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1.
The middle part of the Central Indian Ridge (MCIR) between 8°S and 18°S is representative of mid-ocean ridges in the Indian Ocean but has not previously been systematically surveyed. Here we present results from the first high-resolution mapping survey over both on- and off-axis sections of the MCIR including multibeam bathymetry, magnetics, hydrocasting, and seabed sampling. The 700-km-long MCIR consists of six first-order segments that are offset by > 30 km along well-developed transform faults. Three of the first-order segments are further divided into seven second-order segments with < 30 km offset along non-transform discontinuities. We have recognized for the first time 11 prominent ocean core complexes (OCCs). These occur at nearly all segment ends, corresponding to an occurrence every 60 km of the surveyed ridge. Seafloor spreading model studies using magnetic reversals show that the MCIR is a slow-spreading ridge with average full opening rates ranging from 33.7 to 45.1 mm/yr, increasing from north to south. The highly curved and intermittent axial ridge geometry, rugged flank fabric, variation in the depth and width of the middle valley, and the frequent occurrences of ocean core complexes and non-transform discontinuities demonstrate that asymmetric accretionary processes are dominant along the ridge. The spreading rate symmetry combined with morphotectonic features, reveal that the MCIR segments developed mainly via tectonic extension with little magmatism. Segments with asymmetric accretion controlled by tectonic extension makes up ~ 96% of the MCIR, whereas symmetric accretion controlled by robust magmatism make up < 4%. Hydrothermal plumes with high methane concentrations occur frequently over the OCCs. This finding indicates that abundant OCCs exposed by detachment faults lead to extensive hydrothermal circulation at off-axis areas and that detachment faults are the primary fluid path for hydrothermal fluid circulation at off-axis regions. Serpentinization of mantle-derived rock at OCCs may be one of the major sources of heat and methane in off-axis areas. 相似文献
2.
Chemistry of major and minor elements, 87Sr/86Sr, δD, δ18O and δ34S of brines were measured from Tertiary strata and Quaternary salt lakes in the western Qaidam Basin. The water chemistry
data show that all oilfield brines are CaCl2 type. They were enriched in Ca2+, B3+, Li+, Sr2+, Br−, and were depleted in Mg2+, SO4
2−, which indicated that these brines had the characteristics of deeply circulated water. The relationship between δD and δ18O shows that all data of these brines decline towards the Global Meteoric Water Line (GWL) and Qaidam Meteoric Water Line
(QWL), and that the intersection between oilfield brines and Meteoric Water Lines was close to the local spring and fresh
water in the piedmont in the western Qaidam Basin. The results suggest that oilfield brines has initially originated from
meteoric water, and then might be affected by water-rock metamorphose, because most oilfield brines distribute in the range
of metamorphosing water. The 87Sr/86Sr values of most oilfield brines range from 0.71121 to 0.71194, and was less than that in salt lake water (>0.712), but close
to that of halite in the study area. These imply that salt dissolution occurred in the process of migration. In addition,
all oilfield brines have obviously much positive δ34S values (ranging from 26.46‰ to 54.57‰) than that of salt lake brines, which was caused by bacterial sulfate reduction resulting
in positive shift of δ34S value and depleteed SO4
2− in oilfield brines. Combined with water chemical data and δD, δ18O, 87Sr/86Sr, δ34S values, we concluded that oilfield brines mainly originate from the deeply circulated meteoric waters, and then are affected
by salt dissolution, water-rock metamorphose, sulfate reduction and dolomitization during the process of migration. These
processes alter the chemical compositions of oilfield brines and accumulate rich elements (such as B, Li, Sr, Br, K and so
on) for sustainable utilization of salt lake resources in the Qaidam Basin. 相似文献
3.
The O2 3?-Y3+ center in fluorite-type structures (CaF2 and SrF2) has been investigated at the density functional theory (DFT) level using the CRYSTAL06 code. Our calculations were performed by means of the hybrid B3PW method in which the Hartree–Fock exchange is mixed with the DFT exchange functional, using Becke’s three parameter method, combined with the non-local correlation functionals by Perdew and Wang. Our calculations confirm the stability and the molecular character of the O2 3?-Y3+ center. The unpaired electron is shown to be almost exclusively localized on and equally distributed between the two oxygen atoms that are separated by a bond distance of 2.47 Å in CaF2 and 2.57 Å in SrF2. The calculated 17O and 19F hyperfine constants for of the O2 3?-Y3+ center are in good agreement with their corresponding experimental values reported by previous electron paramagnetic resonance and electron nuclear double resonance studies, while discrepancies are notable for the 89Y hyperfine constants. 相似文献
4.
Mid-Ocean Ridge Basalts (MORB) from the Northern Central Indian Ridge (NCIR) were recovered between latitudes 3° and 11°S and are olivine tholeiite with higher abundances of K and Rb.They are of typical transitional MORB (T-MORB) variety and appear to have been generated from an enriched-mantle peridotite source. The primitive NCIR MORBs having Mg# > 0.68 are the product of partial melting at an estimated pressure of ~ 1 GPa. It is inferred that the magma was subsequently modified at a pressure > 1 GPa by crystal fractionation and spinel was the first mineral to crystallize followed by separation of relatively Fe-rich olivine with subsequent decrease in pressure. During progressive fractionation at lower pressure (between 1-0.5 GPa), the bulk composition of the magma became systematically depleted in MgO, and enriched in ∑FeO, TiO2, P2O5 and Na2O. There was,however, limited gradual depletion in Al2O3 and CaO and concomitant enrichment in K2O. With the progressive fractionation these basalts became gradually enriched in V, Co, Y, Zr and to some extent in Sr, and depleted in Ni and Cr. In addition, the ∑REE of the magma also increased with fractionation,without any change in (La/Yb)n value. 相似文献
5.
Dwijesh RAY Sridhar D. IYER Ranadip BANERJEE Saumitra MISRA M. WIDDOWSON 《《地质学报》英文版》2007,81(1):99-112
1 Introduction The Northern Central Indian Ridge (NCIR) between 3° and 11°S latitudes is joined to the north with the slow spreading Carlsberg Ridge (CR; ~24–26 mm/a, full spreading rate) and to the south with the intermediate spreading Southern Central Indian Ridge (SCIR; ~50 mm/ a) (Fig. 1). Earlier petrological investigations of the Central Indian Ridge were concentrated either on or along the CR to the north and at the southern end of the CIR up to the Rodriguez Triple Junc… 相似文献
6.
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
7.
I. M. Gorokhov A. B. Kuznetsov G. V. Konstantinova G. V. Lipenkov E. O. Dubinina I. V. Bigun 《Doklady Earth Sciences》2018,482(2):1306-1310
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
8.
M. Deiana F. Cervi M. Pennisi M. Mussi C. Bertrand A. Tazioli A. Corsini F. Ronchetti 《Hydrogeology Journal》2018,26(8):2669-2691
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
9.
This work presents the first isotope-geochronological data obtained for granitoid series of the Arminskii block of Central Sikhote Alin: Tatibi gabbromonzodiorite and Olgino leucocratic granite. Every series is characterized by a certain structural position and timing of the formation of ore mineralization. The Tatibi series is characterized by the presence of gold-ore occurrences including the potential Glukhoe goldore occurrence. Granitoids of the Olgino series comprise known tin–sulfide ore occurrences. The conditions for the generation of each series of rocks were revealed on the basis of geochronological-isotope data. 相似文献
10.
Shen Qu Guangcai Wang Zheming Shi Qingyu Xu Yuying Guo Luan Ma Yizhi Sheng 《Hydrogeology Journal》2018,26(5):1443-1453
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
11.
12.
The Ethiopian Rift (a major portion of the Great East African Rift) is characterized by a narrow elongated depression bounded by highlands from both sides. This topographic configuration leads to a monsoon redistribution which resulted in an arid rift floor and humid high rainfall highlands. The rifting and associated volcanism also caused a thinning of the crust and facilitates influx of CO2 and other mantle gases as diffuse sources or along faults from deeper sources. Groundwaters in the rift floor are usually of high mineral content (high F, U, As and salinity) while those on the plateau are of low mineral content. Among many factors, groundwater availability and quality in the rift floor aquifers is the function of their connection to the aquifers in the high rainfall plateau and the residence time of groundwater prior to reaching the rift floor. This entails the need for addressing one basic hydrologic question in such a setting: at what depth and rate does recharge from the high rainfall highland reach the lowland rift aquifers? This study uses spatial variations in trace elements and relates them to 14C variations, thereby investigating the suitability of using trace elements as proxies for residence time estimation of groundwaters of relatively short (1,000–2,000 years) residence time. This work also investigates the behavior of trace element trends along the groundwater flow path in a rifted setting and compares them with such trends in sedimentary aquifers elsewhere. The comparison shows a clear difference in behavior of trace elements along the groundwater flow path when compared with such variations in big sedimentary basins with no prominent rifting and volcanism, suggesting the need of calibrating the relation between trace elements and any direct residence time indicators. An integrated use of major elements, trace elements, and environmental isotopes reveals that the main recharge of the aquifers originates from mountain blocks and that recharge takes place via fractures with no evidence of evaporation prior to recharge. Redox processes appear to play a limited role in trace element geochemistry of groundwaters in the region. Progressive trends in trace element composition along the groundwater flow path suggest continuous groundwater flow from the plateau. 相似文献
13.
V. N. Golubev E. O. Dubinina I. V. Chernyshev T. A. Ikonnikova 《Doklady Earth Sciences》2016,466(1):28-31
The uneven character of the distribution of 18O/16O and 234U/238U values was established in the vertical cross section of the productive sequence of the Dybryn uranium deposit (Vitim uranium-ore region, Buryatia). Both a deficiency and an excess of 234U in relation to the equilibrium 234U/238U ratio in the vertical sequence may provide evidence for the extremely low rate of the infiltration water flow. The behavior of oxygen isotope characteristics for different size fractions of terrigenous rocks provides evidence for active uranium redistribution and openness of the isotope system of this element during interaction of terrigenous–sedimentary rocks with infiltration waters. 相似文献
14.
Yu. O. Larionova L. V. Sazonova N. M. Lebedeva A. A. Nosova V. V. Tretyachenko A. V. Travin A. V. Kargin D. S. Yudin 《Petrology》2016,24(6):562-593
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
15.
Pia Söderlund Tobias Hermansson Laurence M. Page Michael B. Stephens 《International Journal of Earth Sciences》2009,98(8):1835-1851
In order to characterize the post-Svecofennian tectonothermal evolution of the Fennoscandian Shield, 40Ar–39Ar biotite and some 40Ar–39Ar muscovite geochronological data are reported from a total of 30 surface outcrop and 1,000 m long borehole samples at Forsmark,
central Sweden. The 13 surface samples were collected across 3 branches of a major WNW to NW trending system of deformation
zones, whereas the boreholes were drilled within a tectonic lens, in between two of these zones. The 40Ar–39Ar biotite ages indicate that the present erosion surface, in central Sweden, cooled below c. 300°C at 1.73–1.66 Ga, and that
the rocks could have accommodated strain in a brittle manner between 1.8 and 1.7 Ga. The variation in surface ages is suggested
to be due to fault along the large WNW to NW trending deformation zones, following the establishment of a sub-Cambrian peneplain.
The minor variation of ages within a single crustal block may be due to disturbance along ENE to NNE trending fracture zones.
Possible cooling paths, derived from 40Ar–39Ar hornblende, muscovite and biotite ages, were calculated for the time interval from 1.80 to 1.67 Ga, when the area cooled
from c. 500 to 300°C. Cooling rates of 1.9–4°C/m.y. have been attained. Between 1.68 and 1.64 Ga, uplift rates of c. 22 m/m.y.
were calculated from borehole 40Ar–39Ar biotite data. Tectonothermal histories, inferred from the combined cooling and uplift rates, are related to simple cooling
after the Svecofennian orogeny, to crustal movement in response to far-field effects of c. 1.7 Ga orogenic activities further
to the west or to a combination of these possibilities. 相似文献
16.
The ~1,000 km3 Carpenter Ridge Tuff (CRT), erupted at 27.55 Ma during the mid-tertiary ignimbrite flare-up in the western USA, is among the largest known strongly zoned ash-flow tuffs. It consists primarily of densely welded crystal-poor rhyolite with a pronounced, highly evolved chemical signature (high Rb/Sr, low Ba, Zr, Eu), but thickly ponded intracaldera CRT is capped by a more crystal-rich, less silicic facies. In the outflow ignimbrite, this upper zone is defined mainly by densely welded crystal-rich juvenile clasts of trachydacite composition, with higher Fe–Ti oxide temperatures, and is characterized by extremely high Ba (to 7,500 ppm), Zr, Sr, and positive Eu anomalies. Rare mafic clasts (51–53 wt% SiO2) with Ba contents to 4,000–5,000 ppm and positive Eu anomalies are also present. Much of the major and trace-element variations in the CRT juvenile clasts can be reproduced via in situ differentiation by interstitial melt extraction from a crystal-rich, upper-crustal mush zone, with the trachydacite, crystal-rich clasts representing the remobilized crystal cumulate left behind by the melt extraction process. Late recharge events, represented by the rare mafic clasts and high-Al amphiboles in some samples, mixed in with parts of the crystal cumulate and generated additional scatter in the whole-rock data. Recharge was important in thermally remobilizing the silicic crystal cumulate by partially melting the near-solidus phases, as supported by: (1) ubiquitous wormy/sieve textures and reverse zoning patterns in feldspars and biotites, (2) absence of quartz in this very silicic unit stored at depths of >4–5 km, and (3) heterogeneous melt compositions in the trachydacite fiamme and mafic clasts, particularly in Ba, indicating local enrichment of this element due mostly to sanidine and biotite melting. The injection of hot, juvenile magma into the upper-crustal cumulate also imparted the observed thermal gradient to the deposits and the mixing overprint that partly masks the in situ differentiation process. The CRT provides a particularly clear perspective on processes of in situ crystal-liquid separation into a lower crystal-rich zone and an upper eruptible cap, which appears common in incrementally built upper-crustal magma reservoirs of high-flux magmatic provinces. 相似文献
17.
Lorena N. Dias Maurício V. B. Pinheiro Klaus Krambrock 《Physics and Chemistry of Minerals》2009,36(9):519-525
Irradiation techniques are often applied to gem minerals for color enhancement purposes. Natural green, blue and colorless
specimens of rare gemological quality euclase, BeAlSiO4(OH), from Brazil were irradiated with gamma rays in the dose range from 10 to 500 kGy. Although the colors of the different
specimens were not strongly influenced, two different irradiation-induced paramagnetic defect centers were found by electron
paramagnetic resonance (EPR). The first one is an O− hole center interacting with one Al neighbor and the second is a Ti3+ electron center. The EPR angular rotation patterns of both irradiation-induced defects were measured and analyzed. The results
suggest that O− hole centers are formed by dissociation of the hydroxyl ions, similar as in topaz crystals. In euclase the OH− ions interconnect distorted Al octahedra and Be tetrahedra in O5 positions. During irradiation, the electrons are captured by titanium ions (Ti4+ + e−), leading to the formation of paramagnetic Ti3+ ions. From the EPR rotation patterns it is clear that these ions substitute for Al ions. The spin Hamiltonian parameters
of the irradiation-induced defects are analyzed and compared to similar defect centers in other mineral specimens. Thermal
annealing experiments show that the O− hole centers and Ti3+ electron centers are directly connected through the radiation process. 相似文献
18.
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
19.
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
20.
D. E. Martínez E. Fourré O. M. Quiroz Londoño P. Jean-Baptiste M. Glok Galli A. Dapoigny S. I. Grondona 《Hydrogeology Journal》2016,24(5):1107-1120
The Pampa region in Argentina includes vast unconfined–semiconfined aquifers that local economies depend upon, but detailed knowledge of the associated water resources is still lacking. The Pampeano aquifer in the Pampa plain of Argentina covers around 1.5 million km2. In order to achieve a better understanding of the hydrogeological system through the estimation of mean residence times (MRT), water samples were taken from 12 monitoring wells, drilled at different depths in four locations, and analyzed for environmental tracers. The concentrations of 3H, tritiogenic 3He and chlorofluorocarbons (CFCs) can be explained by mixtures of young waters adjusted to exponential piston flow models (EPM) or dispersion models (DM), and different proportions of tracer-free waters (dead water). The sampling site located very close to the water divide shows a dominance of young waters: 85 % of water best represented by a DM model with a MRT of 3 years. For the shallow wells at other sites, best-fitting models result in a DM with MRT between 20 and 35 years, and proportions of dead water between 40 and 60 %. These results lead to important updates in the conceptual model of the Pampeano aquifer. Large proportions of dead water at a few meters depth can be the consequence of upward flows in a multilayered aquifer or diffusive retardation in the inter-bedded clay layers. 相似文献