| 1. |
- Hardisty, Dalton S., et al.
(författare)
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A Holocene History Of Dynamic Water Column Redox Conditions In The Landsort Deep, Baltic Sea
- 2016
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Ingår i: American Journal of Science. - : American Journal of Science (AJS). - 0002-9599 .- 1945-452X. ; 316:8, s. 713-745
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Tidskriftsartikel (refereegranskat)abstract
- The modern Baltic Sea is the world's largest anthropogenically forced anoxic basin. Using integrated geochemical records collected during Integrated Ocean Drilling Program (IODP) Expedition 347 from the deepest and one of the most reducing sub-basins in the Baltic Sea, Landsort Deep, we explore the degree and frequency of natural anoxia through the Baltic Holocene. A marked decrease in carbon-to-sulfur ratios (C/S) from the cores indicate the transition from the Baltic Ice Lake to the current brackish sea, which occurred about 8.5 kyrs B.P. Following this, laminations throughout sediments recording brackish deposition suggest sustained anoxia or extreme low oxygen, while high molybdenum (Mo) concentrations of >100 ppm and iron (Fe) geochemistry suggest water column sulfide accumulation, or euxinia, that persisted beyond seasonal timescales during deposition of two distinct sapropel units. Sedimentary Mo isotope values range from +1.11 to -0.50 permil, which are distinctly fractionated from modern Baltic seawater (+2.26 to -2.67 parts per thousand) and thus indicate that each of the sapropels experienced only weak and/or oscillatory euxinia-in contrast to the more stable euxinic conditions of more restricted basins. A shift in delta Mo-98 starting above the lower sapropel to a distinctly more negative range suggests particularly weak and oscillatory euxinia, with an enhanced contribution of manganese (Mn) redox cycling to Mo deposition relative to the lower portion of the profile. This conclusion is supported by extreme sedimentary Mn enrichments of up to 15 weight percent. We interpret the combined data to indicate episodic but major Baltic inflow events of saline and oxygenated North Sea water into the anoxic Landsort Deep that limited the concentrations and residence time of water column sulfide and caused episodic oxide deposition. Considering the temporal overlap between the most reducing conditions and periods of redox instability, we hypothesize that major Baltic inflows, as is observed today, lead to short-term instability while simultaneously supporting longer-term Baltic anoxia by strengthening the halocline. Ultimately, our results indicate that periods more reducing than the modern Baltic Sea have occurred naturally over the Holocene, but the characteristic dynamic saline inputs have historically prevented the relatively more widespread and stable anoxia observed in other classic restricted basins and will likely continue to do so.
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| 2. |
- Kleine, Barbara I., et al.
(författare)
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RAPID FLUID FLOW ALONG FRACTURES AT GREENSCHIST FACIES CONDITIONS ON SYROS, GREECE
- 2016
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Ingår i: American Journal of Science. - : American Journal of Science (AJS). - 0002-9599 .- 1945-452X. ; 316:2, s. 169-201
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Tidskriftsartikel (refereegranskat)abstract
- Brittle fractures cut through greenschist facies metavolcanic rocks at Delfini on Syros, Greece. An equation for one-dimensional transport by advection along a single fracture and transverse diffusion outwards from this fracture was used to calculate time-averaged fluid velocities and the duration of fluid flow along the brittle fractures. These quartz and carbonate filled fractures are surrounded by symmetrical dark reaction halos. These reaction halos were formed by diffusion of CO2 outwards from the fracture in a hydrous fluid which caused carbonation of the country rock. Changes in concentration of relatively mobile elements (for example K, Na, Cs, Ba, Pb and Sr) occurred. However, little to no changes in most of the major elements and less mobile trace elements were observed. This implies that carbonation was largely isochemical with respect to most non-volatile components. The Sr/Ca ratio was used to model time-averaged fluid velocities and the duration of fluid flow along the fractures. Fluid flow along narrower fractures with discernibly tapering haloes was found to be rapid (10(-6) - 10(-5) ms(-1)) and short lived (0.1-400 years). These are time-averaged values and can therefore alternatively record a series of even shorter and faster pulses, perhaps associated with fracture propagation and associated seismicity. Within the widest fractures with constant halo widths (ca. 60 cm) fluid flow was slower (10(-8) to 10(-6) ms(-1)) and longer lived (100-15000 years). We suspect that the constant width of these haloes reflects a steady state having been reached at which halo width was controlled by the relative rates of fluid flow along the fracture and in the surrounding rock.
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| 3. |
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| 4. |
- Lewerentz, Alexander, et al.
(författare)
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Fluid and carbon flux estimation of regional metamorphic fluid flow in Glen Esk, SE Scottish Highlands : The role of hydrodynamic dispersion for broadening of an isotopic front
- 2018
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Ingår i: American Journal of Science. - : American Journal of Science (AJS). - 0002-9599 .- 1945-452X. ; 318:4, s. 435-457
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Tidskriftsartikel (refereegranskat)abstract
- During metamorphism, large quantities of fluids are mobilized and transported through the crust. Such fluids may interact with surrounding rock and serve as a medium for chemical transport. In this study, we use coupled carbonation and oxygen isotope fronts to estimate fluid and carbon fluxes for metamorphic fluid flow in vein pathways in the Dalradian metasediments that crop out in Glen Esk, SE Scottish Highlands. Comparison of wall rock and vein oxygen Isotope ratios indicate that the Barrovian sequence in Glen Esk was infiltrated by at least two separate fluid flow events, where fluid flow from the North Esk Fault overprints an earlier fluid flow event, for which the syn-metamorphic magmatic intrusions north of Glen Esk are a probable fluid source. Advection is the dominating mode of fluid transportation in veins. By advection-dispersion and advection-diffusion modeling, we estimate a time-averaged fluid flux of 0.0005 to 0.0135 m(3).m(-2).yr(-1) and a carbon flux of 0.04 to 0.71 mol C.m(-2).yr(-1) for fluid flow from the North Esk fault into the Dalradian metasediments. The duration of this fluid flow event is estimated to between 11 and 230 kyr. Our results also indicate that hydrodynamic dispersion was the main reason for broadening of the oxygen isotope front.
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| 5. |
- Lin, Li'na, et al.
(författare)
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GEOCHRONOLOGIC AND GEOCHEMICAL EVIDENCE FOR PERSISTENCE OF SOUTH-DIPPING SUBDUCTION TO LATE PERMIAN TIME, LANGSHAN AREA, INNER MONGOLIA (CHINA) : SIGNIFICANCE FOR TERMINATION OF ACCRETIONARY OROGENESIS IN THE SOUTHERN ALTAIDS
- 2014
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Ingår i: American Journal of Science. - : American Journal of Science (AJS). - 0002-9599 .- 1945-452X. ; 314:2, s. 679-703
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Tidskriftsartikel (refereegranskat)abstract
- The Langshan area in Inner Mongolia is situated in the southern Altaids between the Beishan suture to the west and the Solonker suture to the east. This paper addresses the poorly known tectonic evolution that led to formation of the terminal Solonker suture. Dating of deformed porphyries and undeformed dolerites and gabbros constrains the timing of the relevant NE-E-striking and north-vergent deformation. Deformed granitic-granodioritic porphyries in this area are characterized by high SiO2 (65.38-78.00%), low TFe2O3 (1.29-5.07%), MgO (0.13-0.63%), and variable K2O (0.534.14%) and Na2O (2.05-4.62%). All samples have enriched LREE (La/Yb 6-18) and negative Nb anomalies (Nb-N/Th(N)0.09-0.48), but different Eu anomalies (Eu* <0.7 or similar to 1); these geochemical features can be ascribed to a heterogeneous source in a subduction-related environment. Gabbros and dolerites have 42.33 to 52.03 percent SiO2. All mafic samples have similar La/Yb ratios of 4 to 6 and negative Nb anomalies (Nb-N/Th-N) ratios of 0.2 to 0.8, suggestive of a subduction-related setting. Two granitic porphyries yielded U-238/Pb-206 weighted mean ages of 284.7 +/- 2.1 Ma with MSWD of 1.6 and 291.7 +/- 2: 2.1 Ma with MSWD of 1.14; these ages are consistent with U-235/Pb-207 and U-238/Pb-206 concordia ages of 281 +/- 17 Ma with MSWD of 0.87 and 289.8 +/- 9.2 Ma with MSWD of 0.66. A dolerite yielded concordia ages of 256.2 +/- 2.6 Ma with MSWD of 0.44 and 256 +/- 2.5 Ma with MSWD 0.45. The ages and geochemistry of the deformed porphyries indicate that in the early Permian there was important deformation and recrystallization in a subduction-related setting. The isotopic and geochemical signatures of all the rocks indicate that they formed during subduction-related conditions. We propose that Langshan was a Permian active continental margin arc built on the edge of the North China Craton by southward subduction, which led to closure of the ocean, concomitant formation of the Solonker suture in the late Permian-early Triassic, and termination of the accretion-subduction orogen of the southern Altaids.
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| 6. |
- Villa, Eric M., et al.
(författare)
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Adding reactivity to structure 2 : oxygen-isotope-exchange rates in three isostructural oxide ions
- 2010
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Ingår i: American Journal of Science. - : American Journal of Science (AJS). - 0002-9599 .- 1945-452X. ; 310:7, s. 629-644
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Tidskriftsartikel (refereegranskat)abstract
- To understand how oxide structures react at the molecular scale, rates of steady oxygen-isotope exchanges were followed in three isostructural molecules of similar to 40 atoms as a function of solution composition. These molecules were chosen because the structures in solution are known with complete confidence, yet isotope-exchange reactions can be followed spectroscopically at individual oxygens. The series of molecules differ only in a single Ti(IV) double right arrow Nb(V) substitution in one of the three metal sites, making a series of structures having the stoichiometries: [HxNb10O28]((6-x)-), [HxTiNb9O28]((7-x)-), and [HxTi2Nb8O28]((8-x)-). As in our previous study of the [HxNb10O28]((6-x)-) ion, we find that isotope-ex hange reactions at particular oxygens cannot be understood without considering dynamics of the entire nanometer-size structure, and the interaction of the entire structure with solution. The rates for all reactive oxygens vary similarly with pH within a single molecule, but the relative importance of the proton- or hydroxide-enhanced pathways for isotopic exchange vary systematically across the series, along with Bronsted acid-base properties, and scale like the charge of the unprotonated structure in solution. The local effect on site reactivities of the Ti(IV) substitution is surprisingly small and is of the same order as that due to changes in the counterions. The extents to which the functional-group reactivities reflect global properties of the molecules is striking and emphasizes the importance of having accurate structural information in simulating geochemical reactions. The broad amphoteric chemistry of the rates resembles other classes of oxide reactions, such as ester hydrolysis and mineral dissolution kinetics.
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| 7. |
- Villa, Eric M., et al.
(författare)
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Adding reactivity to structure-reaction dynamics in a nanometer-size oxide ion in water
- 2008
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Ingår i: American Journal of Science. - : American Journal of Science (AJS). - 0002-9599 .- 1945-452X. ; 308:8, s. 942-953
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Tidskriftsartikel (refereegranskat)abstract
- We examine oxygen-isotope exchanges in a nanometer-size oxide molecule in water and, separately, both its rates of dissociation and molecular products. This molecule, the decaniobate ion ([H(x),Nb(10)O(28)]((6-x)-)), is at the same size scale as geochemically interesting features on minerals, such as surface polymers and kink sites on growth steps, although it is structurally quite dissimilar. Unlike mineral surface structures, however, we have complete confidence in the aqueous structure of this molecule and it yields a clear spectroscopic signature as it reacts. We thus can follow proton-enhanced isotope exchanges and base-induced dissociation in unprecedented detail and clarity. The results are surprising and require new thinking about geochemical reactions at the molecular scale. For example, base-induced dissociation of the molecule, which is unprotonated, causes rates of oxygen-isotope exchanges of all structural oxygens to accelerate dramatically. Similarly, protonation of the molecule causes sets of oxygens to react, although protonation is limited. In general, all reactions are via concerted motions of many atoms and the reactivities vary as though the entire structure was responding to changes in solution composition. The site reactivities could not be inferred from the stable structure of the decaniobate molecule because so much of the structure is involved in each exchange event. Thus, computational models must be structurally faithful to an extraordinary degree, and inherently dynamic, or they will miss the essential chemistry.
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| 8. |
- Bogdanova, Svetlana, et al.
(författare)
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Volgo-Uralia: The First U-Pb, Lu-Hf and Sm-Nd Isotopic Evidence of Preserved Paleoarchean Crust
- 2010
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Ingår i: American Journal of Science. - : American Journal of Science (AJS). - 0002-9599. ; 310:10, s. 1345-1383
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Tidskriftsartikel (refereegranskat)abstract
- The crustal segment Volgo-Uralia is the least known part of the East European Craton. Its crystalline crust is hidden beneath a thick Neoproterozoic to Phanerozoic cover but disclosed by thousands of drill holes. In conjunction with the recent "Tatseis" reflection seismic profile, we conducted the first isotopic study of the Bakaly granitoid block in eastern Volgo-Uralia, which represents a subsurface section of the layered upper-middle crust. The study included whole-rock Sm-Nd and ion-probe zircon U-Th-Pb (SIMS) and Lu-Hf (LA-ICPMS) analyses of granitoids from seven drill cores. The Bakaly block was also targeted because its rocks have never been subjected to granulite facies metamorphism, making it possible to date pristine, pre-metamorphic zircon. Our study showed that the four principal suites of granitoids in the Bakaly block are different in age, each corresponding to a particular stage of Archean crustal evolution between 3.3 and 2.6 Ga. The Tashliar monzonitic suite, belonging to an alkaline series yielded zircon ages of 3.3 and 3.2 Ga, which are the oldest ages yet found in Volgo-Urafia. The epsilon(Hf)(T) values of the dated zircon and the epsilon(Nd)(T) values of their host rocks indicate that a Paleo- to Eoarchean protolith with model T-DM ages up to 3.8 Ga had been involved in the formation of the Tashliar melts. Three Neoarchean rock suites, one comprising quartz dioritic and tonalitic gneisses (the Bak 1), another K-rich granodiorites, granites and migmatites (the Bak 2), and the third monzonitic granitoids (the Aktanysh suite) were formed sequentially between 2.72 and 2.60 Ga. The 2.72 Ga Bak 1 suite is chemically diverse. It includes granitoids of the TTG type related to slab/subduction melts as well as rocks formed by the re-melting of older crust with whole-rock Nd T-DM and Hf T-DM model ages of 3.4 to 3.2 Ga. The 2.69 to 2.65 Ga Bak 2 suite was probably associated with a major collisional event, which defined the stacked structure of the Archean crust in Volgo-Uralia and its seismic layering. Our data suggest that the Bak 2 melts originated partly from juvenile sources with epsilon(Hf)(T) zircon values up to +4.8, as well as mixed crustal and juvenile mantle materials. Some crustal contamination of the melts appears to have occurred as evidenced by incorporated xenocrystic zircon. The chemical compositions of Bak 2 granitoids from the different plutons, their zircon epsilon(Hf) values, and the Hf- and Nd T-DM ages all mirror a heterogeneous, collisional, crustal structure. During post-collisional extension at 2.6 Ga, the intrusion of Aktanysh monzonitic granitoids took place. These rocks also bear evidence of a long crustal pre-history with Nd and Hf T-DM model ages of 3.3 to 3.5 Ga. The Aktanysh rocks are coeval with the Tuymazy gabbro-norite-anorthosite intrusions, which are widely distributed along post-collisional shear zones in the Bakaly block. They could have provided the heat necessary to melt the crust at this stage. Altogether, the isotopic evidence suggests several episodes of crustal growth and recycling possibly reaching back to 3.6 and 3.8 Ga. Metamorphic zircon rims show that the Archean crust in the Bakaly block were subjected to several tectonothermal overprints in the Paleoproterozoic between 2.4 and 1.9 Ga ago.
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| 9. |
- Hegner, E., et al.
(författare)
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Mineral Ages and P-T Conditions of Late Paleozoic High-Pressure Eclogite and Provenance of M Lange Sediments from Atbashi in the South Tianshan Orogen of Kyrgyzstan
- 2010
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Ingår i: American Journal of Science. - 0002-9599. ; 310:9, s. 916-950
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Tidskriftsartikel (refereegranskat)abstract
- Ages derived from various isotope systems in high-pressure (HP) rocks of the western Tianshan orogen of NW China have been interpreted as evidence for late Carboniferous and/or Triassic collision of the accretionary margin of the Central Asian Orogenic Belt (CAOB) with the Tarim Craton. In order to elucidate this controversy, we present new P-T data as well as Sm-Nd and (40)Ar/(39)Ar cooling ages for an eclogite sample from Atbashi in the accretionary m lange of the South Tianshan suture in Kyrgyzstan, some 500 km along strike to the west of the controversial locality in the upper Akeyazhi River Valley in NW China. A clockwise P-T path for the eclogite with peak pressures of 18 to 24 kbar at 520 to 600 degrees C is consistent with near-isothermal decompression and exhumation in a subduction zone before collision of the CAOB with the Tarim Craton. Geochemical data and an initial epsilon Nd value of similar to +9 suggest an N-MORB protolith for the eclogite. The high-pressure mineral assemblage of the eclogite yielded a statistically robust Sm-Nd isochron age of 319 +/- 4 Ma (2 sigma, 5 data points, MSWD = 0.4) for equilibration and closure of the Sm-Nd system during HP metamorphism. (40)Ar/(39)Ar dating of phengite from the same sample yielded a cooling age of 316 +/- 3 Ma (2 sigma) implying rapid exhumation. Docking of the Tarim Craton with the southern margin of the Middle Tianshan-North Tianshan blocks in Kyrgyzstan during the late Carboniferous is supported by widespread emplacement of A-type granitoids of early Permian age that suggest a setting of consolidated crust. An tuunetamorphosed and little deformed molasse-type conglomerate of latest Carboniferous age, overlying the HP rocks, indicates that HP metamorphism, exhumation, and exposure of the HP m lange occurred from 320 to 300 Ma. The detrital zircon age spectrum of a metagraywacke sample from the accretionary melange suggests sources in the Tarim Craton and/or from the Middle and North Tianshan that possibly comprise rifted blocks from Tarim.
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| 10. |
- Mitchell, Ross N., et al.
(författare)
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PLATE TECTONICS BEFORE 2.0 Ga: EVIDENCE FROM PALEOMAGNETISM OF CRATONS WITHIN SUPERCONTINENT NUNA
- 2014
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Ingår i: American Journal of Science. - : American Journal of Science (AJS). - 0002-9599. ; 314:4, s. 878-894
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Tidskriftsartikel (refereegranskat)abstract
- Laurentia, the core of Paleo- to Mesoproterozoic supercontinent Nuna, has remained largely intact since assembly 2.0 to 1.8 billion years ago [Ga]. For earlier times, previous paleomagnetic data on poorly dated Paleoproterozoic mafic intrusions yielded ambiguous estimates of the amount of separation between key cratons within Nuna such as the Slave and Superior. Recent developments in paleomagnetism and U-Pb baddeleyite geochronology, including new results reported herein, yield sufficiently precise data to generate partial apparent polar wander paths for both the Slave and Superior craton from 2.2 to 2.0 Ga. Our new apparent polar wander comparison confirms earlier speculations that processes similar to plate tectonics, with relative motion between the Slave and Superior cratons, were operative leading up to the final assembly of supercontinent Nuna.
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