Jaraula C.M.B.,Curtin University Australia |
Schwark L.,Curtin University Australia |
Schwark L.,University of Kiel |
Moreau X.,Energy and Minerals Australia Ltd |
And 3 more authors.
Applied Geochemistry | Year: 2015
We investigated the effect of ionizing radiation on organic matter (OM) in the carbonaceous uranium (U) mineralization at the Mulga Rock deposit, Western Australia. Samples were collected from mineralized layers between 53 and 58.5m depths in the Ambassador prospect, containing <5300ppm U. Uranium bears a close spatial relationship with OM, mostly finely interspersed in the attrinite matrix and via enrichments within liptinitic phytoclasts (mainly sporinite and liptodetrinite). Geochemical analyses were conducted to: (i) identify the natural sources of molecular markers, (ii) recognize relationships between molecular markers and U concentrations and (iii) detect radiolysis effects on molecular marker distributions. Carbon to nitrogen ratios between 82 and 153, and Rock-Eval pyrolysis yields of 316-577mg hydrocarbon/g TOC (HI) and 70-102mg CO2/g TOC (OI) indicate a predominantly lipid-rich terrigenous plant OM source deposited in a complex shallow swampy wetland or lacustrine environment. Saturated hydrocarbon and ketone fractions reveal molecular distributions co-varying with U concentration. In samples with <1700ppm U concentrations, long-chain n-alkanes and alkanones (C27-C31) reveal an odd/even carbon preference indicative of extant lipids. Samples with ≥1700ppm concentrations contain intermediate-length n-alkanes and alkanones, bearing a keto-group in position 2-10, with no carbon number preference. Such changes in molecular distributions are inconsistent with diagenetic degradation of terrigenous OM in oxic depositional environments and cannot be associated with thermal breakdown due to the relatively low thermal maturity of the deposits (Rr=0.26%). It is assumed that the intimate spatial association of high U concentrations resulted in breakdown via radiolytic cracking of recalcitrant polyaliphatic macromolecules (spores, pollen, cuticles, or algal cysts) yielding medium chain length n-alkanes (C13-C24). Reactions of n-alkenes with OH- radicals from water hydrolysis produced alcohols that dehydrogenated to alkanones or through carbonylation formed alkanones. Rapid reactions with hydroxyl radicals likely decreased the isomerization of n-alkenes and decreased alkanone diversity, such that the alkan-2-one isomer is predominant. This specific distribution of components generated by natural radiolysis enables their application as "radiolytic molecular markers". Breaking of C-C bonds through radiolytic cracking at temperatures much lower than the oil window (<50°C) can have profound implications on initiation of petroleum formation, paleoenvironmental reconstructions, mineral exploration and in tracking radiolysis of OM. © 2014 Elsevier Ltd.
Geiger H.,Petrology Ltd. |
Mattsson T.,Petrology Ltd. |
Deegan F.M.,TectonicsUppsala UniversityUppsala Sweden |
Troll V.R.,Petrology Ltd. |
And 2 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2016
The 2014-2015 Holuhraun eruption on Iceland was located within the Askja fissure swarm but was accompanied by caldera subsidence in the Bárdarbunga central volcano 45 km to the southwest. Geophysical monitoring of the eruption identified a seismic swarm that migrated from Bárdarbunga to the Holuhraun eruption site over the course of two weeks. In order to better understand this lateral connection between Bárdarbunga and Holuhraun, we present mineral textures and compositions, mineral-melt-equilibrium calculations, whole rock and trace element data, and oxygen isotope ratios for selected Holuhraun samples. The Holuhraun lavas are compositionally similar to recorded historical eruptions from the Bárdarbunga volcanic system but are distinct from the historical eruption products of the nearby Askja system. Thermobarometry calculations indicate a polybaric magma plumbing system for the Holuhraun eruption, wherein clinopyroxene and plagioclase crystallized at average depths of ∼17 km and ∼5 km, respectively. Crystal resorption textures and oxygen isotope variations imply that this multilevel plumbing system facilitated magma mixing and assimilation of low-δ18O Icelandic crust prior to eruption. In conjunction with the existing geophysical evidence for lateral migration, our results support a model of initial vertical magma ascent within the Bárdarbunga plumbing system followed by lateral transport of aggregated magma batches within the upper crust to the Holuhraun eruption site. © 2016. American Geophysical Union. All Rights Reserved.
Hackley P.C.,U.S. Geological Survey |
Araujo C.V.,Petrobras |
Borrego A.G.,CSIC - National Coal Institute |
Bouzinos A.,54 Energy |
And 27 more authors.
Marine and Petroleum Geology | Year: 2015
Vitrinite reflectance generally is considered the most robust thermal maturity parameter available for application to hydrocarbon exploration and petroleum system evaluation. However, until 2011 there was no standardized methodology available to provide guidelines for vitrinite reflectance measurements in shale. Efforts to correct this deficiency resulted in publication of ASTM D7708: Standard test method for microscopical determination of the reflectance of vitrinite dispersed in sedimentary rocks. In 2012-2013, an interlaboratory exercise was conducted to establish precision limits for the D7708 measurement technique. Six samples, representing a wide variety of shale, were tested in duplicate by 28 analysts in 22 laboratories from 14 countries. Samples ranged from immature to overmature (0.31-1.53% Ro), from organic-lean to organic-rich (1-22wt.% total organic carbon), and contained Type I (lacustrine), Type II (marine), and Type III (terrestrial) kerogens. Repeatability limits (maximum difference between valid repetitive results from same operator, same conditions) ranged from 0.03 to 0.11% absolute reflectance, whereas reproducibility limits (maximum difference between valid results obtained on same test material by different operators, different laboratories) ranged from 0.12 to 0.54% absolute reflectance. Repeatability and reproducibility limits degraded consistently with increasing maturity and decreasing organic content. However, samples with terrestrial kerogens (Type III) fell off this trend, showing improved levels of reproducibility due to higher vitrinite content and improved ease of identification. Operators did not consistently meet the reporting requirements of the test method, indicating that a common reporting template is required to improve data quality. The most difficult problem encountered was the petrographic distinction of solid bitumens and low-reflecting inert macerals from vitrinite when vitrinite occurred with reflectance ranges overlapping the other components. Discussion among participants suggested this problem could not be easily corrected via kerogen concentration or solvent extraction and is related to operator training and background. No statistical difference in mean reflectance was identified between participants reporting bitumen reflectance vs. vitrinite reflectance vs. a mixture of bitumen and vitrinite reflectance values, suggesting empirical conversion schemes should be treated with caution. Analysis of reproducibility limits obtained during this exercise in comparison to reproducibility limits from historical interlaboratory exercises suggests use of a common methodology (D7708) improves interlaboratory precision. Future work will investigate opportunities to improve reproducibility in high maturity, organic-lean shale varieties. © 2014 Elsevier Ltd.
Zhang S.-G.,Central South University |
Zhang S.-G.,Petrology Ltd. |
Xu Y.,Central South University |
Yu X.-H.,Central South University
Kuangwu Yanshi/ Journal of Mineralogy and Petrology | Year: 2010
The metallogenic intensity and scale of skarn-type copper deposit in the middle segment of Nanjing-Zhenjiang is influenced by Yanshanian intermediate-acidic instrusive differentiation evolution. As the growth of plagioclase is conducted during the whole crystallization process of magma, the differentiation evolution characteristics of magmatic process can be revealed by study on the genetic mineralogical properties of plagioclase. Based on comprehensive research on geology, geophysics, geochemistry and remote sensing, combined with the application of optical microscopic observation, X-ray diffraction, microprobe analysis, the genetic mineralogical features of plagioclase phenocryst for a probation of the differentiation evolution characteristics of intrusive bodies in Anjishan, Funiushan, Xiyinkeng, Hongshuidang, Tongshan and Xiashu are studied. The results suggest that the plagioclases are dominated by oligoclase and labradorite with oscillatory zoning. The growth process of plagioclases can be divided into five stages with two growth types,the mafic core one and intermediate core one. The mildly-changing environment, especially in the later stages, tends to a higher crystallization differentiation process. The plagioclases in Xiyinkeng and Tongshan intrusive bodies, whose grades are higher than others and reverse-zoning are more developed,show obvious assimilation and contamination, and the crystallization temperature of intrusive rocks in Xiyinkeng change frequently in the whole stages, leading to the relative activity of K, Na, Ca components fluctuate with high frequency and great range. Analysis of metallogenic element(Cu,Pb, Znjcontents of whole-rock and plagioclase phenocryst of intrusive bodies indicates that Tongshan, Anjishan, Funiushan intrusive bodies, whose crystallization differentiation degree is higher reflected by crystallization temperature and relative activity of K, Na, Ca components with mildly-changing, are the main target to prospect Cu-polymetallic deposits.