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1.
New 87Sr/86Sr, δ13C, and δ18О chemostratigraphic data were obtained for carbonate rocks of the Lower Riphean Yusmastakh and the Vendian Starorechenskaya formations. The δ13С values in dolomites of the Yusmastakh Formation varies from–0.6 to–0.1‰ and in dolomites and dolomitic limestones of the Starorechenskaya Formation, from–1.2 to–0.4‰ PDB, and δ18О values, from 24.4 to 26.4‰ and from 25.3 to 27.6‰ SMOW, respectively. The Rb–Sr systematics of carbonate rocks was studied using the refined method of stepwise dissolution of samples in acetic acid, including chemical removal of up to one-third of the ground sample by preliminary acid leaching and subsequent partial dissolution of the rest of the sample. Owing to this procedure, secondary carbonate material is removed, which enables one to improve the quality of the Sr-chemostratigraphic data obtained. The initial 87Sr/86Sr ratios in carbonate rocks of the Yusmastakh (0.70468–0.70519) and Starorechenskaya (0.70832–0.70883) formations evidence the Riphean–Vendian boundary in the Precambrian sequence of the Anabar Uplift.  相似文献   

2.
With depleted coal resources or deteriorating mining geological conditions, some coal mines have been abandoned in the Fengfeng mining district, China. Water that accumulates in an abandoned underground mine (goaf water) may be a hazard to neighboring mines and impact the groundwater environment. Groundwater samples at three abandoned mines (Yi, Er and Quantou mines) in the Fengfeng mining district and the underlying Ordovician limestone aquifer were collected to characterize their chemical and isotopic compositions and identify the sources of the mine water. The water was HCO3·SO4-Ca·Mg type in Er mine and the auxiliary shaft of Yi mine, and HCO3·SO4-Na type in the main shaft of Quantou mine. The isotopic compositions (δD and δ18O) of water in the three abandoned mines were close to that of Ordovician limestone groundwater. Faults in the abandoned mines were developmental, possibly facilitating inflows of groundwater from the underlying Ordovician limestone aquifers into the coal mines. Although the Sr2+ concentrations differed considerably, the ratios of Sr2+/Ca2+ and 87Sr/86Sr and the 34S content of SO42? were similar for all three mine waters and Ordovician limestone groundwater, indicating that a close hydraulic connection may exist. Geochemical and isotopic indicators suggest that (1) the mine waters may originate mainly from the Ordovician limestone groundwater inflows, and (2) the upward hydraulic gradient in the limestone aquifer may prevent its contamination by the overlying abandoned mine water. The results of this study could be useful for water resources management in this area and other similar mining areas.  相似文献   

3.
Deep-seated landslides are complex systems. In many cases, multidisciplinary studies are necessary to unravel the key hydrological features that can influence their evolution in space and time. The deep-seated Berceto landslide, in the northern Apennines of Italy, has been investigated in order to define the origin and geochemical evolution of groundwater (GW), to identify the slope system hydrological boundary, and to highlight the GW flow paths, transit time and transfer modalities inside the landslide body. This research is based on a multidisciplinary approach that involves monitoring GW levels, obtaining analyses of water chemistry and stable and unstable isotopes (δ18O-δ2H, 3H, 87Sr/86Sr), performing soil leaching tests, geochemical modelling (PHREEQC), and principal component analysis (PCA). The results of δ18O-δ2H and 87Sr/86Sr analyses show that the source of GW recharge in the Berceto landslide is local rainwater, and external contributions from a local stream can be excluded. In the landslide body, two GW hydrotypes (Ca-HCO3 and Na-HCO3) are identified, and the results of PHREEQC and PCA confirm that the chemical features of the GW depend on water–rock interaction processes occurring inside the landslide. The 3H content suggests a recent origin for GW and appears to highlight mixing between shallow and deep GW aliquots. The 3H content and GW levels data confirm that shallow GW is mainly controlled by a mass transfer mechanism. The 3H analyses with GW levels also indicate that only deep GW is controlled by a pressure transfer mechanism, and this mechanism is likely the main influence on the landslide kinematics.  相似文献   

4.
The paper presents the results of study of the Sr, C, and O isotope compositions in Upper Jurassic carbonate rocks of the Baidar Valley and Demerdzhi Plateau in the Crimean Mountains represented by different facies of the carbonate platform at the northern active margin of the Tethys. The 87Sr/86Sr value in them varies from 0.70699 to 0.70728. Based on the Sr chemostratigraphic correlation, the age of massive and layered limestones in the western part of the Ai-Petri and Baidar yailas (pastures) is estimated as late Kimmeridgian–early Tithonian, whereas the age of flyschoids of the Baidar Valley are estimated as late Tithonian–early Berriasian. The nearly synchronous formation of carbonate breccias of the Baidar Valley and Demerdzhi Plateau in late Tithonian–early Berriasian is substantiated. A summary section of Upper Jurassic rocks is compiled based on the Sr chemostratigraphic data. It has been established that δ18O values in the studied carbonate sediments vary from–2.9 to 1.3‰ (V-PDB). At the same time, shallow-water sediments in the internal part and the edge of the Crimean carbonate platform are depleted in 18O (–2.9 to +0.1‰) relative to sediments on the slope and foothill (–0.5 to +1.3‰). It is demonstrated that δ13C values do not depend on the facies properties and decrease in younger carbonate sediments from 3–3.5‰ to 1–1.5‰ in line with the Late Jurassic general trend. The δ13C values obtained for the Crimean carbonate platform turned out to be 0.5–1‰ higher than the values typical of the deep-water marine setting at the western margin of the Tethys. These discrepancies are likely related to peculiarities of water circulation and high bioproductivity in marine waters of the northern Peri-Tethys.  相似文献   

5.
The Warburton Basin of central Australia has experienced a complex tectonic and fluid-flow history, resulting in the formation of various authigenic minerals. Geochemical and geochronological analyses were undertaken on vein carbonates from core samples of clastic sediments. Results were then integrated with zircon U–Pb dating and uraninite U–Th–total Pb dating from the underlying granite. Stable and radiogenic isotopes (δ18O, Sr and εNd), as well as trace element data of carbonate veins indicate that >200 °C basinal fluids of evolved meteoric origin circulated through the Warburton Basin. Almost coincidental ages of these carbonates (Sm–Nd; 432 ± 12 Ma) with primary zircon (421 ± 3.8 Ma) and uraninite (407 ± 16 Ma) ages from the granitic intrusion point towards a substantial period of active tectonism and an elevated thermal regime during the mid Silurian. We hypothesise that such a thermal regime may have resulted from extensional tectonism and concomitant magmatic activity following regional orogenesis. This study shows that the combined application of geochemical and geochronological analyses of both primary and secondary species may constrain the timing of tectonomagmatic events and associated fluid flow in intraplate sedimentary basins. Furthermore, this work suggests that the Sm–Nd-isotopic system is surprisingly robust and can record geologically meaningful age data from hydrothermal mineral species.  相似文献   

6.
Lower ocean crust is primarily gabbroic, although 1–2% felsic igneous rocks that are referred to collectively as plagiogranites occur locally. Recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawater-derived fluids, and thus they may indicate early, high-temperature hydrothermal fluid circulation. To explore seawater–rock interaction prior to and during the genesis of plagiogranite and other late-stage magmas, oxygen-isotope ratios preserved in igneous zircon have been measured by ion microprobe. A total of 197 zircons from 43 plagiogranite, evolved gabbro, and hydrothermally altered fault rock samples have been analyzed. Samples originate primarily from drill core acquired during Ocean Drilling Program and Integrated Ocean Drilling Program operations near the Mid-Atlantic and Southwest Indian Ridges. With the exception of rare, distinctively luminescent rims, all zircons from ocean crust record remarkably uniform δ18O with an average value of 5.2 ± 0.5‰ (2SD). The average δ18O(Zrc) would be in magmatic equilibrium with unaltered MORB [δ18O(WR) ~ 5.6–5.7‰], and is consistent with the previously determined value for equilibrium with the mantle. The narrow range of measured δ18O values is predicted for zircon crystallization from variable parent melt compositions and temperatures in a closed system, and provides no indication of any interactions between altered rocks or seawater and the evolved parent melts. If plagiogranite forms by hydrous partial melting, the uniform mantle-like δ18O(Zrc) requires melting and zircon crystallization prior to significant amounts of water–rock interactions that alter the protolith δ18O. Zircons from ocean crust have been proposed as a tectonic analog for >3.9 Ga detrital zircons from the earliest (Hadean) Earth by multiple workers. However, zircons from ocean crust are readily distinguished geochemically from zircons formed in continental crustal environments. Many of the >3.9 Ga zircons have mildly elevated δ18O (6.0–7.5‰), but such values have not been identified in any zircons from the large sample suite examined here. The difference in δ18O, in combination with newly acquired lithium concentrations and published trace element data, clearly shows that the >3.9 Ga detrital zircons did not originate by processes analogous to those in modern mid-ocean ridge settings.  相似文献   

7.
Isotopic compositions of carbon (δ13C from −51.4 to −10.8 PDB) and oxygen (δ18O from 14.4 to 21.4 SMOW) were studied in rhodochrosite and calcite from manganese ores in the South Faizuly and Kyzyltash deposits of the southern Urals. The geological, petrographic, and isotopic data indicate that the studied carbonates are diagenetic formations. It is suggested that the main ore element (Mn) was delivered to the marine basin with hydrothermal solutions percolating in the oceanic crust. Manganese precipitated on the oceanic bottom as oxides near solution discharge zones. Manganese carbonates formed in sediments as a result of the oxidation of organic matter by manganese oxides. High biological productivity of the environment was caused by proximity to the hydrothermal vent that provided favorable biogeochemical conditions for the development of biocoenosis. Anomalously low 13C values in the South Faizuly deposit testify to the large-scale oxidation of methane in the course of manganese carbonate formation.__________Translated from Litologiya i Poleznye Iskopaemye, No. 4, 2005, pp. 416–429.Original Russian Text Copyright © 2005 by Kuleshov, Brusnitsyn.  相似文献   

8.
The results of isotope-geochemical studies of carbonates of different mineral types from manganese and host rocks of the Famennian manganiferous formation of Pai-Khoi are reported. Kutnahorite ores are characterized by δ13C values from–6.6 to 1.3‰ and δ18O from 20.0 to 27.4‰. Rhodonite–rhodochrosite rocks of the Silovayakha ore occurrence have δ13C from–5.2 to–2.9 and δ18O from 25.4 to 24.3‰. Mineralogically similar rocks of the Nadeiyakha ore occurrence show the lighter carbon and oxygen isotopic compositions: δ13C from–16.4 to–13.1 and δ18O from 24.8 to 22.5‰. Similar isotopic compositions were also obtained for rhodochrosite–kutnahorite rocks of this ore occurrence: δ13C from–13.0 to–10.4‰ and δ18O from 24.6 to 21.7‰. Siderorodochrosite ores differ in the lighter oxygen and carbon isotopic compositions: δ18O from 18.7 to 17.6‰ and δ13C from–10.2 to–9.3‰, respectively. In terms of the carbon and oxygen isotopic compositions, host rocks in general correspond to marine sedimentary carbonates. Geological-mineralogical and isotope data indicate that the formation of the manganese carbonates was related to the hydrothermal ore-bearing fluids with the light isotopic composition of oxygen and carbon dissolved in CO2. The isotopic features indicate an authigenic formation of manganese carbonates under different isotopegeochemical conditions.  相似文献   

9.
Groundwater is of utmost significance to socio-economic development and ecological recovery for the Loess Plateau. However, studies regarding the mechanism governing groundwater recharge over this area appear to be inadequate. This study is to examine the spatio-temporal variations of δ2H and δ18O in precipitation and shallow groundwater. On the basis of this, the mechanisms governing shallow groundwater recharge were explored. Precipitation and groundwater were sampled monthly from May to October during the period 2004–2006 at 13 sites in the Chabagou Catchment (187 km2). In the Caopingxigou Experimental Watershed (0.1 km2), meteorological variables were observed and rainfall larger than 5 mm was sampled immediately after each rain event. Across the area, 90% of the precipitation occurred from May to September primarily in the form of heavy rains or rainstorms with great spatial variability. There were about 30 localized rains in each year. It was indicated that there existed notable seasonality and pronounced spatial variability in precipitation isotopic compositions. Contributing factors and indications of isotopic compositions, as well as their climatic indications such as monsoon intensities and mixing processes of water vapor, were investigated. The δ2H–δ18O relation of groundwater was found to be δ2H = 3.22 × δ18O − 38.1, deviating from the local meteoric water line δ2H = 7.57 × δ18O + 3.9. The range of δ values in groundwater is shrunken to be 15–21% of that in individual precipitations, and groundwater in the middle reaches shows a wider range of δ values. Isotopic results showed that groundwater originates from precipitation with hydrogen and oxygen isotopic compositions being −69 and −9.7‰, respectively, and most groundwater experiences serious evaporation and adequate mixing with old water during infiltration or percolation in the aerated zone. It was also founded that obvious fluctuations of isotopic compositions in groundwater mainly appear in the middle reaches especially at sites that are close to valleys, suggesting varying sources of groundwater from precipitation, precipitation runoff, isotopically enriched surface water and/or lateral recharge of adjacent groundwater.  相似文献   

10.
The Rb–Sr and 147Sm–143Nd age data obtained for sheeted dolerite dykes and rocks of the Platinum Belt of the Urals within the Tagil segment of the paleoceanic spreading structure (Middle Urals) are discussed. The study of the Rb–Sr isotope systematics of gabbro allowed us to reveal errochronous dependencies, which yielded ages of 415 and 345 Ma at (87Sr/86Sr)0 = 0.70385 ± 0.00068 and 0.7029 ± 0.0010, correspondingly. The 147Sm–143Nd isotope age data demonstrate a specific coincidence of the chronometric ages of the sheeted dolerite dyke complex (426 ± 54, 426 ± 34, and 424 ± 19 Ma) and gabbro from the Revda gabbro–ultramafic massif (431 ± 27 Ma) and from screens between dolerite dykes in the sheeted dyke complex (427 ± 32 Ma, 429 ± 26 Ma). The proximity of the 147Sm–143Nd ages of gabbro and dolerite can be explained by the thermal effect of the basaltic melt, which is the protolith for the dyke complex, on the hosting gabbro.  相似文献   

11.
The oxygen isotope ratios (δ18O) of most igneous zircons range from 5 to 8‰, with 99% of published values from 1345 rocks below 10‰. Metamorphic zircons from quartzite, metapelite, metabasite, and eclogite record δ18O values from 5 to 17‰, with 99% below 15‰. However, zircons with anomalously high δ18O, up to 23‰, have been reported in detrital suites; source rocks for these unusual zircons have not been identified. We report data for zircons from Sri Lanka and Myanmar that constrain a metamorphic petrogenesis for anomalously high δ18O in zircon. A suite of 28 large detrital zircon megacrysts from Mogok (Myanmar) analyzed by laser fluorination yields δ18O from 9.4 to 25.5‰. The U–Pb standard, CZ3, a large detrital zircon megacryst from Sri Lanka, yields δ18O = 15.4 ± 0.1‰ (2 SE) by ion microprobe. A euhedral unzoned zircon in a thin section of Sri Lanka granulite facies calcite marble yields δ18O = 19.4‰ by ion microprobe and confirms a metamorphic petrogenesis of zircon in marble. Small oxygen isotope fractionations between zircon and most minerals require a high δ18O source for the high δ18O zircons. Predicted equilibrium values of Δ18O(calcite-zircon) = 2–3‰ from 800 to 600°C show that metamorphic zircon crystallizing in a high δ18O marble will have high δ18O. The high δ18O zircons (>15‰) from both Sri Lanka and Mogok overlap the values of primary marine carbonates, and marbles are known detrital gemstone sources in both localities. The high δ18O zircons are thus metamorphic; the 15–25‰ zircon values are consistent with a marble origin in a rock-dominated system (i.e., low fluid(external)/rock); the lower δ18O zircon values (9–15‰) are consistent with an origin in an external fluid-dominated system, such as skarn derived from marble, although many non-metasomatized marbles also fall in this range of δ18O. High δ18O (>15‰) and the absence of zoning can thus be used as a tracer to identify a marble source for high δ18O detrital zircons; this recognition can aid provenance studies in complex metamorphic terranes where age determinations alone may not allow discrimination of coeval source rocks. Metamorphic zircon megacrysts have not been reported previously and appear to be associated with high-grade marble. Identification of high δ18O zircons can also aid geochronology studies that seek to date high-grade metamorphic events due to the ability to distinguish metamorphic from detrital zircons in marble.  相似文献   

12.
The paper reports detailed data on phlogopite from kimberlite of three facies types in the Arkhangelsk Diamondiferous Province (ADP): (i) massive magmatic kimberlite (Ermakovskaya-7 Pipe), (ii) transitional type between massive volcaniclastic and magmatic kimberlite (Grib Pipe), and (iii) volcanic kimberlite (Karpinskii-1 and Karpinskii-2 pipes). Kimberlite from the Ermakovskaya-7 Pipe contains only groundmass phlogopite. Kimberlite from the Grib Pipe contains a number of phlogopite populations: megacrysts, macrocrysts, matrix phlogopite, and this mineral in xenoliths. Phlogopite macrocrysts and matrix phlogopite define a single compositional trend reflecting the evolution of the kimberlite melt. The composition points of phlogopite from the xenoliths lie on a single crystallization trend, i.e., the mineral also crystallized from kimberlite melt, which likely actively metasomatized the host rocks from which the xenoliths were captured. Phlogopite from volcaniclastic kimberlite from the Karpinskii-1 and Karpinskii-2 pipes does not show either any clearly distinct petrographic setting or compositional differentiation. The kimberlite was dated by the Rb–Sr technique on phlogopite and additionally by the 40Ar/39Ar method. Because it is highly probable that phlogopite from all pipes crystallized from kimberlite melt, the crystallization age of the kimberlite can be defined as 376 ± 3 Ma for the Grib Pipe, 380 ± 2 Ma for the Karpinskii-1 pipe, 375 ± 2 Ma for the Karpinskii-2 Pipe, and 377 ± 0.4 Ma for the Ermakovskaya-7 Pipe. The age of the pipes coincides within the error and suggests that the melts of the pipes were emplaced almost simultaneously. Our geochronologic data on kimberlite emplacement in ADP lie within the range of 380 ± 2 to 375 ± Ma and coincide with most age values for Devonian alkaline–ultramafic complexes in the Kola Province: 379 ± 5 Ma; Arzamastsev and Wu, 2014). These data indicate that the kimberlite was formed during the early evolution of the Kola Province, when alkaline–ultramafic complexes (including those with carbonatite) were emplaced.  相似文献   

13.
Electrical conductivity of saturated soil extracts (ECe) in three reclaimed tideland (RTL) soils on the west coast of Korea decreased with time since reclamation, indicating natural desalinization through leaching of salts by precipitation water. Soil N concentration increased with decreasing ECe. With the increase in soil N concentration, the δ15N decreased, likely caused by the input of 15N-depleted N sources. As N2-fixing plant species were found in the oldest RTL, atmospheric N2 fixation likely contributed to the increase in soil N concentration in the oldest RTL. Negative δ15N (−7.1 to −2.0‰) of total inorganic N (NH4 ++NO3 −) and published data on N deposition near the study area indicate that atmospheric N deposition might be another source of N in the RTLs. Meanwhile, the consistently negative δ15N of soil NO3 − excluded N input from chemical fertilizer through groundwater flow as a potential N source, since NO3 − in groundwater generally have a positive δ15N. The patterns of δ15N of NH4 + (+2.3 to +5.1‰) and NO3 − (−9.2 to −5.0‰) suggested that nitrification was an active process that caused 15N enrichment in NH4 + but denitrification was probably minimal which would otherwise have caused 15N enrichment in NO3 −. A quantitative approach on N budget would provide a better understanding of soil N dynamics in the studied RTLs.  相似文献   

14.
This work reports the results of lithological and isotopic study of carbonate rocks from the Pechishchi stratotype section (Kazan) and three adjacent sections of Kazanian rocks of the Volga-Vyatka region at the eastern Russian Platform. These sections were recovered by the Kremeshki, Popovtsevo, and Chimbulat quarries (near the town of Sovetsk, southeastern Kirov district). Lithological features and wide variations of δ13C (from −6.0 to 6.8‰) and δ18 O (from 22.9 to 33.4 ‰) indicate that the rocks were formed in a shallow-marine basin with rapidly varying conditions of sedimentation which characterize different facies (and/or paleoecological) zones: lagoonal, supralittoral, littoral, shoal. They also suggest processes of postsedimentary alteration (mainly, under supergene conditions). Numerous short-term hiatuses are also recorded.  相似文献   

15.
For the first time 18O and 13C values from carbonates and D values of individual n-alkanes were used to reconstruct palaeohydrological conditions in a lagoon at the southern margin of the Central European Zechstein Basin (CEZB). A 12-m core covering the complete Ca2 interval and adjacent anhydrites (A1 and A2) was analyzed for 18O and 13C values of dolomitized carbonates and D values of individual n-alkanes. 18Ocarb values (+2 to +5 vs. VPDB) were strongly influenced by evaporation and temporal freshwater input into the lagoon. The 13Ccarb values (–1 to +4 vs. VPDB) were controlled mainly by changes in primary production. Both isotopic ratios show an inverse relationship throughout most of the core, contradicting diagenetic alteration, since 13Ccarb values are not altered significantly during dolomitization. Assuming a temperature range of 35–40 °C in the lagoon, 18Ocarb values of +2.5 to +8 (vs. VSMOW) for the lagoonal water can be reconstructed. The lagoon may have desiccated twice during the Ca2 interval, as indicated by very high 18Ocarb and low 13Ccarb values, coinciding with increasing amount of anhydrite in the analyzed sample. These events seem to reflect not just local but a regional intra-Ca2 cyclicity. Measured D values of the short-chain n-alkanes, namely n-C16 and n-C18 which are widely used as indicators for photosynthetic bacterial and algal input, reflect the isotopic composition of the lagoonal water. Assuming constant fractionation during incorporation of hydrogen into lipids of –160, an average D value of +70 (vs. VSMOW) can be reconstructed for the lagoonal water, accounting for very arid conditions. The long-chain n-alkanes n-C27, n-C28, n-C29 and n-C30, thought to be derived from the leaf waxes of terrestrial higher plants, were shown to be depleted in D relative to the short-chain alkanes, therefore indicating a different hydrogen source. Terrestrial plants in arid areas mainly use water supplied by precipitation. By using a smaller fractionation of –120 due to evaporation processes in the leaves, reconstructed values vary between –74 and –9 (vs. VSMOW). These values are not indicating extremely arid conditions, implying that the long-chain n-alkanes were transported trough wind and/or rivers into the lagoon at the Zechstein Sea coast. Dwater values, reconstructed using the n-C16 alkane and 18O water values, independently reconstructed on the same sample using the temperature dependant fractionation for dolomites are good agreement and suggest high amounts of evaporation affecting the coastal lagoon. Altogether, our results indicate that hydrogen isotopic ratios of n-alkanes give information on their origin and are a useful proxy for palaeoclimatic reconstruction.  相似文献   

16.
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17.
New isotope and mineral data on manganese carbonates of the Mazul deposit (Krasnoyarsk region) in combination with morphology of ore bodies suggest that the ores were formed in several stages with the involvement of meteoric solutions through infiltration and, possibly, exfiltration mechanisms. Based on the geological–geochemical data, manganese carbonates of the Mazul deposit may be ascribed to a new genetic subtype of the catagenesis (epigenesis) zone.  相似文献   

18.
Single-crystal electron paramagnetic resonance (EPR) spectra of a gem-quality jeremejevite, Al6B5O15(F, OH)3, from Cape Cross, Namibia, reveal an S = 1/2 hole center characterized by an 27Al hyperfine structure arising from interaction with two equivalent Al nuclei. Spin-Hamiltonian parameters obtained from single-crystal EPR spectra at 295 K are as follows: g 1 = 2.02899(1), g 2 = 2.02011(2), g 3 = 2.00595(1); A 1/g e β e  = −0.881(1) mT, A 2/g e β e  = −0.951(1) mT, and A 3/g e β e  = −0.972(2) mT, with the orientations of the g 3- and A 3-axes almost coaxial and perpendicular to the Al–O–Al plane; and those of the g 1- and A 1-axes approximately along the Al–Al and Al–OH directions, respectively. These results suggest that this aluminum-associated hole center represents hole trapping on a hydroxyl oxygen atom linked to two equivalent octahedral Al3+ ions, after the removal of the proton (i.e., a VIAl–O−–VIAl center). Periodic ab initio UHF and DFT calculations confirmed the experimental 27Al hyperfine coupling constants and directions, supporting the proposed structural model. The VIAl–O−–VIAl center in jeremejevite undergoes the onset of thermal decay at 300 °C and is completely bleached at 525 °C. These data obtained from the VIAl–O−–VIAl center in jeremejevite provide new insights into analogous centers that have been documented in several other minerals.  相似文献   

19.
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20.
Biotite is one of the most common minerals dated by the 40Ar–39Ar method. It frequently shows K contents below the expected stoichiometric value, suggesting the presence of low-K impurities. The most common low-K alteration product of biotite is chlorite. Therefore, it is important to understand the effects of chlorite interlayering on 40Ar–39Ar ages in order to correctly interpret 40Ar–39Ar data. This study examines the outcome of 40Ar–39Ar dating analyses on variably chloritised biotites from Ordovician intrusive rocks. The infrared (IR) laser-probe technique and different gas extraction methods were adopted. Incremental laser-heating data on bulk samples yielded hump-shaped age profiles with meaningless young and old age steps. Both the extent of anomalous old age steps and the degree of discordance of the age spectra were much more pronounced in the more chloritised biotite samples. In contrast, in situ data on rock chips and total-fusion ages on single biotite flakes yielded ages concordant with, or younger than, the inferred emplacement ages. Transmission electron microscopy (TEM) was used to texturally characterise biotite samples at the nanometre scale. It was also used to document the complex decomposition-transformation process affecting interlayered biotite–chlorite during in-vacuo IR-laser heating to temperatures ranging from ~600 to >1,000 °C. TEM results suggest that hump-shaped age profiles result from an interplay between 39ArK redistribution by recoil during sample irradiation and differential release of argon isotopes hosted in three main reservoirs. These reservoirs are (from least to most retentive): extended defects, chlorite and biotite. The final descending age segment is attributed to the progressive release of argon with increasing temperature from large biotite domains for which 39ArK recoil loss was less important. 40Ar–39Ar data support previous findings, which suggest that 40Ar–39Ar ages when recoil effects are minimised, provide minimum estimates that approach the true biotite age, when the pristine domains are analysed. The most effective approach for obtaining meaningful 40Ar–39Ar ages was using individual total-fusion analyses on carefully selected, single flakes previously split along the basal cleavage by wet-grinding and corresponding to a sample mass of a few micrograms.Editorial responsibility: I Parsons  相似文献   

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