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Nitrogen and Carbon Partitioning in Diagenetic and Hydrothermal Minerals from Paleozoic Black Shales, (Selwyn Basin, Yukon Territories, Canada)

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Nitrogen and Carbon Partitioning in Diagenetic and Hydrothermal Minerals from Paleozoic Black Shales, (Selwyn Basin, Yukon Territories, Canada) Chemical Geology 218 (2005) 249–264 www.elsevier.com/locate/chemgeo Nitrogen and carbon partitioning in diagenetic and hydrothermal minerals from Paleozoic Black Shales, (Selwyn Basin, Yukon Territories, Canada) Beate Orbergera,T, Jean-Paul Gallienb, Daniele L. Pintia,1, Michel Fialinc, Laurent Daudinb, Darren R. Grfcked, Jan Pasavae aDe´partement des Sciences de la Terre, CNRS-UMR 8148 IDES, Universite´ Paris Sud, Baˆt. 504, 91405 Orsay Cedex, France bLaboratoire Pierre Su¨e, CEA-CNRS UMR 9956, CEA Saclay, 91191 Gif-Sur-Yvette Cedex, France cCentre CAMPARIS, Universite´ Pierre et Marie Curie-Paris 6, 4, Place Jussieu, 75256 Paris Cedex 05, France dSchool of Geography and Geology, McMaster University, Hamilton, Ontario, Canada L8S 4K1 eCzech Geological Survey, Klarov 131/3, 11821 Praha 1, Czech Republic Received 13 December 2003; accepted 6 January 2005 Abstract Selected mineralized black shales of Devonian age from the Selwyn Basin, Northwest Territories (Canada) were analyzed by Nuclear Reaction Analyses (NRA) and electron microprobe for nitrogen and carbon in silicates, sulfides, phosphates and organic matter in order to give new insights on nitrogen and carbon fractionation processes during diagenesis and hydrothermal infiltration. Hydrothermal feldspars show tri-modal composition: albite, high nitrogen-bearing K-feldspar (~56 mol% buddingtonite (NH4AlSi3O8d 1/2H2O, hydrous ammonium-feldspar, ~51 mol% orthoclase) and hyalophane (~32 mol% celsian). Barium-rich feldspars (hyalophane) contain lowest nitrogen contents. Potassium and nitrogen are positively correlated, while nitrogen and barium are negatively correlated due to the replacement of + 2+ monovalent NH4 by divalent Ba . The Ba-rich K-feldspar rim shows penetrative textures towards an internal K–N-rich core that is interpreted as diffusive overgrowth. These feldspars are interpreted to be deposited from hot hydrothermal Ba- bearing fluids. The second important nitrogen carrier is organic matter (from 0.6 to 0.66 wt.%). Hydrothermal quartz (N=527 ppm), diagenetic biogenic F-rich apatite (conodonts: N=468 ppm,), biogenic Fe–Ni sulfides (N=380–620 ppm) and abiogenic Ni–Fe sulfides (NN440 ppm) contain homogeneously distributed nitrogen with amounts 10-fold lower than those measured in organic matter. A two-step nitrogen-release model is suggested to explain the nitrogen-partitioning in these minerals. Primary organic matter breakdown is considered to liberate nitrogen, phosphate and sulfur to pore fluids and the water column, providing nutrients for vent fauna growth. Sulfurization, due to microbial sulfate reduction, and silicification of the vent fauna releases nitrogen in a second step. Minor nitrogen was trapped as organic molecules in conodonts, while T Corresponding author. Tel.: +33 1 69 15 67 84; fax: +33 1 69 15 48 82. E-mail address: [email protected] (B. Orberger). 1 From September 1st 2004 at GEOTOP-UQAM-McGILL, P.O. Box 8888, Succ. Centre-Ville, Montre´al, QC, Canada H3C 3P8. 0009-2541/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2005.01.012 250 B. Orberger et al. / Chemical Geology 218 (2005) 249–264 the majority was transported by hydrothermal fluids and was incorporated as ammonium in feldspars substituting for potassium. D 2005 Elsevier B.V. All rights reserved. Keywords: Black shales; Hydrothermal activity; Nitrogen; Carbon; Feldspar; Sulfides; Phosphates; Nuclear reaction analysis 1. Introduction silicates included in the oxides. Recent studies of bituminous coal using X-ray Photoelectron Spectro- The geochemical behavior of nitrogen in silicate scopy (XPS) and time of flight SIMS (TOF-SIMS) rocks is not yet fully understood (Boyd, 2001a,b), and have revealed the presence of inorganic nitrogen is in part, related to difficulties in extracting and associated with clay minerals together with expected + analyzing nitrogen at nano-mole levels in silicate organic-derived NH4 in illites (Gong et al., 1997). rocks. Such analytical problems have been overcome Finally, sulfur and sulfides play an important role in the last decade by improving nitrogen-routine during organic matter breakdown suggesting their elemental and isotopic analyses through mass spec- involvement in the release and fixation of nitrogen. trometry (Hashizume and Sugiura, 1992; Boyd et al., However, few datasets are available: for example, oil 1994; Marty, 1995), Selective Ion Mass Spectrometry fields (Thompson, 1994) and Precambrian black (SIMS; Hashizume et al., 2000; Bulanova et al., 2002) shales (Imbus et al., 1992; Watanabe et al., 1997). and through quantification by micro-Fourier Trans- In this study, we provide a quantitative identifica- form Infrared Spectroscopy (FTIR; Busigny et al., tion of the mineral retention sites of nitrogen and 2003b). carbon in metal- and organic matter-rich hydrother- In sedimentary rocks, the breakdown of organic mal-infiltrated black shales from the Selwyn Basin, + matter during diagenesis produces ammonium (NH4 ) Canada (Hulbert et al., 1992). High-resolution spatial (Honma and Itihara, 1981; Itihara and Suwa, 1985), distribution of nitrogen and carbon on different which is equal in charge and similar in ionic radius to mineral phases (silicates, sulfides, phosphates and + potassium. It is usually assumed that NH4 substitutes organic matter) at the micrometric scale was obtained for K+ in minerals such as mica and feldspars (e.g., using NRA (Khodja et al., 2001). Feldspars, contain- Honma and Itihara, 1981). Ammonium is stable at ing more than 0.5 wt.% of nitrogen were systemati- high temperatures, it resists metamorphism and cally analyzed by electron microprobe and the results anatexis, and it can be found in crustal melts (Hall, were compared with NRA. The data generated in this 1999). In sedimentary rocks, nitrogen is commonly study, in comparison to other datasets, provides new found in fossil organic matter such as kerogen insights on the partitioning of nitrogen and carbon (Beaumont and Robert, 1999) and coal (Ader et al., among biogenic, abiogenic, diagenetic and hydro- 1998). In metamorphic rocks, ammonium is trapped in thermal minerals. We go on further to discuss the K-bearing minerals (Mingram and Brauer, 2001) or physico-chemical factors controlling the substitution + N2 occurs in fluid inclusions (De Ronde et al., 2003). of NH4 with alkalis, which is fundamental for There are several other potential retention sites for understanding nitrogen-fractionation processes and nitrogen in rocks. Multi-stepped extraction of nitrogen the nitrogen cycle in a silicate Earth. from Precambrian cherts and banded-iron formations has shown the presence of a high temperature (N1000 8C) nitrogen component, likely included in magnetite 2. Geology and mineralogy of black shales from the (Pinti et al., 2001). Nuclear Reaction Analyses (NRA) Selwyn basin of nitrogen in Archaean cherts from Marble Bar, Australia suggests that nitrogen is associated with Fe– Upper Devonian black shales that were deposited Mn oxi-hydroxides (Gallien et al., 2003). However, it as turbidites on the Mackenzie carbonate platform, a is not clear if this nitrogen is bonded in the Fe-oxide continental margin of the Selwyn Basin, Northwest + structure or occurs as NH4 in microscopic K–Al- Territories of Canada were collected for this study B. Orberger et al. / Chemical Geology 218 (2005) 249–264 251 (Hulbert et al., 1992). Black shales are located at the matter are considered as primary biomass (Fig. 1C). top of a 3- to 20-m-thick series of phosphatic Hydrothermal metal charged fluids (b260 8C) infil- carbonates and cherts. They are mineralized over 10 trated during sedimentation or early diagenesis and cm by crystallized and amorphous pyrite, vaesite, crystallized interstitial to the partly sulfurized organic sphalerite, various Ni–As–Fe- and Mo-sulfides, and matter agglomerations. Na- to K- and K–Ba-feldspars, wurtzite associated with about 1 wt.% of bitumen quartz, apatite, xenotime (HREE-rich), brannerite (Hulbert et al., 1992). Particle Induced X-ray Emis- (UTi2O6), sphalerite and Ag–Cd (Cl) alloys crystal- sion (PIXE) analyses on Fe–Ni, Ni–Fe, Fe- and Zn- lized from these fluids (Fig. 1D) Traces of Ni, Se, Mo, sulfides detected several hundreds to thousands ppm Sb, In, Tl occur in biogenic and abiogenic sulfides. of Cu, As, Se and Mo. Cd (2600 ppm) and In (60 The simultaneous Ni and Se enrichment and the ppm) were found in sphalerite and Sb and Ta (N1700 replacement structure of pyrite by Ni sulfides point to ppm) in Fe–Ni sulfides (Orberger et al., 2003a,b). The a diagenetic origin of these two elements. Platinum mediating process for fixing some of these metals was and Au (~400 ppm) are suggested to occur as alloys in likely the result of biological activity (Orberger et al., Fe–Ni sulfides rather than incorporated in the sulfide 2003a,b). Arseno-pyritized worm tube colonies host structure (Orberger et al., 2003a,b). Cu–As alloys (Fig. 1A) as observed at active and fossil hydrothermal vents (Little, 2002; Maginn et al., 2002). Nano- to micro-metric euhedral sphalerite 3. Analytical methods precipitates along silica worm tubes were also found, probably as the result of bacterial reduction (Orberger Prior to NRA, each sample was carefully studied et al., 2003a,b; Fig. 1B). Similar sphalerite clusters by transmitted and reflected light microscopy and grow in the proteinic axes of the Alvinella worm tubes scanning electron microscope (SEM; Philips XL-30). and are interpreted as being derived from symbiotic Semi-quantitative analyses
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