Textural and geochemical discrimination between xenotime of different origin in the Archaean Witwatersrand Basin, South Africa   总被引：1，自引：0，他引：1  

Natalie Kositcin  Neal J. McNaughton  Ian R. Fletcher  Birger Rasmussen 《Geochimica et cosmochimica acta》2003,67(4):709-731

Xenotime (YPO₄) of detrital, diagenetic, and hydrothermal origin within siliciclastic rocks of the Archaean Witwatersrand Basin, South Africa, has been identified on the basis of petrography and in situ ion microprobe (SHRIMP) age data. The chemical composition of xenotime, determined by in situ electron microprobe analysis, can be correlated with its origin. This allows the origin of any xenotime grain to be assessed by a non-destructive microanalytical method prior to ion microprobe geochronology. The main chemical discriminators are MREE-HREE abundance, normalised HREE slope and Eu anomaly, and, in some cases, U and Th contents. Igneous-detrital xenotime (> 2800 Ma) is distinguished from diagenetic (∼2780 Ma) and hydrothermal (< 2780 Ma) xenotime in having lower Eu, Dy, and Gd concentrations and a distinctively lower Gd/Yb ratio. Hydrothermal xenotime has distinctively lower U and Th concentrations when compared to igneous-detrital and diagenetic xenotime. Three separate hydrothermal fluid events and episodes of post-diagenetic xenotime growth are recognised in the geochemical and geochronological data, which correspond in time to the extrusion of the Ventersdorp lavas at ∼2720 Ma, the emplacement of the Bushveld Igneous Complex at ∼2061 Ma, and an event of unknown affinity at ∼2210 Ma. Although geochemical discrimination of the xenotime types from the Witwatersrand Basin, in combination with careful petrography, appears achievable, universal application of these discriminators to xenotime in other sedimentary basins remains untested.  相似文献   

Abyssal current influence on the southwest Bermuda Rise and surrounding region     

Neal W. Driscoll  Edward P. Laine 《Marine Geology》1996,130(3-4):231-263

Echograms (3.5 kHz) and bottom photographs reveal that the northward flowing Antarctic Bottom Water (AABW) has strongly influenced the modern depositional regime on the southwest Bermuda Rise. The spatial distribution of echo character types, the orientation and nature of current-controlled structures, and limited current meter data show that AABW flows with varying intensities along three primary pathways around and over the southwest Bermuda Rise. The main core of AABW flows clockwise around the eastern and western flanks of the southern Bermuda Rise, roughly parallel to the 5400 m isobath. This current bifurcates at 28°30′N, 69°W where a portion flows northeast over the southwest Bermuda Rise and the remainder continues north along the physiographic boundary between the southwest Bermuda Rise and the Hatteras Abyssal Plain. Secondary ribbons of AABW branch off the main core of AABW during its southerly journey along the southeastern Bermuda Rise, and flow west through fracture zones. Finally, a diffuse, northward flowing AABW sweeps the entire southwest Bermuda Rise.

A progression of current-controlled bedforms occurs beneath the main path of the AABW reflecting the spatially varying current velocities and sediment supply. The main core of AABW flows west through the narrow Vema Gap creating erosional furrows along the border between the southwest Bermuda Rise and the Vema Gap. Current velocities greater than 20 cm s⁻¹ are inferred from the bedforms in this region. Farther north along the southwestern edge of the Bermuda Rise, sediment waves become more prevalent. This transition from erosional to more depositional bedforms results from diminished current velocities (5–15 cm s⁻¹) and increased sediment supply. Although some of these bedforms on the southwest Bermuda Rise appear to be relict, their orientation is consistent with current meter data and abyssal current direction inferred from bottom photographs.  相似文献   

The landscape-scale relationship between lake sediment geochemistry and catchment bedrock composition from the Temagami and Gowganda areas of Northeastern Ontario,Canada     

Dale D. Sprague  Jesse C. Vermaire 《Environmental Earth Sciences》2018,77(12):463

Lake sediments are integrators of watershed wide environmental information that includes bedrock geology, glacial overburden, vegetation, hydrology and land use. Lake sediments were collected from an area covering approximately 13,600 km² from the Gowganda and Temagami regions to determine the geochemical background conditions representing different bedrock types. For the entire dataset, the median value for arsenic in lake sediment was 2.6 mg/kg, well within the prescribed limits indicating that the Canadian national objective for arsenic in sediments for the protection of aquatic life (17 mg/kg). Overall, the vast majority of lake sediment samples (97.2%) were below the 17 mg/kg Canadian objective for the protection of aquatic life, however, catchments with Nipissing Diabase had the highest background levels for arsenic (10 mg/kg) with some lakes ranging up to 30 mg/kg. The major geological controls influencing changes in lake sediment geochemistry were determined using random forest classification and principal component analysis (PCA). Random forest classification was able to identify which geological province the samples were derived from and the dominant rock types in the sample catchment with respectable accuracy. PCA revealed strong spatial relationships between lake sediment geochemistry and bedrock geology, particularly a strong relationship between Nipissing Diabase in the watershed of the lake and the cobalt-type mineralization indicator elements in the lake sediment including naturally higher levels of arsenic. Within the samples that exceeded regulatory standards, over half the samples that exceeded the regulatory objectives were located in watersheds that contained Nipissing Diabase in their catchment. This study demonstrates that unique geochemical assemblages can be associated with specific geological areas which is of interest to exploration geologists, for target generation and prospecting, and to environmental policy makers, for site-specific restoration targets.  相似文献   

Deciphering the Origin of Abiotic Organic Compounds on Earth: Review and Future Prospects     

WANG Chao  TAO Renbiao  Jesse B. WALTERS  REN Tianshi  NAN Jingbo  ZHANG Lifei 《《地质学报》英文版》2023,97(1):288-308

The geologic production of abiotic organic compounds has been the subject of increasing scientific attention due to their use in the global carbon flux balance, by chemosynthetic biological communities, and for energy resources. Extensive analysis of methane(CH₄) and other organics in diverse geologic settings, combined with thermodynamic modelings and laboratory simulations, have yielded insights into the distribution of specific abiotic organic molecules on Earth and the favorable c...  相似文献   

Hydraulic modeling of the 2011 New Madrid Floodway activation: a case study on floodway activation controls   总被引：2，自引：2，他引：0  

Adam Luke  Brad Kaplan  Jeff Neal  Jeremiah Lant  Brett Sanders  Paul Bates  Doug Alsdorf 《Natural Hazards》2015,77(3):1863-1887

  相似文献   

The timing of gold mineralization across the eastern Yilgarn craton using U–Pb geochronology of hydrothermal phosphate minerals     

Noreen Vielreicher  David Groves  Neal McNaughton  Ian Fletcher 《Mineralium Deposita》2015,50(4):391-428

  相似文献   

Enhanced bioturbation on the down‐drift flank of a Turonian asymmetrical delta: Implications for seaway circulation,river nutrients and facies models          下载免费PDF全文

Jesse T. Korus  Christopher R. Fielding 《Sedimentology》2015,62(7):1899-1922

Extant models predict delta front environments down‐drift of river mouths as unfavourable for organisms because of the physico‐chemical stresses caused by sediment and fresh water influx. This study, however, finds evidence for near‐optimal living conditions down‐drift of contemporaneous mouth bars and distributary channels, as well as at the tops of abandoned lobes, in part of the asymmetrical ‘Notom Delta’ complex of the Ferron Sandstone (Turonian, south‐eastern Utah, USA). Presented herein is a sedimentological and ichnological model using thirty‐two detailed measured sections along a 16 km transect through two continuously exposed, ca 10 m thick allomembers containing delta front, mouth bar and distributary channel facies. Azimuths from sedimentary structures show south‐eastward deflection of near‐shore palaeocurrents relative to the inferred north–south shoreline, as well as minor reversal of flow. Two end‐member trace fossil suites are recognized in delta front sandstones: (i) a stressed suite of low abundance, low diversity, diminutive traces reflecting mobile deposit feeding, resting and locomotion behaviours; and (ii) a comparatively unstressed, high abundance, moderate diversity suite with a regular, heterogeneous distribution of deep, vertical or U‐shaped suspension‐feeding burrows which, in places, thoroughly homogenize the sandstones. The down‐drift side of the delta was colonized by suspension feeders during seasonal reversal of the seaway gyre when mud plumes were swept northward or when river‐derived nutrients were sufficiently concentrated relative to fresh water and sediment input. During normal seaway circulation, very high sedimentation rates and mud‐laden, wave‐dampened waters down‐drift of the river mouths heightened the preservation potential of the pervasively bioturbated facies. Up‐drift of the river mouths, these bioturbated facies were either not preserved or not developed until the lobe was abandoned. This alternative model for delta planform asymmetry contributes to the refinement of facies models for deltaic systems and provides a framework for predicting the distribution of bioturbation‐enhanced porosity and permeability in lobe‐deflected deltas.  相似文献   

Climate-driven changes in lakes from the Peruvian Andes     

Neal Michelutti  Colin A. Cooke  William O. Hobbs  John P. Smol 《Journal of Paleolimnology》2015,54(1):153-160

  相似文献   

Sub‐Permil Interlaboratory Consistency for Solution‐Based Boron Isotope Analyses on Marine Carbonates     

Marcus Gutjahr  Louise Bordier  Eric Douville  Jesse Farmer  Gavin L. Foster  Ed C. Hathorne  Brbel Hnisch  Damien Lemarchand  Pascale Louvat  Malcolm McCulloch  Johanna Noireaux  Nicola Pallavicini  James W. B. Rae  Ilia Rodushkin  Philippe Roux  Joseph A. Stewart  Franois Thil  Chen‐Feng You 《Geostandards and Geoanalytical Research》2021,45(1):59-75

Boron isotopes in marine carbonates are increasingly used to reconstruct seawater pH and atmospheric pCO₂ through Earth’s history. While isotope ratio measurements from individual laboratories are often of high quality, it is important that records generated in different laboratories can equally be compared. Within this Boron Isotope Intercomparison Project (BIIP), we characterised the boron isotopic composition (commonly expressed in δ¹¹B) of two marine carbonates: Geological Survey of Japan carbonate reference materials JCp‐1 (coral Porites) and JCt‐1 (giant clam Tridacna gigas). Our study has three foci: (a) to assess the extent to which oxidative pre‐treatment, aimed at removing organic material from carbonate, can influence the resulting δ¹¹B; (b) to determine to what degree the chosen analytical approach may affect the resultant δ¹¹B; and (c) to provide well‐constrained consensus δ¹¹B values for JCp‐1 and JCt‐1. The resultant robust mean and associated robust standard deviation (s*) for un‐oxidised JCp‐1 is 24.36 ± 0.45‰ (2s*), compared with 24.25 ± 0.22‰ (2s*) for the same oxidised material. For un‐oxidised JCt‐1, respective compositions are 16.39 ± 0.60‰ (2s*; un‐oxidised) and 16.24 ± 0.38‰ (2s*; oxidised). The consistency between laboratories is generally better if carbonate powders were oxidatively cleaned prior to purification and measurement.  相似文献   

Imaging mantle transition zone thickness with SdS-SS finite-frequency sensitivity kernels     

Jesse F. Lawrence  Peter M. Shearer 《Geophysical Journal International》2008,174(1):143-158

We invert differential SdS-SS traveltime residuals measured from stacked waveforms and finite-frequency sensitivity kernels for topography on the 410- and 660-km discontinuities. This approach yields higher resolution images of transition zone thickness than previous stacking methods, which simply average/smooth over topographic features. Apparent structure measured using simple stacking is highly dependent upon the bin size of each stack. By inverting for discontinuity topography with a variety of bin sizes, we can more accurately calculate the true structure. The inverted transition zone model is similar to simple stack models with an average thickness of 242 km, but the lateral variations in thickness are larger in amplitude and smaller in scale. Fast seismic velocities in 3-D mantle models such as SB4L18 correlate with areas of thicker transition zone. The elongated curvilinear regions of thickened transition zone that occur near subduction zones are narrow and high amplitude, which suggests relatively little lateral spreading and warming of subducted lithosphere within the transition zone. The anomalously thin transition zone regions are laterally narrow, and not broadly continuous. If these variations in transition zone thickness are interpreted as thermal in nature, then this model suggests significant temperature variations on small lateral scales.  相似文献