| 1. |
- Bogdanova, Svetlana, et al.
(author)
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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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In: American Journal of Science. - : American Journal of Science (AJS). - 0002-9599. ; 310:10, s. 1345-1383
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Journal article (peer-reviewed)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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| 2. |
- Hegner, E., et al.
(author)
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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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In: American Journal of Science. - 0002-9599. ; 310:9, s. 916-950
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Journal article (peer-reviewed)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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| 3. |
- Pisarevsky, S. A., et al.
(author)
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Paleomagnetism Of 1780-1770 Ma Mafic And Composite Intrusions Of Smaland (Sweden): Implications For The Mesoproterozoic Supercontinent
- 2010
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In: American Journal of Science. - : American Journal of Science (AJS). - 0002-9599. ; 310:9, s. 1168-1186
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Journal article (peer-reviewed)abstract
- We present a paleomagnefic study of 1785 to 1770 Ma composite and mafic intrusions in Smaland, Sweden. A positive inverse baked contact test suggests the primary nature of their magnetization. The palaeomagnetic pole (45.7 degrees N, 182.7 degrees E, A(95)=8.0 degrees) may be regarded as a key pole for Fennoscandia. Available paleomagnetic data suggest that Laurentia and Fennoscandia/Balfica drifted together since 1780 Ma until at least 1265 Ma. These two continents could form a core of a hypothetical Mesoproterozoic supercontinent.
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| 4. |
- Villa, Eric M., et al.
(author)
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Adding reactivity to structure 2 : oxygen-isotope-exchange rates in three isostructural oxide ions
- 2010
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In: American Journal of Science. - : American Journal of Science (AJS). - 0002-9599 .- 1945-452X. ; 310:7, s. 629-644
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Journal article (peer-reviewed)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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| 5. |
- Engström, Karin, et al.
(author)
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Directed evolution of an enantioselective lipase with broad substrate scope for hydrolysis of α-substituted esters
- 2010
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 132:20, s. 7038-7042
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Journal article (peer-reviewed)abstract
- A variant of Candida antarctica lipase A (CalA) was developed for the hydrolysis of α-substituted p-nitrophenyl esters by directed evolution. The E values of this variant for 7 different esters was 45−276, which is a large improvement compared to 2−20 for the wild type. The broad substrate scope of this enzyme variant is of synthetic use, and hydrolysis of the tested substrates proceeded with an enantiomeric excess between 95−99%. A 30-fold increase in activity was also observed for most substrates. The developed enzyme variant shows (R)-selectivity, which is reversed compared to the wild type that is (S)-selective for most substrates.
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| 6. |
- Engström, Karin, et al.
(author)
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Enantioselective Kinetic Resolution of p-Nitrophenyl 2-Phenylpropanoate by a Variant of Candida antarctica Lipase A Developed by Directed Evolution
- 2010
-
In: Journal of the American Chemical Society. - : American Chemical Society. - 0002-7863 .- 1520-5126. ; 132:20, s. 7038-7042
-
Journal article (peer-reviewed)abstract
- A variant of Candida antarctica lipase A (CalA) was developed for the hydrolysis of α-substituted p-nitrophenyl esters by directed evolution. The E values of this variant for 7 different esters was 45−276, which is a large improvement compared to 2−20 for the wild type. The broad substrate scope of this enzyme variant is of synthetic use, and hydrolysis of the tested substrates proceeded with an enantiomeric excess between 95−99%. A 30-fold increase in activity was also observed for most substrates. The developed enzyme variant shows (R)-selectivity, which is reversed compared to the wild type that is (S)-selective for most substrates.
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