Search

C record of carbonate rocks (δ 13 C carb ), for the selective extinction of heavily calcifi ed marine animals, and for the abrupt transition in carbonate facies across the extinction horizon, an additional proxy is necessary to distinguish among these options. Calcium isotopes hold promise for differentiating between suggested scenarios for change in seawater carbonate chemistry linked with the end-Permian extinction. The calcium cycle is linked to the carbon cycle through the weathering of limestone and the deposition of calcium carbonate sediments. Calcium isotopes fractionate by ~0.6‰ and ~1.3‰ in calcite and aragonite, respectively, during the precipitation of modern carbonate minerals, meaning carbonate sediments are enriched in the lighter isotope relative to the seawater from which they precipitated (Skulan et al., 1997; Tang et al., 2008). Imbalances between calcium delivery and burial fl uxes and changes in the magnitude of fractionation during carbonate deposition will result in changes in the δ 44/40 Ca of seawater and will be recorded in marine sediments (DePaolo, 2004; Farkao et al., 2007). Therefore, variation in the δ

Preferential flow paths are expected in many groundwater systems and must be located because they can greatly affect contaminant transport. The fundamental characteristics of radiogenic isotope ratios in chemically evolving waters make them highly effective as preferential flow path indicators. These ratios tend to be more easily interpreted than solute-concentration data because their response to water-rock interaction is less complex. We demonstrate this approach with groundwater {sup 87}Sr/{sup 86}Sr ratios in the Snake River Plain aquifer within and near the Idaho National Engineering and Environmental Laboratory. These data reveal slow-flow zones as lower {sup 87}Sr/{sup 86}Sr areas created by prolonged interaction with the host basalts and a relatively fast flowing zone as a high {sup 87}Sr/{sup 86}Sr area.

The reconstruction of Laurentia, Australia, and Antarctica into a Proterozoic supercontinent is evaluated by analyzing the fit of Precambrian provinces defined by isotopic and geochronologic mapping. The analysis is complicated by allochthonous segments of the Antarctic and eastern Australian margins. Removal of the allochthonous provinces produces a closer fit of the continents; there is a match of Early Proterozoic basement between southwestern Laurentia and the only exposure of craton known from the paleo-Pacific margin of Antarctica. In addition, western Laurentia is brought closer to the Australian Gawler block, consistent with provenance interpretations of the Belt Supergroup. Removal of the allochthonous provinces by right-lateral translation relative to the Antarctic craton margin places them in a pre-750 Ma position where they could be southwestward extensions of the Yavapai-Mazatzal and Grenville provinces of southern Laurentia. This modified reconstruction leads to a prediction of extensive Archean basement in Antarctica between the South Pole and Victoria Land, a prediction partly borne out by Archean rocks in the Miller Range of the Transantarctic Mountains; it also predicts the presence of 1.4Ga rapakivi granites in the Transantarctic Mountains basement. This configuration implies assembly of the Australia-Antarctica Gondwana margin by terrane accretion following, or accompanied by, left-lateral translation. This is compatible with a tectonic regime of clockwise rotation of Laurentia relative to Australia and Antarctica after rifting. Thus, the proposed supercontinent, with some modifications, has potential for explaining several aspects of the pattern of Precambrian provinces in the three continents.

We present 238 U- 230 Th - 226 Ra disequilibria and cosmogenic 3 He and 36 Cl data for the Bluewater flow of the Zuni-Bandera volcanic field in western New Mexico. The 238 U- 230 Th disequilibria measured on separated groundmass phases yield an internal isochron age of 68 ka (+24/–20 ka; 2σ). This value cannot be directly compared with surface exposure ages unless erosion rates are known. The apparent (zero erosion) ages determined from both the 3 He concentration (47.5 ± 5 ka; 2σ) and the 36 Cl concentration (41.2 ± 8.8 ka; 2σ) are significantly younger than the U-Th isochron age. When minimum estimates of surface erosion based on flow morphology are considered, the 3 He concentrations indicate a minimum exposure age of 60 ka, in good agreement with the U-Th isochron age, with a minimum erosion rate of 1.7 mm/k.y. and an erosion rate as high as 5 mm/k.y. in other locations. Correcting for erosion has little effect on the model 36 Cl age and, as a result, the 36 Cl age is significantly younger than the U-Th isochron age and erosion-corrected 3 He ages; this discordance is attributed to a lack of closed-system behavior in the 36 Cl system. These new ages have local significance for the geochronology of the Zuni-Bandera volcanic field; however, their larger significance is in their applicability to dating Quaternary basalts and quantifying erosion rates.

The provenance of fore-arc sandstones of the Upper Cretaceous Great Valley Group was investigated by using measurements of Sm-Nd and Rb-Sr isotope ratios and major and trace element concentrations. The objective was to elucidate the erosional history of the adjacent Sierra Nevada arc and its relation to magmatism and fore-arc sedimentation. Relatively detailed provenance information is obtainable because the currently exposed plutonic rocks of the arc (and, by inference, the Cretaceous volcanic rocks) have large geographic gradients in isotopic composition, mainly from west to east across the arc. The relation of ϵ Nd to ϵ Sr and to major and trace elements in Great Valley sandstones is the same as in the plutonic rocks of the Sierra Nevada; this observation confirms that the arc was the primary source of sediment and that the chemistry of the detritus was not significantly disturbed by sedimentary processes. With decreasing ϵ Nd , there is an increase in K 2 O, Rb, Pb, Ba, La, and U, and MgO, Ni, and Cr decrease. The ϵ Nd of sandstones decreases with decreasing depositional age from -0.7 to -5.0 in 97 to 73 Ma strata. The ϵ Nd and age of the sandstones closely match those of the plutonic rocks of the Sierra Nevada. This observation indicates that the location of the mean sandstone source and the active volcanic front were nearly coincident in time and space; thus, the isotopic compositions of the sandstones directly record the temporal and geographic variations in arc magmatism. The volcanic "roof" of the Sierra Nevada arc was evidently chemically and isotopically equivalent to its plutonic roots. Prearc metasedimentary rocks did not significantly contribute to the isotopic or geochemical composition of the fore-arc sandstones.

Authigenic sanidine occurs in almost monomineralic beds in the basal St. Peter Sandstone near Waukau, Wisconsin. The high purity of this K-feldspar indicates that it formed at low temperature either contemporaneously with deposition or during later diagenesis. K-Ca analyses of two samples from the same bed yield discordant ages of 436 +/- 4 and 407 +/- 3 Ma. Because /sup 40/Ca loss is unlikely at the low temperatures to which these minerals have been subjected, these ages are interpreted as the times of formation of the sanidine. Both ages are younger than the estimated stratigraphic age of the St. Peter Sandstone. K-Ar analyses of splits of these same two samples give ages of 402 and 377 Ma, respectively. These ages are lower by about 8% than the K-Ca ages because of the diffusive loss of radiogenic argon over geologic time. An estimate of the diffusion coefficient for argon in the sanidine on the basis of the differences between the K-Ca and K-Ar ages suggests that diffusion of argon is about five orders of magnitude faster at low temperatures than an extrapolation of high-temperature experimental data would indicate. Comparison of K-Ca and K-Ar ages may be useful in general for distinguishingmore » diffusive loss from recrystallization events in feldspars.« less

Several samples of unaltered volcanic biotite were recovered from five biostratigraphically and magnetostratigaphically controlled bentonite horizons in the continuous upper Eocene–Oligocene pelagic carbonate sequence of Gubbio, northern Italy. K/Ar and Rb/Sr dates on these samples offer a unique opportunity to calibrate with closely spaced points the late Paleogene biostratigraphic and magnetostratigraphic time scales. Until now, time scales have been constructed by interpolating between less certain and more widely spaced data points. Our combined K/Ar and Rb/Sr dates are here used to calibrate the following five stratigraphic levels, from top to bottom: Level 5—upper part of Globorotalia opima opima planktonic foraminiferal zone, upper part of indistinct NP23-NP24 nannoplankton zone and CP18 nannoplankton zone, bottom of magnetic anomaly 9N: 28.0 ± 0.3 Ma. Level 4— Globigerina ampliapertura and Cassigerinella chipolensis zone, lower part of NP23 zone and bottom of CP18 zone, upper part of anomaly 12R: 32.0 ± 0.8 Ma. Level 3— Pseudohastigerina micra zone, upper part of NP21 and CP16b zones, top of anomaly 13N: 35.4 ± 0.4 Ma. Level 2—upper part of Globigerinatheka semiinvoluta zone, upper part of NP18 and CP15a zones, top of magnetic anomaly 16N: 36.0 ± 0.4 Ma. Level 1—lower part of G. semiinvoluta zone, between zones NP18 and NP19, middle part of zone CP15a, upper part of anomaly 17N: 36.4 ± 0.3 Ma. From these data, the interpolated age for the much-debated Eocene-Oligocene boundary is 35.7 Ma with a 2-standard-deviation uncertainty of 0.4 Ma.