritic” diamond assemblages sourced from magnesian eclogites.
The snowball Earth hypothesis postulates that the planet was entirely covered by ice for millions of years in the Neoproterozoic era, in a self-enhanced glaciation caused by the high albedo of the ...ice-covered planet. In a hard-snowball picture, the subsequent rapid unfreezing resulted from an ultra-greenhouse event attributed to the buildup of volcanic carbon dioxide (CO(2)) during glaciation. High partial pressures of atmospheric CO(2) (pCO2; from 20,000 to 90,000 p.p.m.v.) in the aftermath of the Marinoan glaciation (∼635 Myr ago) have been inferred from both boron and triple oxygen isotopes. These pCO2 values are 50 to 225 times higher than present-day levels. Here, we re-evaluate these estimates using paired carbon isotopic data for carbonate layers that cap Neoproterozoic glacial deposits and are considered to record post-glacial sea level rise. The new data reported here for Brazilian cap carbonates, together with previous ones for time-equivalent units, provide estimates lower than 3,200 p.p.m.v.--and possibly as low as the current value of ∼400 p.p.m.v. Our new constraint, and our re-interpretation of the boron and triple oxygen isotope data, provide a completely different picture of the late Neoproterozoic environment, with low atmospheric concentrations of carbon dioxide and oxygen that are inconsistent with a hard-snowball Earth.
Mineralogical studies of deep-seated xenoliths and mineral inclusions in diamonds indicate that there is significant variability in oxygen fugacity within the Earth's upper mantle. This variability ...is consistent both with the occurrence of reduced (methane-bearing) or oxidized (CO
2/carbonate-bearing) fluids. Invariably, direct sampling of reduced deep fluids is not possible as they are unquenchable and re-equilibrate with either the surrounding mantle or are affected by degassing. Key information about the nature of such fluids might be found in diamond if it were possible to study a population related to a single source. Usually, diamonds within a kimberlite pipe have different parageneses and can be shown to have formed at different times and depths.
We studied 59 diamonds extracted from a single diamondiferous peridotite xenolith (with a volume of only 27 cm
3), from the Cullinan mine (formerly called the Premier mine) in South Africa. Diamond sizes range from 0.0005 to 0.169 carats (0.1 to 33.8 mg). A correlation between the nitrogen contents of the diamonds (range 40 to 1430 ppm) and their nitrogen aggregation state (varying from 10 to 85% of IaB defects) is compatible with a single growth event. δ
13C-values range from −
4.2‰ to −
0.1‰, with slight internal variability measured in the largest diamonds. Nitrogen isotope measurements show δ
15N ranging from −
1.2‰ to +
7.2‰. On the centimeter scale of this upper mantle rock, the variations for nitrogen content, nitrogen aggregation state, carbon and nitrogen isotopic compositions, respectively, cover 64%, 75%, 15% and 23% of the ranges known for peridotitic diamonds.
In spite of such large ranges, N-content, δ
13C and δ
15N within this diamond population are distinctly coupled. These relationships do not support a mixing of carbon sources, but are best explained by a Rayleigh distillation within the sub-continental mantle at depths
>
150 km and
T
>
1200 °C, which precipitates diamonds from methane-bearing fluid(s). The involvement of this reduced metasomatic agent also suggests that the heterogeneous redox state of Archean cratons may mostly result from the heterogeneous nature of percolating fluids.
The striking variability of the four determined parameters at cm scale may also account for the difficulty in interpreting these parameters in larger productions, such as those from a mine, because in these cases, the diamonds are mixed and sub-populations cannot be disentangled.
Microbial sulfate reduction (MSR) is thought to have operated very early on Earth and is often invoked to explain the occurrence of sedimentary sulfides in the rock record. Sedimentary sulfides can ...also form from sulfides produced abiotically during late diagenesis or metamorphism. As both biotic and abiotic processes contribute to the bulk of sedimentary sulfides, tracing back the original microbial signature from the earliest Earth record is challenging. We present in situ sulfur isotope data from nanopyrites occurring in carbonaceous remains lining the domical shape of stromatolite knobs of the 2.7‐Gyr‐old Tumbiana Formation (Western Australia). The analyzed nanopyrites show a large range of δ34S values of about 84‰ (from −33.7‰ to +50.4‰). The recognition that a large δ34S range of 80‰ is found in individual carbonaceous‐rich layers support the interpretation that the nanopyrites were formed in microbial mats through MSR by a Rayleigh distillation process during early diagenesis. An active microbial cycling of sulfur during formation of the stromatolite may have facilitated the mixing of different sulfur pools (atmospheric and hydrothermal) and explain the weak mass independent signature (MIF‐S) recorded in the Tumbiana Formation. These results confirm that MSR participated actively to the biogeochemical cycling of sulfur during the Neoarchean and support previous models suggesting anaerobic oxidation of methane using sulfate in the Tumbiana environment.
Populations of sulfide inclusions in diamonds from the Orapa kimberlite pipe in the Kaapvaal-Zimbabwe craton, Botswana, preserve mass-independent sulfur isotope fractionations. The data indicate that ...material was transferred from the atmosphere to the mantle in the Archean. The data also imply that sulfur is not well mixed in the diamond source regions, allowing for reconstruction of the Archean sulfur cycle and possibly offering insight into the nature of mantle convection through time.
In order to better constrain the origin of volatile variability of mid ocean ridge basalt (MORB), we have performed crushing analyses on 33 fresh basaltic glasses. Most samples originate from the ...Southwest Indian Ridge (i.e. 78–49°E, EDUL cruise, August 97), the Southeast Indian Ridge, the Central Indian Ridge and the Rodriguez Triple Junction. δ13C and δ18O of CO2, δ15N of N2 together with C, N and Ar contents were determined. δ15N values vary between −5.9‰ and +2.1‰ while δ13C values range between −11.4 and −4.3‰. C/N2 ratios vary by one order of magnitude (316–3900). Most N2/Ar ratios fall within a narrow range of values (48–90) but four samples yield higher values up to 220. Overall, the data for δ13C, δ18O, δ15N, C/N2 and N2/Ar are within the range of those previously reported for Pacific and Atlantic oceans. No volatile DUPAL anomaly has been detected in the present study. The correlations between N2/Ar, C/Ar and C/N2 together with δ13C and δ15N show that major volatile signatures, including δ15N, are more influenced by degassing-induced fractionation than by mantle heterogeneity and/or late atmospheric contamination. A two stage degassing model (a closed-system degassing followed by a Rayleigh distillation) can be used to explain the data set. This model gives initial Indian MORB values similar to those for the Atlantic and Pacific oceanic basalts δ13C0∼−4.5‰, δ15N0∼−6.0‰, (C/N2)0∼130, (C/Ar)0∼14 000 and (N2/Ar)0∼110 and C0 between 1100 and 5000 ppm C. This large range of possible initial carbon concentrations (due to the lack of constraints on the extent of degassing under closed-system conditions) results in a large range of mantle flux estimates (i.e. from 0.4 to 1.8×1013 mol/yr). The correlations induced by these degassing processes permit an estimation of the relative solubilities of C, N and Ar: SC/SN2∼5 and SAr/SN2∼1.2 as well as an evaluation of the fractionation of nitrogen isotopes between the vesicles and the melt: ΔN=−1.6‰. The present study proposes a coherent data set for volatile mantle fluxes with φC∼1.3±0.1×102φN2∼1.4±0.1×104φAr (molar).
The present paper reports recent improvements in sealed tube combustion technique used for the determination of N isotopic composition in various rocks characterized by low N contents (i.e. few ppm). ...Nitrogen is extracted from samples by combustion in quartz tubes sealed under vacuum. The nitrogen gas purified using Cu, CuO and CaO, is quantified as dinitrogen N
2 by capacitance manometry in ultra-high vacuum line. Nitrogen isotopic analysis is performed on a triple-collector static vacuum mass spectrometer, allowing measurement of nanomole quantity of N
2. Nitrogen amount and isotopic composition of the analytical blanks are low and describe Gaussian distribution with mean values of 0.65
±
0.30 nmol N and −
3.7‰
±
2.7‰,
respectively (2
σ). Systematic analyses of international and internal standards demonstrate that this technique provides accurate and precise results. The precision on N content and isotopic composition are better than ±
8% and ±
0.5‰ respectively, even for samples containing less than 2 ppm N. The sealed tube combustion technique is shown to apply successfully to rocks of various lithologies such as metagabbros, metaperidotites and altered basalts. It is thus suitable for studying oceanic crust in a perspective to better constrain N exchanges between Earth mantle and surface reservoirs. The investigation can also be extended to analysis of small size samples, particularly when little sample exists and when high spatial resolution is required.
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