Mark D. Schmitz Copyright (c) 2011 All rights reserved. http://works.bepress.com/mark_schmitz Recent documents in Mark D. Schmitz en-us Sat, 17 Sep 2011 01:30:21 PDT 3600 Neoarchean Paleoweathering of Tonalite and Metabasalt: Implications for Reconstructions of 2.69 Ga Early Terrestrial Ecosystems and Paleoatmospheric Chemistry http://works.bepress.com/mark_schmitz/9 http://works.bepress.com/mark_schmitz/9 Thu, 15 Sep 2011 10:29:29 PDT Field and laboratory investigations of a 2690.83 Ma (207Pb/206Pb age of Saganaga Tonalite) unconformity exposed in outcrop in northeastern Minnesota, USA, reveal evidence for development of a deep paleoweathering profile with geochemical biosignatures consistent with the presence of microbial communities and weakly oxygenated conditions. Weathering profiles are characterized by a 5–50 m thick regolith that consists of saprolitized Saganaga Tonalite and Paulson Lake succession basaltic metavolcanic rocks retaining rock structure, which is cross-cut by a major unconformity surface marking development of a successor basin infilled with alluvial deposits. The regolith and unconformity are overlain by thick conglomerate deposits that contain both intrabasinal (saprock) as well as extrabasinal detritus. Thin-section microscopy and electron microprobe analyses reveal extensive hydrolysis and sericitization of feldspars, exfoliation and chloritization of biotite, and weathering of Fe-Mg silicates and Cu-Fe sulfides; weathering of Fe-Ti oxides was relatively less intense than for other minerals and evidence was found for precipitation of Fe oxides. Geochemical analyses of the tonalite, assuming immobile TiO2 during weathering (τTi,j), show depletion of SiO2, Al2O3, Na2O, CaO, MgO, and MnO, and to a lesser degree of K2O, relative to least-weathered parent materials. Significant Fe was lost from the tonalite. A paleoatmospheric pCO2 of 10–50 times PAL is estimated based on geochemical mass-balance of the tonalite profile and assuming a formation time of 50–500 Kyr. Interpretations of metabasalt paleoweathering are complicated by additions of sediment to the profile and extensive diagenetic carbonate (dolomite) overprinting. Patterns of release of P and Fe and retention of Y and Cu in tonalite are consistent with recent laboratory experiments of granite weathering, and with the presence of acidic conditions in the presence of organic ligands (produced, for example, by a primitive microbial community) during weathering. Cu metal in the profile may document lower pO2 than present day at the surface. Comparison with previous studies of weathered tonalite and basalt (Denison, 2.45–2.22 Ga) in Ontario, Canada, reveal general similarities in paleoweathering with our study, as well as important differences related to lower paleoatmospheric pO2 and terrestrial biosignature for the older Minnesota profile. A falling water table in the Alpine Lake locality is presumed to have promoted formation of this gossan-like deep-weathering system that extends to 50-m depth.

]]> Steven G. Driese et al. Mesozoic Magmatism and Deformation in the Northern Owyhee Mountains, Idaho: Implications for Along-Zone Variations for the Western Idaho Shear Zone http://works.bepress.com/mark_schmitz/8 http://works.bepress.com/mark_schmitz/8 Thu, 15 Sep 2011 10:29:28 PDT The northern Owyhee Mountains of southwestern Idaho contain granitoid rocks that are the same age as the Cretaceous western border zone of the Idaho batholith to the north of the Snake River Plain. They contain a well-developed and consistently oriented 020° foliation, zircon yielding U-Pb dates of ca. 160–48 Ma, and initial 87Sr/86Sr isotopic compositions that show a steep west-to-east transition in values from 0.704 to 0.708 over a distance of ∼30 km. The rocks of the northern Owyhee Mountains are interpreted to be the southward continuation (Owyhee segment) of the western Idaho shear zone. Similar to a well-studied section of the western Idaho shear zone by McCall (McCall segment), the Owyhee segment displays steep foliation and lineation orientations, deformation of 98–90 Ma plutons, steep Sr isotopic gradients, and syntectonic tonalite intrusions. However, the Owyhee segment has three major differences from the McCall segment: (1) significantly less well-developed solid-state strain fabric foliations; (2) trend of 020° rather than 000°; and (3) a wider transition zone in initial Sr ratios from 0.704 to 0.708. We present a simple tectonic model to explain these differences, assuming a 20° along-zone difference in the initial orientation of the western margin of the Laurentia, a rigid-body collision, homogeneous material behavior, and transpressional kinematics. For the Owyhee segment, the model predicts a lower oblique-convergence angle, less convergent displacement, more dextral transcurrent displacement, and an overall lower finite strain relative to the McCall segment.

]]> B. Benford et al. Ecological Persistence in the Late Mississippian (Serpukhovian, Namurian A) Megafloral Record of the Upper Silesian Basin, Czech Republic http://works.bepress.com/mark_schmitz/7 http://works.bepress.com/mark_schmitz/7 Fri, 10 Jun 2011 10:50:38 PDT The Serpukhovian (Namurian A) stratigraphy of the Ostrava Formation, Upper Silesian Coal Basin, Czech Republic, consists of coal-bearing paralic sediments underlain by marine deposits in a cyclothemic nature similar to those in the Pennsylvanian of Euramerica. The thickness of the formation exceeds 3000 m, in which >170 coals are identified in a foreland basin setting. Fifty-five genetic cycles are identified in the present study, using transgressional erosional surfaces as lower and upper boundaries. Terrestrial plant-macrofossil assemblages are preserved within each cycle, mostly associated with coals, and these represent a sampling of the coastal plain vegetation. New high-precision isotope dilution–thermal ionization mass spectrometry U-Pb ages on zircons from tonsteins of two coals provide chronometric constraints for the Serpukhovian. Unweighted Pair Group Method with Arithmetic Mean clustering and Bayesian statistical classification group macrofloral assemblages into four distinct stratigraphic clusters, with assemblages persisting for <18 cycles>before compositional change. Cycle duration, based on Ludmila (328.84±0.16 Ma) and Karel (328.01±0.08 Ma) tonsteins, overlaps the short-period (100 kyr) eccentricity cycle at the 95% confidence interval. These dates push the beginning of the Serpukhovian several million years deeper in time. An estimate for the Visean-Serpukhovian boundary is proposed at 330 Ma. Late Mississippian wetland ecosystems persisted for >1.8 million years before regional perturbation, extirpation, or extinction of taxa occurred. Significant changes in the composition of macrofloral clusters occur across major marine intervals. These results accord with other estimates of Carboniferous tropical wetland community persistence. Hence, vegetational persistence was characteristic of peat-accumulating and mineral-substrate wetland ecosystems beginning in the Late Mississippian, when there is evidence for the first appearance of eustatically controlled cyclothems during the buildup of Gondwanan glaciation.

]]> Robert A. Gastaldo et al. High-Precision U-Pb Calibration of Carboniferous Glaciation and Climate History, Paganzo Group, NW Argentina http://works.bepress.com/mark_schmitz/6 http://works.bepress.com/mark_schmitz/6 Mon, 23 May 2011 11:06:20 PDT

The duration and geographic extent of Carboniferous glacial events in southern Gondwana remain poorly constrained despite recent evidence for a more dynamic glacial history than previously considered. We report 10 high-precision (2σ ± <0.1%) U-Pb ages for the Permian-Carboniferous Paganzo Group, NW Argentina, that redefine the chronostratigraphy of the late Paleozoic Paganzo and Río Blanco Basins, and significantly refine the timing of glacial events and climate shifts in the western region of southern Gondwana. Radiometric calibration of the Paganzo Group indicates three pulses of Carboniferous glaciation in the mid-Visean, the late Serpukhovian to earliest Bashkirian, and between the latest Bashkirian to early Moscovian.

An abrupt shift in depositional style from high-sinuosity single-storied fluvial deposits and clay-rich paleosols to low-sinuosity multistoried feldspathic fluvial deposits intercalated with eolianites and calcic paleosols is constrained to the latest Moscovian and earliest Kasimovian. These constraints indicate a relatively abrupt climate shift from humid-subhumid to nonseasonal semiarid regional climate conditions that occurred significantly earlier than previously inferred (Early Permian). This period of high-latitude aridity was contemporaneous with a shift to dryland depositional environments and a major vegetation regime shift documented throughout the Pangean paleotropics in the Pennsylvanian.

]]> E. L. Gulbranson et al. Sm–Nd Isotope Systematics by Laser Ablation-Multicollector-Inductively Coupled Plasma Mass Spectrometry: Methods and Potential Natural and Synthetic Reference Materials http://works.bepress.com/mark_schmitz/5 http://works.bepress.com/mark_schmitz/5 Tue, 10 May 2011 12:36:01 PDT An improved method for precise and accurate in situ determination of Sm–Nd isotopes, with high spatial resolution, and high sample throughput, in light rare earth element enriched accessory minerals by laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) is discussed. The accuracy of the method is demonstrated by comparison of LA-MC-ICPMS analyses of several natural minerals (Durango apatite, Hondo Canyon titanite, Daibosatsu allanite, Mae Klang monazite, and Trebilcock monazite) to isotope dilution-thermal ionization mass spectrometry (ID-TIMS) analyses of the same minerals. The TIMS analyses demonstrate that each of these minerals, with the possible exception of Durango apatite, is more homogeneous in Nd isotopic composition than can be measured based upon the internal precision of the LA-MC-ICPMS analyses in this study. Thus, these natural minerals may be considered as isotopically homogeneous reference materials for Nd isotopic determinations. In addition two synthetic glasses produced specifically for in situ Sm–Nd isotopic determination and external calibration are presented here.

The three main obstacles in obtaining accurate and precise Sm–Nd isotopic measurements by LA-MC-ICPMS are the isobaric interference correction of 144Sm on 144Nd, determination of the Sm mass bias, and accurately measuring the 147Sm/144Nd which is imperative for producing robust initial Nd isotopic compositions. The 144Sm interference correction was calculated using the measured 149Sm and recently published Sm isotopic abundances. The Sm mass bias was determined using an exponential law and the 147Sm/149Sm measured in the sample. Determination of 147Sm/144Nd was done by calibration to an external glass reference material synthesized specifically for this purpose. This approach produced 147Sm/144Nd that is in agreement with ID-TIMS analysis. The MC-ICPMS instrument configuration used in this study also allows for the determination of Eu and Gd, thereby permitting simultaneous determination of Eu anomalies along with Sm–Nd isotopic composition. The high spatial resolution and sample throughput at a geologically useful level of accuracy and precision possible with LA-MC-ICPMS offers numerous potential geochemical applications including provenance tracing, terrane reconstruction, crustal growth studies, fluid composition and evolution, metamorphic reaction pathways, and silicic magma generation and evolution.

]]> Christopher M. Fisher et al. Calibrating the Cryogenian http://works.bepress.com/mark_schmitz/4 http://works.bepress.com/mark_schmitz/4 Thu, 11 Nov 2010 08:59:33 PST The Neoproterozoic was an era of great environmental and biological change, but a paucity of direct and precise age constraints on strata from this time has prevented the complete integration of these records. We present four high-precision U-Pb ages for Neoproterozoic rocks in northwestern Canada that constrain large perturbations in the carbon cycle, a major diversification and depletion in the microfossil record, and the onset of the Sturtian glaciation. A volcanic tuff interbedded with Sturtian glacial deposits, dated at 716.5 million years ago, is synchronous with the age of the Franklin large igneous province and paleomagnetic poles that pin Laurentia to an equatorial position. Ice was therefore grounded below sea level at very low paleolatitudes, which implies that the Sturtian glaciation was global in extent.

]]> Francis A. Macdonald et al. High-precision U-Pb Zircon Age Calibration of the Global Carboniferous Time Scale and Milankovitch Band Cyclicity in the Donets Basin, Eastern Ukraine http://works.bepress.com/mark_schmitz/3 http://works.bepress.com/mark_schmitz/3 Wed, 27 Oct 2010 12:56:03 PDT High-precision ID-TIMS U-Pb zircon ages for 12 interstratified tuffs and tonsteins are used to radiometrically calibrate the detailed lithostratigraphic, cyclostratigraphic, and biostratigraphic framework of the Carboniferous Donets Basin of eastern Europe. Chemical abrasion of zircons, use of the internationally calibrated EARTHTIME mixed U-Pb isotope dilution tracer, and improved mass spectrometry guided by detailed error analysis have resulted in an age resolution of <0.05%, or ∼100 ka, for these Carboniferous volcanics. This precision allows the resolution of time in the Milankovitch band and confirms the long-standing hypothesis that individual high-frequency Pennsylvanian cyclothems and bundles of cyclothems into fourth-order sequences are the eustatic response to orbital eccentricity (∼100 and 400 ka) forcing. Tuning of the fourth-order sequences in the Donets Basin to the long-period eccentricity cycle results in a continuous age model for the Middle to Late Pennsylvanian (Moscovian-Kasimovian-Ghzelian) strata of the basin and their record of biological and climatic changes through the latter portion of the late Paleozoic Ice Age. Detailed fusulinid and conodont zonations allow the export of this age model to sections throughout Euramerica. Additional ages for Mississippian strata provide among the first robust radiometric calibration points within this subperiod and result in variable lowering of the base ages of its constituent stages compared to recent global time scale compilations.

]]> Vladimir I. Davydov et al. Derivation of Isotope Ratios, Errors, and Error Correlations for U-Pb Geochronology Using 205Pb-235U-(233U)-spiked Isotope Dilution Thermal Ionization Mass Spectrometric Data http://works.bepress.com/mark_schmitz/2 http://works.bepress.com/mark_schmitz/2 Fri, 16 Jan 2009 14:31:02 PST A comprehensive treatment of the derivation of U-Pb isotope ratios and their corresponding uncertainties from isotope dilution thermal ionization mass spectrometric measurements is presented. Standard parametric statistical methods of error propagation are utilized to convolve uncertainties associated with instrumental mass fractionation, tracer subtraction, blank Pb and U subtraction, and initial common Pb correction. Derivations include errors and error correlations for total sample U/Pb and Pb isotope ratios (including radiogenic and initial common Pb) for two- and three-dimensional isochron calculations, radiogenic U/Pb and Pb isotope ratios for concordia and radiogenic model age calculations, and the propagation of model age errors from radiogenic isotope ratios.

]]> Mark D. Schmitz