Characterisation of primary and secondary carbonates in hypabyssal kimberlites: an integrated compositional and Sr-isotopic approach     

Castillo-Oliver  Montgarri  Giuliani  Andrea  Griffin  William L.  O&#;Reilly  Suzanne Y. 《Mineralogy and Petrology》2018,112(2):555-567

Carbonates in fresh hypabyssal kimberlites worldwide have been studied to understand their origin [i.e. primary magmatic (high T) versus deuteric (‘low T’) versus hydrothermal/alteration (‘low T’)] and identify optimal strategies for petrogenetic studies of kimberlitic carbonates. The approach presented here integrates detailed textural characterisation, cathodoluminescence (CL) imaging, in situ major- and trace-element analysis, as well as in situ Sr-isotope analysis. The results reveal a wide textural diversity. Calcite occurs as fine-grained groundmass, larger laths, segregations, veins or as a late crystallising phase, replacing olivine or early carbonates. Different generations of carbonates commonly coexist in the same kimberlite, each one defined by a characteristic texture, CL response and composition (e.g., variable Sr and Ba concentrations). In situ Sr isotope analysis revealed a magmatic signature for most of the carbonates, based on comparable ⁸⁷Sr/⁸⁶Sr values between these carbonates and the coexisting perovskite, a robust magmatic phase. However, this study also shows that in situ Sr isotope analysis not always allow distinction between primary (i.e., magmatic) and texturally secondary carbonates within the same sample. Carbonates with a clear secondary origin (e.g., late-stage veins) occasionally show the same moderately depleted ⁸⁷Sr/⁸⁶Sr ratios of primary carbonates and coexisting perovskite (e.g., calcite laths-shaped crystals with ⁸⁷Sr/⁸⁶Sr values identical within uncertainty to those of vein calcite in the De Beers kimberlite). This complexity emphasises the necessity of integrating detailed petrography, geochemical and in situ Sr isotopic analyses for an accurate interpretation of carbonate petrogenesis in kimberlites. Therefore, the complex petrogenesis of carbonates demonstrated here not only highlights the compositional variability of kimberlites, but also raises concerns about the use of bulk-carbonate C-O isotope studies to characterise the parental melt compositions. Conversely, our integrated textural and in situ study successfully identifies the most appropriate (i.e. primary) carbonates for providing constraints on the isotopic parameters of parental kimberlite magmas.

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Tellurium,selenium and cobalt enrichment in Neoproterozoic black shales,Gwna Group,UK: Deep marine trace element enrichment during the Second Great Oxygenation Event          下载免费PDF全文

Joseph G. T. Armstrong  John Parnell  Liam A. Bullock  Magali Perez  Adrian J. Boyce  Jorg Feldmann 《地学学报》2018,30(3):244-253

Black shales of the late Neoproterozoic Gwna Group (570–580 Ma), UK, contain enrichments of tellurium (Te), selenium (Se) and cobalt (Co) relative to average shale compositions. The Te and Co enrichments bear comparison with those of ferromanganese crusts in the modern deep ocean. Gwna Group deposition coincides with the Second Great Oxidation Event, which had a significant effect on trace element fixation globally. Selenium and Te concentrations within these black shales indicate increased continental weathering rates, high biological productivity and corresponding increases in atmospheric O₂ concentrations. Cobalt, nickel (Ni) and arsenic (As) enrichments in this succession are secondary mineralisation phases. Demand for many of the trace elements found enriched in the Gwna Group black shales make their mechanisms of accumulation, and variations through the geological record, important to understand, and suggests that new resources may be sought based on black shale protoliths from this period.  相似文献   

Assessing the performance of WRF model in predicting high-impact weather conditions over Central and Western Africa: an ensemble-based approach     

Igri  P. Moudi  Tanessong  Roméo S.  Vondou  D. A.  Panda  Jagabandhu  Garba  Adamou  Mkankam  F. Kamga  Kamga  A. 《Natural Hazards》2018,93(3):1565-1587

Natural Hazards - For numerical weather prediction over a particular region, it is important to know the best combination of physical parameterizations available in the considered modelling frame...  相似文献   

Ziminaite,Fe3+VO4, a new howardevansite-group mineral from the Bezymyannyi volcano,Kamchatka, Russia     

Pekov  Igor V.  Siidra  Oleg I.  Yapaskurt  Vasiliy O.  Polekhovsky  Yury S.  Kartashov  Pavel M. 《Mineralogy and Petrology》2018,112(3):371-379

Mineralogy and Petrology - The new mineral ziminaite, ideally Fe3+VO4, was found in fumarole sublimates at the Bezymyannyi volcano, Kamchatka, Russia. Ziminaite occurs as lamellar, tabular or...  相似文献   

Early growth fault activity: new evidence for extension in the upper Cambrian Owen Group,Proprietary Peak,western Tasmania,Australia     

M. J. O'Mara  S. A. Mahmud  M. Hall 《Australian Journal of Earth Sciences》2018,65(2):209-220

The coarse-grained, upper Cambrian Owen Group of western and northern Tasmania is a prominent feature of the Tasmanian landscape and regional map series. The group has previously been divided into four informal formations (Lower Owen Conglomerate, Middle Owen Sandstone, Middle Owen Conglomerate and Upper Owen Sandstone) that have been correlated across the state over tens to hundreds of kilometres. The deposition of these sediments is largely believed to have occurred during extensional tectonics, but some authors continue to argue a compressional tectonic regime. Detailed mapping and sedimentological work around Proprietary Peak on the Mount Jukes massif, 10 km south of Queenstown, Tasmania, has identified significant depositional variations controlled by early growth faulting and paleotopography. Discontinuity of stratigraphic units (L6–L13) across two growth faults on the north face of Proprietary Peak shows the strong effect on sediment deposition in the area. Paleotopography is also evident with most stratigraphic units (L8–L13 and U1) gradually onlapping basement during their deposition. Significant paleotopography has also been identified on East Jukes Peak, where lower Owen Group sedimentary units onlap basement volcanics, with no evidence for tectonically controlled deposition. Field evidence strongly supports the deposition of the Owen Group during extensional tectonics, after a period of prolonged erosion of the underlying Mount Read Volcanics. The distinct variation in vertical and lateral extent of stratigraphic units within the Owen Group in the Proprietary Peak area suggests that widespread lithostratigraphic correlation of older Owen Group sedimentary units across Tasmania may not be feasible.  相似文献   

Paleo-environment and reservoir evolution of the Middle Ordovician Kelimoli Formation in the northern Tianhuan area,Ordos Basin,China: implications for high-quality reservoir development     

S. W. Mao  J. M. Li  W. Liu  Z. C. Wang  Z. L. Huang 《Australian Journal of Earth Sciences》2018,65(4):535-555

The Middle Ordovician Kelimoli Formation carbonate reservoirs in the northern Tianhuan area, Ordos Basin, China are main exploration targets. Subsurface core samples, logging, drilling and production data from the area were used to characterise the carbonate reservoir and to construct a genetic model for exploration. The sedimentary facies identified include trough–continental rise, upper–lower slope, platform margin reef-shoal, open platform and evaporation-restricted platform. The slope and platform margin facies are potential zones for high-quality reservoirs. Porosity in the study area comprises intergrain, intercrystal, intragrain and intracrystal pores, fractures and vughs. The Sr/Ba (0.40–4.87) and V/(V + Ni) (0.64–0.97) ratios indicate deposition in a brackish water-dominated environment under reducing conditions, associated with sea-level fluctuations during the deposition of the Kelimoli Formation. The Sr/Ba, V/(V + Ni), ⁸⁷Sr/⁸⁶Sr ratios, δ¹⁸O values and crystal texture of dolomite samples suggest that meteoric water was involved in the diagenetic fluid in the near-surface depositional environment. Isotopes (δ¹³C and δ¹⁸O) and trace elements (Fe and Mn) allowed the identification of seepage-reflux and mixed-water dolomitisation. The crystal textures of the samples consist of micritic, bioclastic and reefal limestones, and dolomite with gypsum, which were easily dissolved during the early diagenetic stage. The epidiagenetic stage was the key period for the development of high-quality reservoirs because of large-scale bedding-parallel karstification from meteoric water. The reservoir zones, dominated by partially filled and unfilled vughs, and fractures, are favourable exploration targets in the northern Tianhuan area.  相似文献   

Tectonothermal events in the Olympic IOCG Province constrained by apatite and REE-phosphate geochronology     

A. R. Cherry  V. S. Kamenetsky  J. McPhie  J. M. Thompson  K. Ehrig  S. Meffre 《Australian Journal of Earth Sciences》2018,65(5):643-659

The Olympic iron oxide–copper–gold province in South Australia contains numerous deposits and prospects, including the Olympic Dam Cu–U–Au–Ag deposit and the Acropolis prospect. The Acropolis prospect comprises massive, coarse-grained magnetite–apatite veins partly replaced by a hematite-stable assemblage. The apatite grains in the veins contain zones with abundant inclusions of other minerals (including monazite and xenotime) and low trace-element concentrations relative to the inclusion-free zones. The inclusion-rich apatite zones are interpreted to be formed from the recrystallisation of the inclusion-free apatite and remobilisation of U, Th and rare earth element (REE) from apatite into monazite and xenotime. Apatite, monazite and xenotime are all established U–Th–Pb geochronometers and offer the potential to constrain the alteration history of the Acropolis prospect. The LA-ICPMS U–Pb age of inclusion-free apatite is within error of the age of the host volcanic units (ca 1.59 Ga). Inclusion-rich apatite yields both near-concordant analyses that are within error of the inclusion-free apatite as well as highly disturbed (discordant) analyses. The most concordant analyses of monazite (Th–Pb) inclusions and xenotime (U–Pb) inclusions and rim grains indicate an alteration event occurred at ca 1.37 Ga and possibly also at ca 500 Ma. The disparity in age of the inclusion-rich apatite and the REE-phosphate inclusions (and rim grains) is suggested to be owing to the apatite being initially recrystallised at ca 1.59 Ga and modified again by a later event that also formed (or coarsened) most of the inclusions. Partial resetting of the majority of the monazite inclusions as well as the presence of significant amounts of common Pb has complicated the interpretation of the monazite results. In contrast, xenotime is a more robust geochronometer in this setting. The ages of the two post-1.59 Ga events that appear to have affected the Acropolis prospect do not correspond to any events known to have occurred in the Gawler Craton. The earlier (ca 1.37 Ga) age instead corresponds best with metamorphic–magmatic–hydrothermal activity in Laurentia, consistent with the proximity of Laurentia and the Gawler Craton inferred from palaeogeographic reconstructions. The later (ca 500 Ma) event corresponds to the Delamerian Orogeny and has been shown by prior studies to have also affected the Olympic Dam deposit.  相似文献   

Petrophysical characterisation of tight sandstone gas reservoirs using nuclear magnetic resonance: a case study of the upper Paleozoic strata in the Kangning area,eastern margin of the Ordos Basin,China     

Y. Y. Huyan  T. S. Liu  F. J. Jiang  X. Z. Chen  X. Q. Ma 《Australian Journal of Earth Sciences》2018,65(6):863-875

Residual and movable porosity are significant parameters for characterising petrophysical properties, especially in tight reservoirs. Eight tight sandstone samples from the upper Paleozoic gas-bearing strata in the Kangning area, from the eastern margin of the Ordos Basin, were analysed using nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), petrography, and porosity and permeability tests. The lithology and pore types were identified and classified using petrography and SEM. The residual and movable porosity were obtained with NMR. In addition, NMR was used to visualise pore structure and pore size distribution. The results suggest that the upper Paleozoic sandstones in the study area mainly comprise feldspathic litharenite and litharenite. The sandstone porosity and permeability are low, with means of 5.9% and 0.549 mD, respectively. Four pore types exist in the tight sandstones: residual primary pores, grain dissolution pores, micropores (clay-dominated) and microfractures. The T₂ spectra under water-saturated conditions correlate with pore size and can be used to distinguish small and large pores based on the transverse relaxation time cutoff value of 10 ms, which corresponds to a pore diameter of 0.232 μm. Small pores account for 72% of the pores in the tight sandstones. The continuous bimodal T₂ spectra suggest good connectivity between small and large pores, despite the low porosity and permeability. In this study, the movable porosity of the major tight sandstone gas reservoirs is higher than the residual porosity, which confirms the effective evaluation of movable porosity to tight sandstone reservoirs, based on NMR experiments.  相似文献   

Empirical assessment of the role of geospatial technology in delivering governance and strengthening democracy in Nigeria     

Ganiy I. Agbaje  Innocent E. Bello  Adebayo Gbenga Ojo 《GeoJournal》2018,83(4):743-756

The paper examined the concept of development in line with the role of geospatial technology applications in governance and geospatial management in Nigeria. It also evaluates the previous efforts made by successive governments in managing governance process through geospatial technology initiatives using Lagos State as a proof-of-concept. It empirically demonstrates the role of monitoring spatial development initiatives using Landsat multi-dates satellite images 1985, 1990, 1995, 2000, 5005, 2010 and 2016 and Geographic Information System technique. The paper then identified, mapped and examined the settlement growth of Lagos using Ikorodu Division as an empirical proof-of-concept. The study reveals a geometric increase in uncoordinated human activities resulting from sporadic spatial development (builtup area) from 27.67 sq km (7.7%) in 1985–313.02 sq km (87.3%) spatial coverage in 2016. Based on the regression line, the study reveals a strong correlation between increasing settlement development and time. Low level of community involvement and lack of sense of ownership of development project initiatives in developing a local “vision” and strategy are identified as a problem. The paper re-emphasized that sustaining democratic processes as well as the fight against poverty will be won or lost mostly in the rural and less in the urban areas. The rural area is home to about 70% of the world’s poor due to lack or inadequate government services and basic amenities. The study suggest that making the future better for the yet unborn requires appreciable fundamental dataset, technologies, human resources and reliable government structures.  相似文献   

What does the occurrence of Sporormiella (Preussia) spores mean in Australian fossil sequences?          下载免费PDF全文

John Dodson  Judith H. Field 《第四纪科学杂志》2018,33(4):380-392

Understanding the loss of the final few species of Australian megafauna is beset by a paucity of data on human arrival, well‐provenanced megafauna, human/megafauna population range and distribution (coexistence and interaction), and the range, scale and impact of environmental changes spanning the human–megafauna period. To overcome these shortcomings, the occurrence and decline of coprophilous fungal spores of Sporormiella in sediments have been used as a proxy for extinct megaherbivores. The Sporormiella evidence is presented as the key indicator of extinction timing and these reports are often from locations where there is no known archaeological record or megafauna remains. However, interpreting fungal spore occurrence is not straightforward, as demonstrated by studies investigating taphonomy, taxonomy and the types of animal dung where Sporormiella occurs. No detailed studies on these problems exist for Australia and no evidence supporting the use of Sporormiella as a valid proxy has been reported. Here we examine the occurrence of Sporormiella spores from Cuddie Springs in south‐eastern Australia. Despite a well‐preserved suite of megafauna fossils, Sporormiella occurrence is sporadic and frequencies are low. We conclude that using Sporormiella alone as an indicator for the presence of megafauna is premature for the Australian context.  相似文献