Late Minoan Jar ca. 1450-1400 bce

Elemental Geochemistry of BlackShales--Statistical Comparison of Low-Calcic Shales with Other Shales

M.S. Quinby-Hunt and Pat Wilde,
Marine Sciences Group, University of California, Berkeley, CA 94720
C.J. Orth,
Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545
W.B.N. Berry,
Marine Sciences Group, University of California, Berkeley, CA 94720

Modified for the World Wide Web April 1997 After U. S. Geological Circular 1037 Metalliferous Black Shales and Related Ore Deposits,
R. I. Grauch and J. S. Leventhal (eds.) p. 8-15 (1989)


Introduction

Anoxic sedimentary environments of the past are recorded in black shales. Wedepohl (1968) noted that "average" shales reflect shallow-water sediments that accumulated under oxidizing conditions. Thus, the composition of so-called "average" shales does not represent past conditions where anoxic water and sediments were a significant component of the potential geologic record. Secondarily, as sediments resulting in black shales can be deposited under both oxic and anoxic conditions and in shallow to deep water, a black-shale composite better indicates oceanic conditions during the time of deposition than composites based primarily on shallow-water oxic conditions. Accordingly, we analyzed 287 samples of stratigraphically well documented low calcic Paleozoic and Mesozoic black shale (table 1), using neutron activation analysis, for: Na, Mg, Al, K, Ca, Sc, Ti, V, Cr Mn, Fe, Co, Zn, Ga, As, Se, Rb, Sb, Cs, Ba, Hf. Ta, Ti and U. (table 2a)and for the rare-earth elements La, Ce, Nd, Sm. Eu, Tb, Dy, Yb, and Lu (table 2b).

This study expands the compilation of North American black shales by Vine and Tourtelot (1970) by the addition of 16 elements, including the first compilation of the rare-earth-element series for black shales. In this report, our data are compared with the black-shale compilation (V&T:BSC) of Vine and Tourtelot (1970) and with other general shale compilations such as Turekian and Wedepohl (1961) and Gromet and others (1984).

This study (called the Marine Sciences Group Black-Shale Composite or MSG:BSC) emphasizes the Paleozoic (Ordovician, Silurian, and Carboniferous) and middle Mesozoic (Jurassic), as most black shales were deposited then (Berry and Wilde, 1978). The MSG:BSC study includes samples (total 287, table 1) from the Cambrian (1), Lower Ordovician (90), Middle Ordovician (33), and Upper Ordovician (49), Lower Silurian (37), Carboniferous (19), and Jurassic (58). Samples were collected from continental Europe, Scandinavia, and North and South America. The samples were identified, stratigraphically and paleontologically, by W.B.N. Berry or were from well-documented collections (for example, Sedgwick Museum, Oxford, England; Museum of Paleontology, Berkeley, Calif.). The MSG:BSC study focused on field-identified black shales; nearly 90 percent of the samples contained less than 5 percent CaCO3. The study specifically excluded shaly limestone or limy shale containing greater than approximately 15 percent CaCO3 (Pettijohn, 1949, p. 291), in order to focus on the composition of the argillaceous black-shale facies. (MSG data available as Lotus 123 file).

Analytical Procedures

Neutron Activation Analysis

A suite of 41 elements was determined in the samples (table 2). All elemental abundances were determined at the Los Alamos National Laboratory using automated neutron activation analysis (Minor and others, 1982). Use of a single laboratory minimized the possibility of error commonly introduced by variations in analytical techniques or laboratories (Fairbairn and others, 1951; Quinby-Hunt and others, 1986). Most of the elemental concentrations were determined using conventional reduction of gamma-ray spectra of the radioactive isotopes.

Uranium was determined by delayed neutron counting. The automated system was calibrated using a collection of U.S. Geological Survey, U.S. National Bureau of Standards, and Canadian Geological Survey standard rocks. The system was checked periodically for stability against these standards.

Statistical Methods

Frequency distributions for each element were plotted against the midpoints of distribution bins used in determining the distribution (fig.1). Means and modes of elements in the MSG:BSC study are listed in table 2 with mean concentrations of Turekian and Wedepohl (1961) and Gromet and others (1984) for shales and means and modes of the V&T:BSC study. In determining the mode and to facilitate comparison with the study of black shales by Vine and Tourtelot (1970, p. 257), we used their geometric concentration ranges. Each bin (fig. 1) has boundaries of 1.2, 1.8, 2.6, 3.8, 5.6, 8.3, 12, 18 ppm, and so on. For example, the midpoint of the bin containing data ranging from 0.83 to 1.2 ppm is 1; the midpoint for the bin containing data from 1.2-1.8 ppm is 1.5 ppm. The next consecutive midpoints are 2, 3, 5, 7, 10 ppm, and so on. The mode, as the midpoint in the frequencydistribution bin, is presented because the mean is sensitive to a few extreme values and may not indicate the central tendency in a distribution.

If less than 60 percent of the samples did not contain detectable levels of an element, we have presented the mean of the detectable concentrations; for the mode, we have listed the percentage of all samples containing less than the detectable concentration. For example, 134 (47 percent) of the 287 samples contained measurable amounts of calcium. The average calcium concentration for samples in which it was detected (fig.1) was 17,000 ppm. However, the average sample containing undetectable quantities of calcium contained less than 800 ppm. In fact, 60 percent of all samples contained less than 2,600 ppm. Thus, the mode is given as "60 percent < 2,600 ppm" in table 2.

Discussion

The low-calcic shales analyzed for this study showed less variability of composition for the unimodal elements than was observed by Vine and Tourtelot (1970) because, in this study, the samples were basically clays and other detrital minerals having little carbonate to complicate the mineralogy. The multimodality seen in these low-calcic shale samples suggests that factors affecting composition of black shales, other than source composition, are present during deposition and diagenesis.

This study demonstrates that the black-shale facies is chemically complex and contains several chemofacies. Ideally, if the black-shale facies observed in the field was a single chemical-sedimentary quantity, the elements characteristic of that facies would show a unimodal distribution. When compared with the classic shale composites (Turekian and Wedepohl, 1961; Gromet and others, 1984; Taylor and McLennan, 1985) and with the study of black shales by Vine and Tourtelot (1970), this study shows excellent agreement for the elements generally associated with detrital minerals, such as aluminum, titanium, and scandium. The trimodality of manganese concentrations indicates the possibility of three redox zones in black shales. Multimodality in volatile elements such as arsenic and antimony suggests that, additionally, a volcanic source may have an important chemical impact on the black-shale faeies. The mean concentrations of elements of the organic and volatility indicators, vanadium, zinc, bromine, strontium, molybdenum, antimony, barium, and uranium, are significantly higher than the means for the shale composites and the Vine and Tourtelot (1970)

compilation. This difference is due to the inclusion of the Dictyonema-bearing black shales of Balto-Scandia in our composite. Several authors have noted that certain black shales are enriched in various metals (Goldschmidt, 1954; Wedepohl, 1964; Vine and Tourtelot, 1970; Tardy, 1975; Holland, 1979; Berry and others, 1986). The exclusion of these black shales from alternative compilations biases projections of the global oceanic conditions in the Paleozoic and Mesozoic. We prefer to include these samples, noting both the mean and the strong multimodal character of their frequency distributions. This approach identifies elements typifying an average black shale but also identifies elements with multiple modes that show that black shales can form under two or more chemical environments.

Conclusions

Black shales, which seem to be an easily identifiable and distinct sedimentary facies, represent a complex chemical system that contains several chemofacies and includes a wide range of redox conditions. The variability of composition observed in this study of lowcalcic black shales shows the need to examine distributional modes other than the mean when characterizing chemofacies associated with depositional environments. Comparisons with other composites, with respect to redox sensitive elements, indicate that the black-shale facies have a wider range of redox conditions than seen in published general composites. As such composites (for example, the North American shale composite) are used for normalization of chemical data, their usefulness as a standard, particularly for investigations of non-oxic shales, must be questioned. On the other hand, a well-documented black-shale composite may be a more valid standard for paleo-oceanographic and comparative chemical studies, especially in Paleozoic and earlier times when the redox conditions in the oceans were not as uniform or as oxic as in the modern oceans.

References

Berry, W.B.N., and Wilde, Pat, 1978, Progressive ventilation of the oceans--an explanation for the distribution of the Lower Paleozoic black shales: American Journal of Science, v. 278, p. 257-275.

Berry, W.B.N., Wilde, Pat, Quinby-Hunt, M.S., and Orth, C.J., 1986, Trace element signatures in Dictyonema shales and their geochemical and stratigraphic significance: Norsk Geologisk Tidsskrift, v. 66, p. 45-51.

Fairbairn, H.W., and others, 1951, A cooperative investigation of precision and accuracy in chemical, spectrochemical and modal analysis of silicate rocks, in Contributions to geochemistry, 1950-51: U.S. Geological Survey Bulletin 980, p. 1-71.

Goldschmidt, V. M., 1954, Geochemistry: Oxford, England, Oxford University Press, 730 p.

Haskin, M.A., and Haskin, L.A., 1966, Rare earths in European shales--A redetermination: Science, v. 154, p. 507-509.

Holland, H.D., 1979, Metals in black shales--A reassessment: Economic Geology, v. 74, p. 1676-1680.

Minor, M.M., Hensley, W.K., Denton, M.M., and Garica, S.R., 1982, An automated activation analysis system: Journal of Radioanalytical Chemistry, v. 70, p. 459-471.

Pettijohn, F. J., 1949, Sedimentary rocks: New York, Harper and Brothers, 526 p.

Quinby-Hunt, M.S., McLaughlin, R.D., and Quintanilha, A.T., 1986, Instrumentation for environmental monitoring, Volume 2, in Greenberg, A.E., and Morton, G.A., eds., Water (2nd ed.): New York, Wiley, 982 p.

Tardy, Yves, 1975, Element partition reties in some sedimentary environments, I. Statistical treatments, II. Studies on North American black shales: Strasbourg Sciences Geologiques Bulletin, v. 28, p. 59-95.

Taylor, S.R., and McLennen, S.M., 1985, The continental crust--Its composition and evolution: Oxford, England, Blackwell Scientific Publication, 3i2 p.

Turekian, K.K., and Wedepohl, K.H., 1961, Distribution of the elements in some major units of the Earth's crust: Geological Society of America Bulletin, v, 72, p. 175-191.

Vine, J.D., and Tourtelot, E.B., 1970, Geocbemistry of black shale deposits--A summary report: Economic Geology, v. 65, p. 25~272.

Wedepohl, K.H., 1964, Untersuchen am Kupferschiefer in Nordwestdeutschland; Ein Beitrag zur Deutung der Genese bituminoser Sedimente: Geochimica et Cosmochimica Acta, v. 28, p. 305-364.

Wedepohl, K.H., 1968, Chemical fractionation in the sedimentary environment, in Ahrens, L.H., ea., Origin and distribution of the elements: Oxford, England, Pergamon Press, p. 999-1016.

Table 1 Summary of samples for study of elemental chemistry of black shales
AgeLocationSamplesSource*
JurassicOxfordianSwitzerland6WBNB
LiassicEngland, UK52WBNB
CarboniferousWesphalianWales, UK9RAR
SilurianLandoveryScotland, UK4WBNB
LandoveryScotland, UK3SMCU
LandoveryWales, UK2WBNB
LandoveryNorway1WBNB
LandoveryNew Brunswick, Canada1WBNB
LandoveryMaine, USA1WBNB
OrdovicianAshgillIdaho1WBNB
AshgillScotland, UK48WBNB
AshgillScotland, UK1SMCU
upper-middleNew York, USA4WBNB
upper-middleNew Jersey, USA7UCMP
middleWales, UK1WBNB
middlePenna, USA6LP
middlePenna, USA5UCMP
middleNorway3WBNB
middleNew York4PW
middleNew Jersey1WBNB
middleMaine1WBNB
early-middleNewfoundland1SMCU
TremadocNorway23WBNB
TremadocSweden21LUC
TremadocLevis, Canada15WBNB
TremadocWales, UK14WBNB
TremadocBolivia6BCPC
TremadocNew York, USA4WBNB
TremadocEstonia2SMCU
TremadocBelgium2SMCU
TremadocNew Brunswick, Canada2SMCU
TremadocDenmark1SMCU
Cambrianupper-middleNorway1WBNB
* Sources:
WBNB: William B. N. Berry, University of California, Berkeley
SMCU: Sedgwick Museum, Cambridge University (mainly from theBulman collection)
UCMP: Museum of Paleontology, University of California, Berkeley
LP: Lucien Platt, Bryn Mawr University
RAR: Robert A. Raisewell, Leeds University
PW: Pat Wilde, University of California, Berkeley
LUC: Lund University Collection, Sweden
BCPC: Bolivian California Petroleum Company, La Paz, Bolivia

Table 2a Marine Science Group Composite compared to other shale and black shale composites
NON-RARE EARTH ELEMENTS
Marine Sciences Group Black Shale Composite
Black Shale
Shale
ELEMENTThis ReportV&T (1970) T&W (1961)T&M (1985)NASC (1984)
MeanMode*MinimumMaximumNMeanMode*MeanMeanMean
Na5,2607,00038024,8002877,00010,0009,6008,9007,500
Mg10,40010,0002,49040,2002847,0007,00015,00013,00017,000
Al82,100070,00014,700129,90028770,00070,00080,000100,00089,000
Cl240+<120 (75)301,78099NRNRNRNRNR
K29,90020,0003,50083,60028620,00030,00026,60031,00032,000
Ca17,100+<2,600 (60)74073,30013415,00010,00022,1009,30025,000
Sc15.6151.530.72871010131614.9
Ti4,3405,0008507,3602862,0002,0004,6006,0004,200
V500150396,260187150150130150NR
Cr111100104182781007090110124.5
Mn383200153,780287150100850850465
Fe36,80050,0002,50099,80028720,00030,00047,20050,00040,000
Co16.9200.41082871015192325.7
Zn310+<38 (65)73,800106<300++9585NR
Ga21.9+<38 (80)7.843.211920201920NR
As28.8201.3152.6279NRNRNRNR28.4
Se5.6+<5.6 (80)1.029.876NRNRNRNRNR
Br4.0+<2.6 (76)0.936.1106NRNRNRNR0.69
Rb13115018322287NRNRNR160125
Sr310+<260 (75)15053021200200300200142
Zr230+<380 (90)391,0109970100160210200
Mo65+<18 (65)0.160011810++2.61.0NR
In0.21+<0.18 (76)0.080.666NRNRNRNRNR
Sb5.71.50.270.6260NRNRNRNR2.09
Cs8.6100.519.6287NRNRNR155.16
Ba1,12030014050,200279300300139650636
Hf4.350.818.4287NRNRNR5.06.3
Ta0.90.70.31.8275NRNRNRNR1.12
W3.3+<3.8 (82)1.012.561NRNRNR2.72.1
Au0.023**0.0070.05314NRNRNRNRNR
Th11.6101.132.3287NRNRNR14.612.3
U15.231.2442.6287NRNRNR3.12.66

EXPLANATION OF ANNOTATION
Concentrations in parts per million.
NR, not reported;
<, less than;
( ), percent of samples containing less than the modal value of the element
* The mode is the midpoint of distribution bins used for determining the frequency distribution by the method of Vine and Tourtelot (1970) (see text). If less than 60 percent of the samples contained detectable amounts of the element, the frequency distribution of the detection limit was examined in conjunction with the detectable concentrations and the percentage of samples below a certain concentration determined.
+ These values are means of the samples in which less than 60 percent of the samples contained detectable concentrations.
++ For these elements Vine and Tourtelot (1970) indicated a majority of samples less than the lower limit of detection using spectrography.
** Too few values available for calculation of meaningful mode.

SOURCES OF SAMPLES
This Report: Quinby-Hunt and others, (1989) Data available as Lotus 123 file.
V&T 1970: Black Shale Composite Vine and Tourtelot (1970).
T&W 1961: Shale Composite Turekian and Wedepohl (1961).
T&M 1985: Post-Archean Australian shales Taylor and McLennan (1985).
NASC 1984: North American Shale Composite, Gromet and others (1984).
H&H 1966: European shales, Haskin and Haskin (1966)
Chondrite: Rare Earth Reference, Haskin and others (1966).

Table 2b Marine Science Group Composite compared to other shale and black shale composites
RARE EARTH ELEMENTS
Marine Sciences Group Black Shale Composite
Black Shale
Shale
Chondrite
ELEMENTThis ReportV&T (1970) T&M (1985)NASC (1984) H&H (1966) H et al. (1966)
MeanMode*MinimumMaximumNMeanMode*MeanMeanMeanMean
La44501011128630303831.141.10.30
Ce8010013197282NRNR8066.781.30.84
Nd555014130214NRNR3227.440.10.58
Sm6.270.827.5280NRNR5.65.597.30.21
Eu1.271.50.215.26285NRNR1.11.181.520.074
Tb0.9510.062.42265NRNR0.770.851.050.049
Dy4.8550.2510.73285NRNR4.4NRNR0.31
Yb3.1031.296.21286NRNR2.83.063.290.17
Lu0.470.50.070.90270NRNR0.430.4560.580.031

EXPLANATION OF ANNOTATION
Concentrations in parts per million.
NR, not reported;
* The mode is the midpoint of distribution bins used for determining the frequency distribution by the method of Vine and Tourtelot (1970) (see text). If less than 60 percent of the samples contained detectable amounts of the element, the frequency distribution of the detection limit was examined in conjunction with the detectable concentrations and the percentage of samples below a certain concentration determined.

SOURCES OF SAMPLES
This Report: Quinby-Hunt and others, (1989) Data available as Lotus 123 file.
V&T 1970: Black Shale Composite Vine and Tourtelot (1970).
T&W 1961: Shale Composite Turekian and Wedepohl (1961).
T&M 1985: Post-Archean Australian shales Taylor and McLennan (1985).
NASC 1984: North American Shale Composite, Gromet and others (1984).
H&H 1966: European shales, Haskin and Haskin (1966)
Chondrite: Rare Earth Reference, Haskin and others (1966).