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
a
Département des Sciences de la Terre, CNRS-UMR 8148 IDES, Université Paris Sud, Bât. 504, 91405 Orsay Cedex, France
b
Laboratoire Pierre Süe, CEA-CNRS UMR 9956, CEA Saclay, 91191 Gif-Sur-Yvette Cedex, France
c
Centre CAMPARIS, Université Pierre et Marie Curie-Paris 6, 4, Place Jussieu, 75256 Paris Cedex 05, France
d
School of Geography and Geology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
e
Czech 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
monovalent NH4+ by divalent Ba2+. 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 Babearing 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, Montréal, 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
The geochemical behavior of nitrogen in silicate
rocks is not yet fully understood (Boyd, 2001a,b), and
is in part, related to difficulties in extracting and
analyzing nitrogen at nano-mole levels in silicate
rocks. Such analytical problems have been overcome
in the last decade by improving nitrogen-routine
elemental and isotopic analyses through mass spectrometry (Hashizume and Sugiura, 1992; Boyd et al.,
1994; Marty, 1995), Selective Ion Mass Spectrometry
(SIMS; Hashizume et al., 2000; Bulanova et al., 2002)
and through quantification by micro-Fourier Transform Infrared Spectroscopy (FTIR; Busigny et al.,
2003b).
In sedimentary rocks, the breakdown of organic
matter during diagenesis produces ammonium (NH4+)
(Honma and Itihara, 1981; Itihara and Suwa, 1985),
which is equal in charge and similar in ionic radius to
potassium. It is usually assumed that NH4+ substitutes
for K+ in minerals such as mica and feldspars (e.g.,
Honma and Itihara, 1981). Ammonium is stable at
high temperatures, it resists metamorphism and
anatexis, and it can be found in crustal melts (Hall,
1999). In sedimentary rocks, nitrogen is commonly
found in fossil organic matter such as kerogen
(Beaumont and Robert, 1999) and coal (Ader et al.,
1998). In metamorphic rocks, ammonium is trapped in
K-bearing minerals (Mingram and Brauer, 2001) or
N2 occurs in fluid inclusions (De Ronde et al., 2003).
There are several other potential retention sites for
nitrogen in rocks. Multi-stepped extraction of nitrogen
from Precambrian cherts and banded-iron formations
has shown the presence of a high temperature (N1000
8C) nitrogen component, likely included in magnetite
(Pinti et al., 2001). Nuclear Reaction Analyses (NRA)
of nitrogen in Archaean cherts from Marble Bar,
Australia suggests that nitrogen is associated with Fe–
Mn oxi-hydroxides (Gallien et al., 2003). However, it
is not clear if this nitrogen is bonded in the Fe-oxide
structure or occurs as NH4+ in microscopic K–Al-
silicates included in the oxides. Recent studies of
bituminous coal using X-ray Photoelectron Spectroscopy (XPS) and time of flight SIMS (TOF-SIMS)
have revealed the presence of inorganic nitrogen
associated with clay minerals together with expected
organic-derived NH4+ in illites (Gong et al., 1997).
Finally, sulfur and sulfides play an important role
during organic matter breakdown suggesting their
involvement in the release and fixation of nitrogen.
However, few datasets are available: for example, oil
fields (Thompson, 1994) and Precambrian black
shales (Imbus et al., 1992; Watanabe et al., 1997).
In this study, we provide a quantitative identification of the mineral retention sites of nitrogen and
carbon in metal- and organic matter-rich hydrothermal-infiltrated black shales from the Selwyn Basin,
Canada (Hulbert et al., 1992). High-resolution spatial
distribution of nitrogen and carbon on different
mineral phases (silicates, sulfides, phosphates and
organic matter) at the micrometric scale was obtained
using NRA (Khodja et al., 2001). Feldspars, containing more than 0.5 wt.% of nitrogen were systematically analyzed by electron microprobe and the results
were compared with NRA. The data generated in this
study, in comparison to other datasets, provides new
insights on the partitioning of nitrogen and carbon
among biogenic, abiogenic, diagenetic and hydrothermal minerals. We go on further to discuss the
physico-chemical factors controlling the substitution
of NH4+ with alkalis, which is fundamental for
understanding nitrogen-fractionation processes and
the nitrogen cycle in a silicate Earth.
2. Geology and mineralogy of black shales from the
Selwyn basin
Upper Devonian black shales that were deposited
as turbidites on the Mackenzie carbonate platform, a
continental margin of the Selwyn Basin, Northwest
Territories of Canada were collected for this study
B. Orberger et al. / Chemical Geology 218 (2005) 249–264
(Hulbert et al., 1992). Black shales are located at the
top of a 3- to 20-m-thick series of phosphatic
carbonates and cherts. They are mineralized over 10
cm by crystallized and amorphous pyrite, vaesite,
sphalerite, various Ni–As–Fe- and Mo-sulfides, and
wurtzite associated with about 1 wt.% of bitumen
(Hulbert et al., 1992). Particle Induced X-ray Emission (PIXE) analyses on Fe–Ni, Ni–Fe, Fe- and Znsulfides detected several hundreds to thousands ppm
of Cu, As, Se and Mo. Cd (2600 ppm) and In (60
ppm) were found in sphalerite and Sb and Ta (N1700
ppm) in Fe–Ni sulfides (Orberger et al., 2003a,b). The
mediating process for fixing some of these metals was
likely the result of biological activity (Orberger et al.,
2003a,b). Arseno-pyritized worm tube colonies host
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
precipitates along silica worm tubes were also found,
probably as the result of bacterial reduction (Orberger
et al., 2003a,b; Fig. 1B). Similar sphalerite clusters
grow in the proteinic axes of the Alvinella worm tubes
and are interpreted as being derived from symbiotic
bacterial reduction of sulfides (Zbinden et al., 2001).
Phosphate fossil fragments (conodonts) and organic
251
matter are considered as primary biomass (Fig. 1C).
Hydrothermal metal charged fluids (b260 8C) infiltrated during sedimentation or early diagenesis and
crystallized interstitial to the partly sulfurized organic
matter agglomerations. Na- to K- and K–Ba-feldspars,
quartz, apatite, xenotime (HREE-rich), brannerite
(UTi2O6), sphalerite and Ag–Cd (Cl) alloys crystallized from these fluids (Fig. 1D) Traces of Ni, Se, Mo,
Sb, In, Tl occur in biogenic and abiogenic sulfides.
The simultaneous Ni and Se enrichment and the
replacement structure of pyrite by Ni sulfides point to
a diagenetic origin of these two elements. Platinum
and Au (~400 ppm) are suggested to occur as alloys in
Fe–Ni sulfides rather than incorporated in the sulfide
structure (Orberger et al., 2003a,b).
3. Analytical methods
Prior to NRA, each sample was carefully studied
by transmitted and reflected light microscopy and
scanning electron microscope (SEM; Philips XL-30).
Semi-quantitative analyses (EDX-PGT; Ge detector)
were performed at 20–30 kV. The CEA-CNRS Pierre
Sqe Laboratory nuclear microprobe was used to
Fig. 1. SEM microphotographs showing: (A) sulfurized worm-tube colonies; (B) hydrothermal lenses interstitial to the biogenic sulfides
composed of hydrothermal feldspar; the bright rims are Ba-rich K-feldspar overgrown on K–N–Ba feldspars. Filamentous biogenic silica
contains micron-large sphalerite alignments; (C) Conodont fragment mineralized to F-rich Ca-phosphate; (D) hydrothermal lens, interstitial to
biogenic sulfides Ba-rich K-feldspars, shows penetrative texture towards the inner part of the N-rich alkali feldspars. Euhedral apatite
crystallized on the edge of the hydrothermal lens, while quartz, REE-rich xenotimes and brannerite crystallized in its central part.
252
B. Orberger et al. / Chemical Geology 218 (2005) 249–264
achieve carbon and nitrogen determination in minerals
from black shales (Khodja et al., 2001). The facilities
allow simultaneous measurements of the light element
abundances (12C, 14N, 16O, 28Si, 32S and 27Al) by
NRA and heavier elements (Ni, Fe, Zn, K, and Ca
among others) by Particle Induced X-ray Emission
(PIXE). A 1.9-MeV deuteron (2H+) incident beam is
used to achieve the maximum sensitivity for nitrogen
detection. 12C(d,p 0)13C and 14N(d,p 0)15N nuclear
reactions are used to determine carbon and nitrogen,
respectively. The deuteron beam is focused to 3!3
Am2 and intensity is close to 0.8 nA. Nuclear reaction
light product detection was performed by a 130-m Sr,
1500-Am depletion depth annular silicon surface
barrier detector located at 1708. X-rays were measured
using a 95-mm2 collimated high-purity germanium
detector with a 50-Am Mylar filter to stop most
charged particles. Backscattered particles are stopped
in the Mylar-screen, but higher energy charged
particles derived from nuclear reactions go through.
These analytical conditions led to a counting rate of a
few hundred particles detected per second depending
on the mineral composition and results in very low
deadtime of 1–2%.
A 30-Am-thick sample was mounted on ultra-pure
SUPRASIL glass to avoid simultaneous analysis of
trace elements of the target carrier, due to the beam
penetration depth of several tenths of a micron (e.g.,
for 1.9 deutons: 24-Am penetration depth in quartz).
The sample surface was diamond polished and
subsequently carbon coated (~10 nm) to ensure charge
collection. Since we focused on the quantification of
subsurface bulk carbon, carbon coating does not
influence the results when depth-resolved analysis
techniques like NRA are used. Consequently, surface
carbon and nitrogen is ignored and the signal coming
from 1 to ~9 Am deep is used to achieve quantification. NRA spectra with sufficient statistics are also
necessary to achieve elemental quantification (data
acquisition is stopped when total counts in the
14
N(d,p 0)15N region reaches a few hundred and
reflects the nuclear cross section shape).
With such heterogeneous samples, a scanning
mode was used to achieve the most accurate view of
the sample in order to select the most suitable (in
terms of size and thickness) area for nitrogen and
carbon determination. The MPAWIN data acquisition
software is run in the blistcard-coincidentQ mode and
the generated data files are post-processed with the
homemade RISMIN software (Daudin et al., 2003).
Special care was taken to extract data (X-ray and
NRA spectra) corresponding to a homogeneous phase.
When performing NRA analysis on mineral
samples, the spectrum is the sum of numerous nuclear
reactions (d,p) or (d,a) (fundamental and excited
states) for all blightQ elements (Zb17, isotopes
included). With respect to our analytical conditions
(beam energy, detection angle), the lack of NRA
cross-section data for the contributing isotopes leads
us to use standards and perform an EXCEL processing of the spectra. The following standards were used
to achieve quantification: Al2O3, SiO2, Mg, FeS2,
UO2, CaPO4(OH)5, TiN and CaCO3. The overall
spectrum is fitted and backgrounds are determined
especially for carbon determination. Deadtime, accumulated charge and stopping powers are taken into
account for standards and samples, leading to the
carbon and nitrogen quantification. Scanned areas
range from 50!50 Am2 to 500!500 Am2. During a
measurement time of a few hours, beam current
fluctuation effects on scanned areas were minimized
by using a high scanning rate (1 kHz). Nitrogen
quantitative microanalysis mapping requires high
micro-beam current (800 pA, 5!5 Am2) to reach 20Am-accumulated charge in a realistic time (7 h). The
detection limit of nitrogen by NRA is about 100 ppm.
However, point analyses require about 1 AC charge
with similar detection limits. A typical nitrogen NRA
spectrum is shown in Gallien et al. (2004). Electron
microprobe analyses (CAMEBAX SX 50) were
performed at the Centre CAMPARIS of the Université
Pierre et Marie Curie on feldspars and on Caphosphates (conodont fragments). As NRA on feldspars showed nitrogen and carbon amounts in the
range of 1–3 wt.%, we adapted the sample preparation
and the analytical conditions to measure nitrogen,
carbon and simultaneously the major (K, Na, Al, Si)
and trace elements (Ba, Sr, Fe, Mg, Ca, Mn). Samples
were first coated with carbon. Feldspars were analyzed only for nitrogen, major and trace elements. The
detection limit of nitrogen is 0.5 wt.%. The standard
used for nitrogen measurements is BN. For carbon
analyses, the samples were coated with 50 Am Au–Pd
after complete removal of the carbon coating by repolishing followed by an ultrasonic bath for 1 min.
Data of nitrogen and major and trace elements in the
Table 1
Electron microprobe analyses of (a) a hydrothermal feldspars and their structural formula calculated on the basis of 8 oxygens; (b) Ca-phosphates (conodonts)
SiO2 (wt.%)
TiO2
Al2O3
FeOt
MgO
CaO
Na2O
K2O
BaO
N
C
NH4
Total
F22
F23
F24
F25
F26
F27
F28
F29
F30
F31
64.33
bd.l.
16.43
0.09
bd.l.
bd.l.
0.20
5.26
10.80
1.65
0.58
2.12
99.33
65.70
bd.l.
19.52
0.67
0.15
bd.l.
9.76
1.13
1.53
bd.l.
1.42
bd.l.
100.03
66.14
bd.l.
20.11
0.21
0.19
bd.l.
10.87
0.53
0.38
b0.5
0.68
b0.5
99.21
67.10
bd.l.
17.96
0.13
bd.l.
bd.l.
0.08
8.15
1.96
2.36
1.16
3.04
98.95
68.87
bd.l.
16.79
0.61
0.05
0.01
0.04
7.17
2.84
2.15
1.58
2.76
100.11
67.79
0.05
18.29
bd.l.
0.02
0.03
bd.l.
8.34
1.07
2.51
0.57
3.23
98.66
64.14
bd.l.
19.06
0.16
bd.l.
0.04
0.10
8.09
2.67
2.91
2.68
3.74
99.92
67.67
0.07
19.51
0.06
bd.l.
bd.l.
11.87
0.28
0.00
b0.5
1.04
b0.5
100.51
60.53
bd.l.
20.10
0.08
bd.l.
bd.l.
0.18
7.54
7.28
2.49
0.54
3.20
98.75
62.47
bd.l.
19.57
0.15
bd.l.
0.03
0.14
7.36
5.10
3.01
0.59
3.86
98.40
63.34
bd.l.
19.47
0.00
bd.l.
0.06
0.29
8.87
3.05
2.67
1.49
3.43
99.24
67.59
0.07
19.55
0.00
bd.l.
bd.l.
11.69
0.07
bd.l.
0.06
0.63
0.07
99.65
59.49
bd.l.
20.18
0.12
bd.l.
bd.l.
0.66
7.45
8.49
1.96
0.62
2.52
98.97
65.05
0.04
19.74
0.21
bd.l.
bd.l.
0.05
8.57
1.70
2.40
1.31
3.09
99.09
65.76
bd.l.
19.16
0.08
bd.l.
bd.l.
0.11
8.58
2.00
2.44
0.48
3.14
98.68
65.98
bd.l.
18.74
bd.l.
bd.l.
bd.l.
0.04
8.47
1.15
3.07
0.80
3.94
98.26
64.57
bd.l.
18.65
0.06
bd.l.
bd.l.
0.12
8.39
3.14
2.44
0.68
3.14
98.05
67.81
bd.l.
18.58
0.01
0.02
0.02
11.31
0.04
0.00
0.00
1.42
b0.5
99.64
55.24
bd.l.
20.40
0.05
0.04
bd.l.
0.27
6.57
13.60
1.60
0.62
2.05
98.39
59.88
bd.l.
17.90
0.12
bd.l.
0.03
0.24
5.95
13.53
1.29
0.61
1.65
99.57
63.56
bd.l.
16.74
0.13
bd.l.
0.02
0.21
4.89
13.59
1.05
0.68
1.35
100.86
66.63
bd.l.
19.59
0.06
0.01
0.15
11.76
0.06
0.09
bd.l.
1.16
b0.5
99.50
66.36
0.02
17.36
0.21
0.01
bd.l.
0.13
7.34
3.60
2.88
0.64
3.70
98.55
65.04
bd.l.
19.83
0.19
0.01
bd.l.
8.67
1.78
4.00
b0.5
bd.l.
b0.5
99.91
68.22
bd.l.
19.11
0.10
bd.l.
bd.l.
11.07
0.13
0.28
b0.5
bd.l.
b0.5
99.09
66.34
bd.l.
19.85
bd.l.
0.01
bd.l.
9.89
1.01
3.53
b0.5
bd.l.
b0.5
100.78
54.58
bd.l.
21.56
0.36
bd.l.
bd.l.
1.47
5.61
14.11
1.06
bd.l.
1.36
98.80
Structural formula calculated on the basis of 8
Si
2.794 2.878 3.043 2.98 3.15
Ti
–
–
–
0.002 –
Al
1.073 1.065 1.003 1.013 0.948
Fe
0.001 0.019 0.011 0.005 0.004
Mg
–
–
0.001 –
–
Ca
0.001 –
–
0.002 –
Na
0.014 0.008 0.012 0.968 0.019
K
0.422 0.437 0.378 0.021 0.328
Ba
0.156 0.077 0.078 0.008 0.207
N
0.540 0.518 0.474 –
0.345
Or (mol%) 37.32 42.01 40.09 2.07 36.51
Ab
1.21 0.73 1.26 96.89 2.08
An
0.05 –
–
0.22 –
Ce
13.74 7.40 8.29 0.82 23.03
Bud
48.86 45.84 46.25 –
41.36
N (at.%)
4.25 4.04 3.65 bd.l. 2.61
C (at.%)
1.01 1.31 1.24 1.12 1.08
C/N
0.237 0.324 0.340 –
0.412
oxygens
2.97
–
1.041
0.025
0.010
–
0.857
0.065
0.027
–
6.88
90.27
–
2.85
–
0.18
2.40
13.115
2.95
–
1.056
0.008
0.012
–
0.939
0.030
0.007
–
3.09
96.20
0.02
0.68
–
bd.l.
1.15
–
3.05
–
0.964
0.005
–
–
0.007
0.473
0.035
0.461
48.48
0.76
–
3.58
38.21
3.52
2.02
0.574
3.17
–
0.910
0.023
0.003
–
0.004
0.421
0.051
0.422
46.83
0.44
0.04
5.70
34.89
3.17
2.72
0.858
3.05
0.002
0.969
–
0.001
0.001
0.000
0.478
0.019
0.483
48.68
0.04
0.12
1.91
44.36
3.74
0.99
0.265
2.89
–
1.014
0.006
–
0.002
0.009
0.466
0.047
0.562
42.90
0.82
0.18
4.36
34.21
4.26
4.58
1.075
2.96
0.002
1.007
0.002
–
–
1.008
0.016
–
–
1.55
98.45
0
0
0
bd.l.
1.73
–
2.81
–
1.099
0.003
–
–
0.016
0.446
0.132
0.495
40.94
1.46
0.01
12.14
45.78
3.89
0.97
0.249
2.83
–
1.046
0.006
–
0.001
0.012
0.426
0.091
0.584
38.28
1.07
0.11
8.14
50.54
4.61
1.05
0.228
2.86
–
1.035
–
–
0.003
0.025
0.510
0.054
0.516
46.07
2.27
0.24
4.87
37.17
4.00
2.56
0.640
2.97
0.002
1.014
–
–
–
0.997
0.004
–
0.011
0.37
98.59
0
0
0.92
0.08
1.05
13.125
2.81
–
1.123
0.005
–
–
0.061
0.449
0.157
0.397
42.19
5.71
0
14.78
37.72
3.09
1.13
0.366
2.94
0.001
1.053
0.008
–
–
0.004
0.495
0.030
0.466
49.73
0.44
0.02
3.03
36.89
3.57
2.27
0.636
2.968
0
1.019
0.003
–
–
0.010
0.494
0.035
0.471
48.88
0.97
0
3.50
43.52
3.67
0.84
0.229
2.932
0
0.981
0
–
–
0.004
0.480
0.020
0.584
44.13
0.34
0
1.84
46.92
4.58
1.38
0.301
2.956
–
1.006
0.002
–
–
0.010
0.490
0.056
0.479
47.33
0.99
0
5.43
42.17
3.72
1.21
0.325
3.034
–
0.980
0.000
–
0.001
0.982
0.002
–
0
0.20
99.72
0.07
0
0
bd.l.
2.38
–
2.750
–
1.197
0.002
0.003
–
0.026
0.417
0.265
0.340
39.75
2.49
0
25.30
36.33
2.64
1.19
0.451
2.995
–
1.055
0.005
–
0.002
0.023
0.380
0.265
0.275
40.18
2.41
0.19
28.07
33.11
2.09
1.16
0.555
3.181
–
0.988
0.005
–
0.001
0.020
0.312
0.267
0.225
37.83
2.42
0.12
32.31
31.02
1.66
1.37
0.825
2.952
–
1.023
0.002
0.001
0.007
1.010
0.003
0.001
–
0.34
98.82
0.70
0.14
0
bd.l.
1.94
–
3.006
0.001
0.927
0.008
–
–
0.012
0.424
0.064
0.558
40.11
1.10
0
6.04
49.03
4.36
1.12
0.257
2.977
–
1.070
0.007
–
–
0.769
0.104
0.072
–
11.01
81.39
0.02
7.58
0
bd.l.
bd.l.
–
3.030
–
1.000
0.004
–
–
0.953
0.008
0.005
–
0.78
98.72
0
0.50
0
bd.l.
bd.l.
–
2.972
–
1.048
–
–
–
0.859
0.058
0.062
–
5.91
87.76
0
6.33
0
bd.l.
bd.l.
–
2.720
–
1.266
0.015
–
–
0.142
0.356
0.275
0.225
35.66
14.24
0
27.56
0
bd.l.
bd.l.
–
60.16
bd.l.
19.61
0.01
bd.l.
0.01
0.15
7.13
8.55
2.71
0.55
3.49
99.01
63.12
bd.l.
19.82
0.49
bd.l.
bd.l.
0.09
7.51
4.31
2.66
0.74
3.41
98.71
65.79
bd.l.
18.40
0.28
0.01
bd.l.
0.13
6.40
4.31
2.40
0.70
3.08
98.41
66.98
0.07
19.32
0.14
bd.l.
0.05
11.21
0.36
0.47
b0.5
0.66
b0.5
99.27
B. Orberger et al. / Chemical Geology 218 (2005) 249–264
(a) Representative electron microprobe analyses of hydrothermal feldspars from black shales (Yukon territories, Canada, Centre Camparis, Université Paris VI)
F1
F2
F3
F4
F5
F6
F7
F8
F9
F10 F11 F12
F13 F14 F15 F16
F17 F18 F19 F20 F21
(b) Electron microprobe analyses of conodont fragments (Centre Camparis, Université Pierre et Marie Curie)
P2O5
CaO
Na2O
MgO
SrO
FeO
K2O
MnO
Cl
F
Total
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
C13
41.37
51.62
0.04
0.04
0.06
0.10
bd.l.
bd.l.
0.04
3.72
96.99
41.88
51.67
bd.l.
bd.l.
0.12
bd.l.
0.01
0.10
0.07
3.70
97.55
41.55
51.66
bd.l.
bd.l.
bd.l.
0.09
bd.l.
bd.l.
0.02
3.12
96.44
41.81
52.65
0.01
0.01
bd.l.
0.09
0.03
bd.l.
0.03
3.61
98.24
41.70
50.29
0.04
0.04
bd.l.
0.07
0.07
0.01
bd.l.
3.13
95.35
41.96
49.30
bd.l.
bd.l.
0.04
0.09
0.10
0.04
bd.l.
3.60
95.13
41.95
51.21
0.01
0.01
0.06
0.11
0.02
bd.l.
0.03
3.80
97.20
41.56
52.42
0.02
0.02
0.08
0.07
bd.l.
bd.l.
0.04
3.43
97.64
42.98
52.70
0.27
0.05
bd.l.
0.01
bd.l.
0.02
bd.l.
3.44
99.47
41.99
53.16
0.27
0.01
0.04
0.06
0.02
bd.l.
0.05
3.45
99.05
41.39
52.21
0.16
bd.l.
0.02
0.15
bd.l.
bd.l.
0.02
3.25
97.20
42.49
52.42
0.57
bd.l.
0.15
0.03
0.02
0.06
0.04
3.57
99.35
42.56
52.69
0.36
0.05
0.15
0.06
bd.l.
bd.l.
bd.l.
2.83
98.70
253
d.l.: detection limit.
254
B. Orberger et al. / Chemical Geology 218 (2005) 249–264
two runs, carbon and Au–Pd coated ones gave
comparable results (Table 1). For carbon, the graphite
standard was used. Nitrogen and carbon were
analyzed at 10 kV and 40 nA. Due to the very fragile
minerals, a defocused beam was used (20 Am) and
counting times were fixed to 10 s to avoid degassing
of the volatile elements. The other elements were
analyzed at 15 kV, 10 nA (Table 1). The following
standards were used for major and trace elements in
feldspars and conodonts: orthoclase (Al, Si, K);
MnTiO3; (Mn, Ti); albite (Na); anorthite (Ca); barite
(Ba); diopside (Mg); hematite (Fe); SrSiO3 (Si);
apatite (P); CaF2 (F); scapolite (Cl). Two hundredfifty feldspar analyses were performed however, only
60 were considered for the calculation of the structural
formula. Representative analyses are shown in Table
1a. Feldspar analyses with excess silica were omitted
and not considered for interpretation. Presently, it is
impossible for us to attribute the non-stochiometry of
feldspars to: (1) analytical errors; (2) the presence of
univalent cations such as NH4+; or (3) the occurrence
of a solid solution of NaAlSi3O8–KAlSi3O8 and
Schwantke’s molecule CaAl2Si6O16 (Barker, 1964;
Carman and Tuttle, 1963). Nitrogen and carbon
contents obtained by NRA and by electron microprobe have been calculated on an 8-oxygen basis and
due to the high contents of Ba and nitrogen, a fivecomponents system (orthoclase, albite, anorthite,
celsian and buddingtonite) was calculated (Table
1a). High nitrogen- and carbon-bearing feldspars
might contain small amounts of zeolitic water, as
suggested by Erd et al. (1964) and Barker (1964). The
only previous measurements of in situ nitrogen by
electron microprobe were performed on hyalophanes
from pegmatites from Bosnia containing up to 0.15
wt.% NH4+ (Beran et al., 1992). The Ca-phosphate
analyses, conodonts composed of apatites/hydroxyapatites are presented in Table 1b. It must be noted that
nitrogen contents obtained by NRA correspond to
14
N, while electron microprobe analyses correspond
Fig. 2. PIXE and NRA scan showing hydrothermal Na–K-feldspar and quartz replacing organic carbon. Sulfides are composed of heterogeneous
distributed Ni and Fe. The K and Si scans represent hydrothermal quartz and feldspars; N scan shows its heterogeneous distribution in
hydrothermal feldspars.
255
B. Orberger et al. / Chemical Geology 218 (2005) 249–264
Table 2
N and C contents analysed by NRA and compiled data from the literature
Rock/sediment
Age
C, wt.%
N, wt.%
C/N atomic
Ref.
Hydrothermal feldspar
Hydrothermal feldspar
Hydrothermal feldspar
Hydrothermal feldspar
Hydrothermal feldspar
Hydrothermal feldspar
Hydrothermal quartz
Biogenic apatite (Conodont)
Biogenic Fe–Ni sulfides
Biogenic Fe–Ni sulfides
Biogenic Fe–Ni sulfides
Abiogenic Ni–Fe sulfides
Organic carbon
Organic carbon
Devonian
Devonian
Devonian
Devonian
Devonian
Devonian
Devonian
Devonian
Devonian
Devonian
Devonian
Devonian
Devonian
Devonian
0.26
0.25
0.22
3.26
0.26
0.25
0.88
1.07
0.87
0.56
1.05
0.88
~100
~100
0.37
1.23
0.96
2.05
2.37
1.23
0.053
0.047
0.038
0.039
0.062
0.044
0.663
0.6
0.82
0.24
0.27
1.86
0.13
0.24
19.37
26.56
26.71
16.75
19.76
23.33
174.80
193.28
NRA
NRA
NRA
NRA
NRA
NRA
NRA
NRA
NRA
NRA
NRA
NRA
NRA
NRA
Organic matter in sediments
Laminated black marlstone
Bioturbated white limestone
Black claystone
Green claystone
Homogeneous black claystone
Laminated limestone
Turbidite (Madeira Abyssal Plain)
Turbidite (Madeira Abyssal Plain)
Turbidite (Madeira Abyssal Plain)
Turbidite (Madeira Abyssal Plain)
Calcareous pelagic sediments
Argillaceous pelagic sediments
Siliceous pelagic sediments
Lake sediments
Lake sediments (Lugano, swiss)
Lake sediments (Lugano, swiss)
Sapropel (eastern Mediterranean)
Cretaceous
Cretaceous
Cretaceous
Cretaceous
Cretaceous
Cretaceous
Modern/oxic
Modern/anoxic
Quaternary/oxic
Quaternary/anoxic
Modern
Modern
Modern
Modern
Modern/oxic
Modern/anoxic
Modern
2.40
0.07
11.34
0.25
1.87
1.04
0.29
1.34
0.22
1.19
0.25
0.21
0.36
4.41
0.11
0.003
0.499
0.020
0.077
0.047
0.06
0.13
0.057
0.116
0.014
0.016
0.016
0.36
–
–
25.5
27.2
26.5
14.6
28.3
25.8
5.6
12.0
4.5
12.0
20.8
15.3
26.3
14.3
~8
~10
8.84
[1]
[1]
[1]
[1]
[1]
[1]
[2]
[2]
[3]
[3]
[4]
[4]
[4]
[4]
[5]
[5]
[6]
Kerogens
Kerogens in chertsa
Kerogens in chertsa
Kerogen in shales (wt.% of OM)
Kerogen in mudstone (wt.% of OM)
Kerogen in mudstone (wt.% of OM)
OM in anthracites
Archean
Proterozoic
Archean
Proterozoic
Cretaceous
Carboniferous
0.04
0.022
64–78
93.51
0.5–2.2
0.004
0.002
0.12–0.16
0.72
0.01–0.3
–
204.5
154.1
500–1000
166.7
2.8–137.7
16–250
[7]
[7]
[8]
[9]
[10]
[11]
3.9
9–32.4
23.4–42.7
2.9–14.3
6.2–47
[12]
[13]
[13]
[12]
[14]
Fossils
Fossilized bacteria
Conodonts
Conodonts
Marine bacteria
Zooplankton from hydrothermal vent
(particulate flux in mg/m2/day)
MOR-TAG
Ordovician
Devonian
Modern
Modern
–
–
–
–
–
–
–
5.9–1359
–
–
–
–
0.2–238
References: [1] Rau et al., 1987; [2] Cowie et al., 1998; [3] De Lange, 1998; [4] Wlotzka, 1969; [5] Lehmann et al., 2002; [6] Struck et al., 2001;
[7] Beaumont and Robert, 1999; [8] Itihara et al., 1986; [9] Itihara and Aihara, 1987; [10] Williams et al., 1995; [11] Ader et al., 1998; [12] AlHanbali et al., 2001; [13] Marshall et al., 2001; [14] Bayona et al., 2002.
a
The reported C, N amounts and C/N ratios for kerogens are average values.
B. Orberger et al. / Chemical Geology 218 (2005) 249–264
4. Results
Optical microscope investigation of the black
shales allowed selection of four mineralized zones
representing potential trapping sites for nitrogen: (1)
non-deformed worm tubes and surrounding diagenetically derived sulfides and organic matter; (2) diagenetically deformed worm tubes surrounded by late
diagenetically precipitated Ni sulfides and inter-grown
hydrothermal Na–K–Ba-feldspars and quartz (Fig.
1A); (3) hydrothermal lenses, interstitial to sulfurized
organic matter, composed essentially of feldspars and
quartz; and (4) conodont fragments are composed of
F-rich apatite/hydroxyapatite (F=2.83–3.8 wt.%; Fig.
1C, Table 1b). PIXE scanning shows heterogeneous
Fe and Ni distribution in sulfide lenses that are
embedded in organic carbon (Fig. 2); sphalerite (ZnS)
is inter-grown with hydrothermal quartz and feldspars,
suggesting a simultaneous precipitation from the same
fluid. Feldspars and quartz indicate that they partially
replace organic carbon (Figs. 1B and 2).
NRA scanning on these four distinct zones
identified six nitrogen- and carbon-bearing phases
(Table 2) with nitrogen as the main carrier in feldspar
(from 1 to 3.39 wt.%; Tables 1 and 2). Electron
microprobe analyses of these feldspars indicate trimodal feldspar compositions, albite, K-feldspar and
hyalophane (Fig. 1B,C) and confirm the semi-quantitative SEM–EDX investigations by Orberger et al.
(2003a). The barium-rich feldspar shows penetrative
textures towards internal rims, or sharp crystal rims of
several tenths of microns thicknesses form around the
potassium–nitrogen-rich varieties (Fig. 1B,D). Fig. 1B
shows that albite is clearly separated from K-feldspar
by a Ba-rich rim. NRA high-resolution scans indicate
a heterogeneous nitrogen distribution at the mineral
scale (Fig. 2). The central portion of feldspars is richer
in nitrogen than the outer rim (Fig. 2). Electron
microprobe analyses show that these heterogeneities
are related to the three main feldspar populations: (1)
albite, composed rarely of 3 mol% of orthoclase and
2.8 mol% of celsian, nitrogen amounts generally
being below the detection limit of the electron
microprobe and the NRA (b100 ppm of N); (2)
feldspars, composed of almost an equal amount of
orthoclase and the NH4+-rich end-member buddingtonite; and (3) K-bearing feldspars composed of 15 to
32.3 mol% of celsian. Anorthose with increasing
amounts of celsian and buddingtonite were rarely
observed (Table 1). The main nitrogen carrier (i.e. the
K–Ba-bearing feldspars) shows a positive correlation
between potassium and nitrogen (Fig. 3A). This
correlation is also observed in whole rock analyses
(Mingram and Brauer, 2001; Busigny et al., 2003a)
and it reflects the genetic relation between K+ and the
replacing NH4+ ion (Honma and Itihara, 1981).
However, at the crystal edges, where the Ba content
increases, the nitrogen amount decreases leading to a
negative correlation between Ba and nitrogen (Fig.
3B). The carbon contents vary from about 0.5 wt.%
up to 1.8 wt.%. NRA reveals higher concentrations,
even up to ~3 wt.% of carbon. For this extreme value,
5
A
K-NH4+-Ba feldspars
4
NH4+, wt%
to total nitrogen contents. The 14N isotopic abundance
of 99.63% allows comparing the NRA data with those
obtained by electron microprobe (total nitrogen).
3
2
1
Ba-rich
outer rim
4
5
6
7
8
9
K2O, wt%
5
B
K-NH4+-Ba feldspars
4
NH4+, wt%
256
3
Ba 2
+
pro
gre
2
1
ssiv
e re
cem
ent
0
5
Ba-rich
outer rim
pla
of N
H +
4
10
15
20
BaO, wt%
Fig. 3. Diagrams showing (A) K2O versus NH+4 ; (B) BaO versus
NH+4 in hydrothermal albites and K–Ba-feldspars. Data obtained by
electron microprobe analyses (Centre Camparis, Université Pierre et
Marie Curie).
257
B. Orberger et al. / Chemical Geology 218 (2005) 249–264
biogenic Fe–Ni sulfides (380–620 ppm) and abiogenic Ni–Fe sulfides (up to 440 ppm) contain
homogeneously distributed nitrogen and carbon,
about 10 times lower than the amounts measured in
the organic matter (Table 2). It must be noted that pure
Ni sulfides do not contain nitrogen. Carbon is in the
same order of magnitude in all analyzed phases (0.2 to
about 1 wt.%), except for organic carbon (Table 2).
Hydrothermal feldspars show significantly lower
C/N atomic ratios (C/N=0.13–13; average is 1.1)
it cannot be ruled out that part of the carbon is related
to inter-grown organic matter at a depth of a few
microns. Carbon contents in albite are in the same
order of magnitude as those in the K–Ba–N feldspars.
At relatively constant nitrogen contents, considerable
variations in carbon are observed (Fig. 4).
The second important nitrogen carrier is organic
matter (from 0.6 to 0.66 wt.%), where nitrogen is
homogeneously distributed. Hydrothermal quartz (527
ppm), biogenic F-rich apatite (conodonts: 468 ppm),
F el ds pa rs
A
0.4
H yd ro th er m al
N, wt.%
5.0
4.0
3.0
2.0
1.0
0.5
0.3
0.2
Sediments
0.1
Organic Matter
0.0
0
1
2
3
4
10 12 14
C, wt.%
B
0.6
MD 84641
Monterey Bay
Northern Albany
ODP 1085A
Jet Rock
N, wt.%
0.5
0.4
0.3
0.2
0.1
0
0
2
4
6
8
10
12
14
C, wt.%
Fig. 4. (A,B) Nitrogen versus carbon weight percent (wt. %) data compiled from the literature including this study. NRA: nuclear reaction
analyses; EM: electron microprobe analyses. References: OM in Proterozoic and Archean kerogens (Beaumont and Robert, 1999); OM in oxic
and anoxic marine sediments (Rau et al., 1987; De Lange, 1998; Cowie et al., 1998; Calvert, 2004); OM in sapropels (Milder et al., 1999; Struck
et al., 2001); black shales (Rau et al., 1987; Gröcke et al., 2001; Jenkyns et al., 2001); and mudstones (Williams et al., 1995).
258
B. Orberger et al. / Chemical Geology 218 (2005) 249–264
compared to the sulfides and phosphates (C/N: 17–
26.7; Table 2). The latter show C/N ratios comparable
to those reported for Mesozoic and Quaternary oxic
and anoxic sediments and Paleozoic conodonts (Table
2 and Fig. 4A,B). The highest C/N ratios (174.8 and
193.28, respectively) have been measured in organic
matter.
5. Discussion
The origin of the high metal concentration in black
shales is not fully understood. Syn-sedimentary
enrichment of metals from sluggish seawater under
anoxic, sulfate reducing conditions which was episodically replenished by upwelling oxidizing seawater
was proposed as the formation of the poly-metallic
sulfides, phosphorites, barite and sapropelic beds of
the Yangtse platform in southeastern China (Mao et
al., 2002). Syn-sedimentary noble metal enrichment
was also proposed for the black shales of the
Mackenzie platform in Canada (Horan et al., 1994).
This model is based on 187Os/186Os and 187Re/186Os
values and similar inter-element patterns in black
shales and average seawater, which shows an enrichment factor of 106 to 108 in black shales for a broad
redox-sensitive element spectrum (Horan et al., 1994;
Mao et al., 2002). Syn-sedimentary infiltration of
diffuse dense metal and hydrocarbon-rich hydrothermal fluids into the black shales has been proposed
by Hulbert et al. (1992) and Grauch et al. (1991) for
the Selwyn basin in northern Canada, by Lott et al.
(1999) and Steiner et al. (2001) for the Yangtse
platform in southeastern China, and by Canet et al.
(2004) for the Prades Mountain black shales in
northeastern Spain. Various arguments favor this
model, for example, brecciated ore textures associated
with quartz veinlets host fluid inclusions indicating
99–263 8C fluid temperatures (Lott and Coveney,
1996). PAAS and NASC normalized REE pattern
show a clear positive Eu anomaly in the Prades
Mountain and Chinese black shales (Canet et al.,
2004; Steiner et al., 2001), as described from modern
hydrothermal vents (Michard et al., 1984). Sulfurisotope compositions of ore material in Chinese black
shales have been interpreted as being derived from
hydrothermal leaching of the underlying sediments
with low y34S values (Steiner et al., 2001; Coveney
and Chen, 1991). Our micro-mineralogical and
mineral-chemistry data (Orberger et al., 2003a,b)
suggested a combined model of two metal sources,
infiltration of syn-sedimentary hydrothermal alkaline
silica-rich hot fluids (b265 8C) accompanied or
succeeded by early diagenetic fluid.
The new data compiled in this study based on
feldspar, Ba, nitrogen and carbon chemistry clearly
support a hydrothermal fluid influx and metasomatism
of the black shales of the Selwyn basin. We observe
mainly tri-modal feldspar compositions, albite, dominantly K-feldspars and hyalophanes. K-feldspars are
sometimes completely surrounded by a Ba-rich rim
(Fig. 1B) and only a few analyses indicate anorthose
composition (Table 1a). The absence of continuous
solid solution between albite and K-feldspars may
indicate that these feldspars crystallized from different, successive fluids. Successive Na and K-metasomatism leading to high-temperature hydrothermal
albite and K-feldspar crystallization was reported
from the Batalha Granite, Brazil (Juliani et al.,
2002). Bimodal low-temperature hydrothermal albite
and adularia are known from the Pantelleria caldera
geothermal system, Italy (Fulignati et al., 1997).
However, in sedimentary environments, a diagenetic
origin of albite is common: 3Al2Si2O5(OH)4 (kaolinite)+2KAlSi3O8 (K-feldspar)+2Na+=2KAl4Si3O10
(OH)2 (illite)+2NaAlSi3O8 (albite) H++3H2O (Bjorlykke et al., 1995) and diagenetic K-feldspar overgrowth on detrital K-feldspar grains are observed.
Diagenetic xenotimes and apatites were also described
from Ordovician black shales in England (Lev et al.,
1998). The apatite and xenotimes are anhedral and
considerably larger (some tenths of microns) than the
apatite in the vicinity of the K-Ba-feldspars of the
Selwyn black shales (some microns and euhedral).
Furthermore, the co-existence and inter-growth relationship between xenotime and brannerite, a hightemperature U-oxide, as well as the hyalophane outer
rim of the K-feldspar supports a hydrothermal origin
of, at least, the K- and K–Ba-feldspars.
Feldspar-rich layers are known from many exhalative deposits worldwide. Some are rich in albite
(e.g., Sullivan, Canada (Shaw and Jodgson, 1986)), or
rich in anorthite (Prades Mountain (Canet et al.,
2004); Dachang, China (Pan and Amstutz, 1993)).
Based on the Eu–La enrichment, the anorthite layer of
the Prades Mountain black shales was related to hot
B. Orberger et al. / Chemical Geology 218 (2005) 249–264
hydrothermal reducing fluids that mixed with seawater (Canet et al., 2004). Ba-rich feldspars (celsian
or hyalophane), similar to those observed in the
Selwyn black shales, are also characteristic for
exhalative ores. For example, they were found in
association with Zn–Cu–Pb sedex deposits of the
Sudbury Basin, Canada (Whitehead et al., 1992). The
penetrative texture towards the inner parts of high Babearing feldspars can be interpreted as a diffusive
overgrowth related to the final fluid phase, which
was responsible for the crystallization of quartz,
phosphates and the U–Ti-oxides. High-temperature
diffusion experiments of Ba in compositionally
homogenous alkali feldspar and plagioclase show that
Ba diffusivities in the plagioclase and sanidine do not
differ more than an order of magnitude over a
temperature range from 775 to 1000 8C (Cherniak,
2004). However, at lower temperatures (e.g., 500 8C),
Ba-diffusion in sanidine is about two to four orders of
magnitude smaller than in plagioclase. In addition to
ion size and charge considerations, elastic properties
of the mineral lattice might influence the diffusion rate
(Cherniak, 2002; Blundy and Wood, 1994). Slower
diffusion rates imply longer periods of complete reequilibration, thus, Ba-zonation is preserved for
longer. For example, heating experiments showed that
at 8008, a 10-Am Ba-zone in sanidine persists over 108
years (Cherniak, 2002), and much longer preservation
time can be extrapolated for hydrothermal temperatures in the order of 260 8C. These observations
would explain why Ba-zonation is not observed in
albite of Selwyn black shales, while a clear Bazonation is still preserved in the K-feldspars.
The incorporation of Ba in the feldspar structure,
rather than forming barite, points to reducing fluids
with respect to sulfur, hence in this organic-matterrich environment, bacteria are dominantly reducing
seawater sulfate. The high buddingtonite component
of the K-feldspars provides further evidence for a
hydrothermal source. Buddingtonite is characteristic
of epithermal hot-spring systems (Krohn et al., 1993),
and has been found as pseudomorphosis after plagioclase in Quaternary andesites of Lake County,
California containing up to 8 wt.% NH4+, together
with ~3.5 wt.% of zeolitic water (H2O+) and small
amounts of OH-bound water (0.8 wt.%; Erd et al.,
1964). For the origin of this nitrogen, several
possibilities have to be taken into consideration,
259
namely (1) hydrothermal fluids; and (2) shallow-water
volcanic hydrothermal vents (T: 40–70 8C, pH: 3.13)
which can contain appreciable amounts of NH4+ (up to
42.5 AM/kg; Tarasov et al., 1999). However, deepwater hydrothermal vents contain significantly lower
contents of nitrogen (0.5–16 AM/kg; Von Damm,
1995). According to Krohn et al. (1993), modern hot
springs contain from 0.5 to 3.5 wt.% of nitrogen,
while fossil hydrothermal systems have lower nitrogen content (from 200 ppm to 2.5 wt.%). The lower
amount in fossil systems could be explained by
nitrogen loss during post-depositional alteration. The
nitrogen source at hydrothermal venting could be
magmatic nitrogen or alternatively derived from the
decomposition of organic matter associated within the
sediments (Lilley et al., 1993).
Our microscopic and PIXE investigations show
textural evidences that K–Ba-feldspar replaces
organic matter, the primary nitrogen carrier (Figs.
1B and 2). Thus, we suggest that the majority of NH4+
was produced during the replacement of the organic
matter and was directly incorporated into the Kfeldspar crystallizing from a hot hydrothermal fluid
(Fig. 5). High-temperature experiments on NH4+
incorporation in feldspars by Barker (1964) have
shown that NH4+ is more easily incorporated in the
presence of water, due to the expansion of the
structure of feldspar, which can then accommodate
the larger NH4+ ion. Under anhydrous conditions, NH4+
feldspar is metastable at 700–600 8C and can break
down to mullite, silica, ammonia and water. These
experiments suggest that water, which is reversibly
driven off at about 300 8C, is probably zeolitic. In
many minerals, substitution of the NH4+ causes a
change in the water content of the silicate (Barrer et
al., 1953; Hey and Bannister, 1962). Feldspars in the
Selwyn black shales may contain zeolitic water or
other volatiles (CH4, CO2) because most analyses are
below 100% (Table 1a). Carbon was detected up to
2.8 wt.% independently of the feldspar composition
(Table 1, Fig. 4). We can suggest similar sources for
nitrogen and carbon, organic matter or magmatic
carbon in the hydrothermal fluids, but additionally,
carbonates must be taken into consideration as a
carbon source. Carbon could be localized in fluid
inclusions or in structural sites and could also replace
water in volatile bearing feldspars. Unfortunately,
fluid inclusions have not been observed because of the
260
,C
4
sulfurized
tube worms
N, P
silicified
tube worms
OM
P,
OM
black shales
N,
OM
OM
OM
,S
S
OM
P,
hydrothermal
feldspar
FDSP
Qz
P
N,
conodonts
Ca phosphates
S
S
,S
P,
N, P
N,
hydrothermal
fluids
N,
hydrothermalism
tube worms
,C
and
NH
NH 4
sedimentation
NH4, HS-, H2S, CH4
NH4, HS-, H2S, CH4
P
E A R L Y
sea water
N,
D I A G E N E S I S
B. Orberger et al. / Chemical Geology 218 (2005) 249–264
Fig. 5. A schematic model for the release of nitrogen during sedimentation, early diagenesis and simultaneous hydrothermal venting. OM: OM;
Qz: quartz. OM decay releases nitrogen (N), phosphorous (P) and sulfur (S), into the pore fluids and to the hydrothermal fluids. N and P provide
nutrients for the vent fauna such as the worm tubes, which mineralized as sulfides and silica with aligned sphalerite clusters. Bio-mineralization
led to a new N-release. N and C are finally incorporated into hydrothermal minerals, preferentially K–Ba-feldspars, and in minor amount in
quartz. Sulfides and phosphates might have retained N as organic molecules.
opacity of the shales, thus, at this stage we cannot
discriminate between these two potential trapping
sites for carbon.
It is interesting to note that buddingtonite phase in
K-feldspar is characterized by a direct relationship
between the amount of K2O and the replacing NH4+
(Fig. 3a), while Ba-rich hyalophane phase shows an
inverse relationship (Fig. 3b). This can be interpreted
as the progressive replacement of monovalent NH4+ by
divalent Ba2+ during the diffusive intergrowth phase.
The penetrative texture observed in some samples
argues for a cation exchange process between Ba2+
and NH4+. Ba has an ionic radius of 0.144 nm which is
very similar to that of NH4+ (0.143 nm; Honma and
Itihara, 1981), while K+ ionic radius is of 0.133 nm.
The replacement of NH4+ by Ba has important
consequences because it is commonly assumed that
NH4+, after replacing K in the lattice sites of K-bearing
silicates is tightly retained (Sawhney, 1972). Here, we
show that, at least during high temperature hydrothermal circulation, NH4+ can be remobilized from
feldspars.
Nitrogen data on sulfides and phosphates are rare
in the literature. Biogenic and abiogenic sulfides
formed during the hydrothermal infiltration in the
Selwyn black shales. The high amount of organic
matter (1 wt.% bitumen) suggests that during decay,
considerable quantities of nitrogen were liberated.
Lückge et al. (1999) studied the diagenetic alteration
of organic matter by sulfate reduction in sediments
from the northeastern Arabian Sea. Lückge et al.
(1999) show that up to 60 % of total nitrogen and
more than 50% of total phosphorus were generated
during a burial depth interval between 0.4 and 2–3
m. Sulfate reduction in this narrow interval accounts
for the decay of 70% of the total organic matter
content. The nitrogen, phosphorus and sulfur liberated during organic matter decay in the Selwyn black
shales became available as nutrients for worm tubes
that, during growth, sulfurized and silicified (Fig. 5).
The mineralization of these worm tubes once again
liberated nitrogen and phosphorus to the water
column, although traces of nitrogen and carbon were
trapped in mineralizing sulfides and silica (Fig. 5). In
what chemical form the nitrogen was made available
to the water column is uncertain. PIXE analyses did
not show traces of K in sulfides that could be the
indirect evidence for the presence of NH4+, thus,
B. Orberger et al. / Chemical Geology 218 (2005) 249–264
nitrogen might be present in a stable form within
organic molecules as observed in conodonts of
various ages (Marshall et al., 1999). The C/N atomic
ratios can potentially be used as evidence for the
redox conditions of the deposition environment in
which these conodonts are found. Marshall et al.
(1999) suggest that C/N ratios N100 indicate that
decay and diagenesis of organic matter were
produced under reducing conditions, while low C/N
atomic ratios (b100) point to its production under
oxidizing conditions. As conodonts in the black
shales have C/N ratios of 25, this would suggest that
organic nitrogen was incorporated during early diagenesis and the oxidation of organic matter. The
reported nitrogen and carbon wt.% amounts measured in organic matter from mudstones, marine
sediments, sapropel, black shales and Precambrian
kerogens indicate that organic matter from oxic
marine sediments is characterized by a lower C/N
ratio than organic matter from reducing environments
(Fig. 4 and Table 2). Even with organic matter
contents in mudstones and black shales from a
variety of settings (Fig. 4b), the trend between
percent nitrogen and carbon remains relatively
consistent. Although the slope of the linear trends
varies between sites and geological time periods,
none are that distinct to suggest preferential loss or
burial of either component (see Calvert, 2004).
Variation about the linear trends is potentially caused
by localized/regional environmental conditions. In
Fig. 4a, the results generated from this study of
individual minerals have been included for comparison, which show that C/N ratios in hydrothermal
feldspars are different from those observed in other
mineral phases. These contrasting C/N ratios can be
attributed to two factors; (1) the favored capacity to
accommodate NH4+ in the feldspar structure compared to carbon and a higher local oxygenated
environment; and (2) higher localized oxidizing
conditions supported by the co-precipitation of
brannerite, apatite and xenotimes, a typical pegmatitic mineral assemblage.
Further studies are required to determine whether
the high C/N ratios in sulfides and biogenic diagenetic
apatite (Table 1) are more indicative of a reducing
environment and/or a differentiation of nitrogen
species preserved in organic molecules. The wholerock amount of nitrogen in the Selwyn black shales
261
can be estimated by assuming 5–10% of K-feldspar
containing an average 2 wt.% of N, 10–12 wt.% of
organic carbon with 400 ppm of nitrogen and 78–85
wt.% of sulfides, phosphates and quartz with 7000
ppm of nitrogen. The total nitrogen content range
from 0.2 to 3 wt.% and a C/N range between 21 and
34 from other organic-matter rich sediments (Table 2)
are comparable to that measured in the Toarcian
oceanic anoxic event and Kimmeridgian black shales
(Scholten, 1994; Gröcke, 2001).
6. Conclusions
Nitrogen fractionation in sedimentary–hydrothermal environments is a complex and dynamic process
depending on the availability of organic matter and
its evolution under the physico-chemical conditions
of the infiltrating hydrothermal fluids. The present
scenario suggests that hot hydrothermal fluids
infiltrated the black shales during sedimentation
and early diagenesis (Fig. 5). We propose that the
primary marine or terrestrial organic matter experienced microbial sulfate reduction and organic degradation (Fig. 5). During this process, nitrogen, P
and S were released to the pore fluids and the water
column. Hot hydrothermal fluids carried Ba and also
nitrogen, P and S liberated from buried decomposing
organic matter. The fossilization of conodonts into Frich Ca-phosphates occurred under oxidizing conditions, incorporating traces of nitrogen, possibly as
part of organic molecules. The simultaneous infiltration of the hot hydrothermal fluids favored the
growth of typical vent fauna, similar to that
occurring in present-day hydrothermal vents in deep
oceanic (Hessler and Kaharl, 1995) or in shallow
coastal settings (Tarasov et al., 1999). The previously
released nitrogen, P and S were used as nutrients by
the developing vent fauna, additional nutrients being
provided by the hydrothermal vents themselves.
Sulfurization and silicification of the vent fauna led
to a successive release of nitrogen. Part of this
nitrogen and the associated carbon was fixed in
sulfides. Hydrothermal feldspars assimilate NH4+
during the replacement of organic matter. However,
the final Ba–P- and REE-enriched fluid phase
enhanced the incorporation of Ba into the feldspar
structure, causing the observed negative correlation
262
B. Orberger et al. / Chemical Geology 218 (2005) 249–264
between Ba and nitrogen (Fig. 3b). Alkaline acidic
K–Ba-rich hydrothermal fluid infiltration into an
anoxic environment such as black shales leads to a
considerable nitrogen mobilization and fractionation
at mineral scale and to local variations of the redox
conditions.
Acknowledgements
This study was supported by Orsayterre FRE 2566
(CNRS-UPS), the bExobiologyQ (CNRS-INSUECNES), the bTransmetQ (CNRS), the PAI Barrandeprojects and the Ministry of Education, Youth and
Sports, Czech Republic (ME-44 grant). It is a
contribution to the IGCP 429 bOrganics in major
environmental issuesQ. We thank L. Hulbert (Canadian
Geological Survey) for the samples, the staff of the
Pierre Sqe Laboratory for helping during NRA, L.
Delabesse for the preparation of the microphotographs
and G. Roche for the drawing. We thank two
anonymous reviewers for their useful comments.
[CA]
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