Chichester, West Sussex, P019 1EB, England. Thepublisher's colophon is reproduced from lames. Gillison's drawing of the ancient Market Cross, Chichester.
.:
,~-
dety, aims to gather together e. The original series of e and illustrate the common ling the reader to identify been enlarged to include emes.
s usefulness to the practising of the gaps in this knowledge, lion.
tlIogy, University of Exeter . D. BRASIER, .. Department
Lecturer in of Geology,
ord
••. Department
of Geology,
CONODONTS: Investigative Techniques and Applications
icoIogical Studies, Plymouth
Editor:
RONALD L. AUSTIN, .. MURRA Y, Professor of
B.Sc.,Ph.D.
Senior Lecturer University of Southampton
also more advanced works important books recently
E.E.HUMPHRIES
of the State Museum for
~ Published by
ELLIS HORWOOD LIMITED Publishers· Chichester Ph=ing. Wye College
for
THE BRITISH MICROPALAEONTOLOGICAL
SOCIETY
First published in 1987 ELLIS HORWOOD LIMITED Market Cross House, Cooper Street, Chichester, West Sussex, P019 1EB, England The publisher's colophon is reproduced from lames Gillison's drawing of the ancient Market Cross, Chichester.
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Contributors . . . Preface. . . . . . . pter
1 I
tee
© 1987 The British
Micropalaeontological Society/ Ellis Horwood Limited
British Library Cataloguing in Publication Data Conodonts: investigative techniques and applications. (British Micropalaeontological Society series) 1. Conodonts 1. Austin, R. L. 11. Series 562'.2 QE899 Library of Congress Card No. 86-20061 ISBN 0-85312-907-X (Ellis Horwood Limited) ISBN 0-470-20697-7 (Halsted Press) Phototypeset in Times by Ellis Horwood Limited Printed in Great Britain by Butler & Tanner, Frome, Somerset
COPYRIGHT NOTICE All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the permission of Ellis Horwood Limited, Market Cross House, Cooper Street, Chichester, West Sussex, England.
1 Reo
Sec. 14.1]
14 The electron spin resonance technique in conodont studies
(
When nuclei wi are present in pa electron magnetic nuclear magnetic splits the ESR sign hyperfine splitting). ting is described by stant A which is I spectra by the sej peaks in terms of m (mT) or gauss (G» monly presented a derivative of the j against the magne .
Z. Belka, K. Miaskiewicz and T. Zydorowicz
Conodonts from Ordovician to Triassic age deposits have been investigated by electron spin resonance. This technique permits detection of the type and relative number of paramagnetic centres having different absorption signals. Conodonts exhibit signals induced by natural background radiation. The intensity of the signals is dependent upon the radiation dose received by the sample as well as upon the temperature. Electron spin resonance is not recommended for dating the age of conodonts but can be used for determining the age of completion of the latest heating event (i.e. most frequently the time ofthe last upheaval of the host-rocks). Because there is a correlation between the conodont colour alteration index (CAI) and the electron spin resonance spectral structure, it seems that this method may be applied to testing CAI values.
14.1 INTRODUCTION Electron spin resonance (ESR) (or electron paramagnetic resonance) is a branch of spectroscopy in which microwave-frequency radiation is absorbed by molecules, ions or atoms possessing unpaired electrons. This technique
A
enables detection of the presence and relative abundance of various paramagnetic centres. These centres include radicals, vacant lattice points and transition metals. ESR spectroscopy utilizes the different energy states which arise from the interaction of the magnetic moment of an unpaired electron with a static magnetic field, and the transition between them upon the application of microwave radiation. The basic equation is hv = g~BB where v is the fixed frequency of the microwave radiation, B is the strength of the static magnetic field at resonance, ~B is the electron Bohr magneton, and g is the Lande factor, which is a unique property of the electron as a whole. If the electron spin is the only source of the magnetism, then g = 2.0023. In atoms or ions an orbital angular momentum may be also present, in which case g is different from the free spin value. The g factor is measured from the spectra by the use of standards (commonly diphenylpicrylhydrazyl, with g = 2.0036) and the relationship B standard g g _ standard B
~ (a)
dA -(
dB
~
(b)L--
r
_
Fig. 14.1 - T spectrum presei and (l
The g factor and h are generally anis values along three second-rank tens
Sec. 14.1]
rresence and relative aramagnetic centres. meals, vacant lattice tilizes the different rom the interaction of an unpaired electron . and the transition application of micro-
l
231
samples the anisotropy is averaged, but in solid samples the anisotropy significantly alters the shape of the ESR spectrum. Comprehensive reviews of ESR spectroscopy have been published by Atherton (1973), Drago (1977) and Symons (1978). ESR spectroscopy has occasionally been applied to geological materials. Ikeya and Miki (1980) reported that only a few geological materials are suitable for dating by ESR, provided that the materials possess defects and traps of high thermal stability, e.g. apatites. ESR spectroscopy has been used to date stalactites (Ikeya, 1975), animal and human bones (Ikeya and Miki, 1980), and flints (Robins et al., 1978; Garrison et al., 1981). As a dating technique, it has been applied to conodonts by Morency et al. (1970). In addition, ESR spectroscopy has been used to A derive information on the formation of manganese nodules (Wakeham and Carpenter, 1974) and the distribution of Mn2+ in carbonates (Schindler and Ghose, 1970; Wildeman, 1970; Low and Zeira, 1972). Cubbitt and Wilkinson (1976) used ESR spectroscopy to classify Kansas shales, and Gilinskaya et al. (1982) have lalL_ classified Siberian apatites. The potential of B ESR spectroscopy in studies of clay minerals has been discussed by Friedlander et al. (1963). dA Conodonts consist of carbonate apatite, dB approximately of the composition of the mineral francolite (Pietzner et al., 1968). In addition to the mineral phase, conodonts contain an organic soft tissue characterized by cells, fibrous material and lumina (cf. Fahraeus and Fahraeus-van Ree, 1985). It is assumed that paramagnetic centres in conodonts can be Ibl L-------~-------r.:~ related to both the inorganic and the organic B materials. When the paramagnetic centres consist of organic free radicals and/or electron Fig. 14.1 -- The shape of a typical ESR traps, the relative number of the centres is spectrum presented as (a) absorption curve and (b) derivative curve. dependent upon external factors, such as the total radiation or temperature. The purpose of this study is to determine the The g factor and hyperfine coupling constant A dependence of the presence and intensity of are generally anisotropic, with three principal conodont ESR signals upon sample age in addivalues along three orthogonal axes (i.e. they are tion to temperature and the micromorphologisecond-rank tensors). In gaseous and liquid cal composition. Since heat alters conodont
When nuclei with spin value I not equal to 0 are present in paramagnetic molecules, the electron magnetic moment can interact with nuclear magnetic moments. This interaction splits the ESR signal on some lines (so-called hyperfine splitting). The magnitude of the splitting is described by the hyperfine splitting constant A which is measured directly from the spectra by the separation of the component peaks in terms of magnetic field units (millitesla (mT) or gauss (G)). The ESR spectra are commonly presented as derivatives, e.g. the first derivative of the absorption curve is plotted against the magnetic field strength B (Fig. 14.1).
conodont
y of the microwave of the static magne. the electron Bohr de factor, which is a on as a whole. If uy source of the magIn atoms or ions an n may be also present, r from the free spin measured from the dards (commonly rith g = 2.0036) and
Introduction
232
The electron spin resonance technique in conodont studies
colour (Epstein et al., 1977) and other investigations (cf. Hutton and Troup, 1966) have shown that the colour of many materials is related to paramagnetic centres, we have also compared the conodont ESR spectra with their colour alteration index (CAI) values. The study was performed on 17 conodont samples (see Table 14.1) which range in age from Ordovician to Triassic and were obtained from various localities in Poland, North America, Germany and Svalbard. The results presented are preliminary and indicate only qualitative analyses of the spectra. 14.2 EXPERIMENTAL CONDITIONS The conodont preparation was identical with that for strati graphic work. The minimum recommended weight of the conodont sample is
Table 14.1Sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
[Ch. 14
5 mg, i.e. about 300 conodont elements of various growth stages. The ESR spectra were recorded on an SE/X 2542 spectrometer (Radiopan, Poland), using the X band with 100kHz modulation. All spectra were recorded using the same modulation amplitude (0.1 mT) and microwave energy. This procedure does not damage the conodont material. 14.3 SPECTRAL STRUCTURE Four distinct signals can usually be observed in the ESR spectra of conodonts (Fig. 14.2). The signals have been labelled A, B, C and D in order of descending g factor values (Table 14.2). In addition, some spectra indicate hyperfine splitting, with a peak split into four component peaks (indicated by asterisks (*) in Fig.
Sec. 14.4]
14.2). The splitting i: fine splitting con la intensity ratio of 1: The structure of between samples. wl was always identical marked g factor anis 14.4 SOURCES OF Our preliminary allowed identificario netic centres within can only indicate therefore the signal question.
Conodont samples used in this study. Locality Erratic boulder from Poland Mojcza, Holy Cross Mountains, Poland Borehole Solarnia IG-1, depth of 845 m, Silesia, Poland Amsdell Creek, New York, USA Borehole ZMZ-23, depth of 281 m, Cracow Upland, Poland Harz Mountains, Germany Plucki, Holy Cross Mountains, Poland Dule, Holy Cross Mountains, Poland Borehole BK-318, depth of 662m, Cracow Upland, Poland Borehole B-1, depth of 940m, Cracow Upland, Poland Galezice, Holy Cross Mountains, Poland Lower Silesia, Poland Strzelce Opolskie, Lower Silesia, Poland Strzelce Opolskie, Lower Silesia, Poland Strzelce Opolskie, Lower Silesia, Poland Gasiorowice, Lower Silesia, Poland Botneheia, Isfjorden, Svalbard
Age
Conodont CAI
Arenig Llanvirn
1 1
Givetian Frasnian
4 3
Frasnian Famennian Frasnian Famennian
3.5 4 2 2
Famennian
2.5
Tournaisian Visean Ladinian Anisian Anisian Anisian Anisian Ladinian
2 2 1 1 1 1 1 2
J_
A
""I
~
OP
t Fig. 14.2 - ESR recognized signals
--~-
es
[Ch. 14
Sec. 14.4]
lont elements
of vari-
14.2). The splitting is characterized by a hyperfine splitting constant A of 2.31mT, and an intensity ratio of 1:3:3:l. The structure of signals B, C and D varied between samples, whereas signal A, if present, was always identical in shape and showed a very marked g factor anisotropy.
recorded on an SE/X opan, Poland), using nodulation. All spec:he same modulation microwave energy. lamage the conodont
TURE
Sources of signals
Peak asymmetry, as shown by signal A, is related to the mineral content of the conodont. Signals with g factors very similar to those of peaks Band C were noted in the spectra obtained from the apatites from various metamorphic rocks of Siberia (Gilinskaya et al., 1982). The signals were reported to have been produced by the radicals SO; and PO~-. Therefore, signal B is probably derived from the SO; radical, the signal of which has a congruent g factor. PO~- radicals originate from damage of the apatite structure when uranium ions replace calcium ions (Gilinskaya and Shcherbakova, 1975; Gilinskaya et al., 1982), the concentration of the radicals being
14.4 SOURCES OF SIGNALS
l
ually be observed in onts (Fig. 14.2). The d A, B, C and D in factor values (Table Jectra indicate hypersplit into four compoasterisks (*) in Fig.
Our preliminary investigations have not allowed identification of the specific paramagnetic centres within the conodonts. The data can only indicate possible signal sources, and therefore the signal source remains an open question.
!/c
t
Conodont
233
~
~ ~o
CAI
1
V
1 4 3
\f
11
,
vD
11\
0
~
\
3.5
4 2 2 2.5
2 2 1 1 1 1 1 2
I
, DPPH
1~
B
()
t \I
1mT '---J
t
Fig. 14.2 - ESR spectrum obtained for conodonts of the Amsdell Creek locality (sample 4), to show the recognized signals A, B, C and D present in the ESR spectra of conodonts: *, an additional signal split on four component peaks; DPPH, diphenylpicrylhydrazyl.
/'
234
The electron spin resonance technique in conodont studies
Table 14.2 - g Factor values of signals present in conodont ESR spectra. Signal
g factor
A
2.051±0.00l 2.004±0.OOl 2.000±0.001 1.999±0.001
B
C D
proportional to the uranium content. It is unlikely that signal C could originate from PO~radicals, because uranium was not detected in the conodont crowns, and the presence of uranium in the conodont bases is doubtful (Pietzner et al., 1968). We attribute signal D, the only centre which is stable to 1000°C (cf. table 14.3), to the presence of organic matter. A signal split into four component peaks (see Fig. 14.2) indicated the presence of three equivalent nuclei with a spin value I of !, the nucleus 19Fbeing a probable source.
14.5 SIGNAL INTENSITY AS A FUNCTION OF TIME AND TEMPERATURE Geological dating by ESR spectroscopy is based on the general principle that the signals emitted by certain geological materials (bones, flints, apatites, etc.) are proportional to the total radiation received. By assuming a constant radiation dose, the signals are therefore proportional to the duration of radiation. Conodonts contain very low concentrations of radioactive elements. Their ESR spectra were therefore induced by natural background radiation, i.e. cosmic rays and radiation from radioactive elements present in the host-rocks. The ESR spectra obtained from the conodonts investigated do not indicate such a simple dependence. It appears that the spectral structure is influenced by both time and temperature
[Ch. 14
and that the observed signal intensity is dependent upon both the radiation dose received by the sample and the temperature. (a) Influence oftime Although a dependence of the signal intensities on the sample age was not observed, the spectral structure enables a subdivision into three groups to be made. Spectra obtained from Triassic conodonts are distinct in that signal A is not exhibited. Only signal B is present, with signals C and D being either very weak or absent (Fig. 14.3). The second category of spectra consists of Devonian and Carboniferous conodonts. This category is characterized by the presence of peak B, and also signals A, C and D which are usually strong although, in some samples, signals A or C may be absent. The absence of the signals is more a function of heating than time. The third group consists of the spectra of Ordovician conodonts in which signals C and D are very weak relative to the strong signals of peaks A and B (see Figs. 14.3 and 14.4). (b) Temperature influence Two samples, samples 1 and 4, were heated in open air from 100-1000°C for a period of 0.5-96h. The resultant thermal annealing caused changes in the intensity of particular signals, with eventual disappearance of signals except peak D, the intensity of which had almost doubled by a temperature of 1000 °C (cf. Table 14.3 and Fig. 14.4). Because thermal annealing generates the decay of signals A, B and C, the intensity of these signals in altered conodonts with an advanced thermal history will be proportional to the time of the last geological upheaval and not to the total age of the sample. For such conodonts, the intensity of signal D will always be greater than that produced by the background radiation during the total geological age of the sample. Signal A, absent in the spectra obtained from Triassic conodonts (even those which are unaltered), occurs in the spectra of Palaeozoic
Sec. 14.5]
samples (cf. Figs. 1radiation persisting necessary to produ responsible for sign
®
----®
1
Fig. 14.3 - ESl showing a depei
~--------~
!S
[Ch. 14
ial intensity is depen:ion dose received by rarure.
Signal intensity as a function of time and temperature
Sec. 14.5]
samples (cf. Figs. 14.3 and 14.5). It appears that radiation persisting for longer than 225 ma was necessary to produce the paramagnetic centres responsible for signal A. Therefore, all Palaeo-
235
zoic conodonts which do not emit peak A must have been heated at a time after the Carboniferous Period.
f the signal intensities
observed, the specubdivision into three ectra obtained from inct in that signal A is al B is present, with either very weak or :ond category of specI and Carboniferous characterized by the lsosignals A, C and D ~ although, in some may be absent. The more a function of of the spectra of hich signals C and D the strong signals of l-t.3 and 14.4).
e md 4. were heated in =C for a period of thermal annealing nensity of particular appearance of signals ensity of which had erature of 1000 QC(cf. . Because thermal Iecay of signals A, B tese signals in altered need thermal history the time of the last lot to the total age of ants, the intensity of rreater than that proI radiation during the ! sample. the spectra obtained even those which are spectra of Palaeozoic
@
/
B
/
/0
®
I
/A B
/
1mT
li
I~
L..--.I
Fig. 14.3 - ESR spectra for conodonts from Triassic, Carboniferous, Devonian and Ordovician deposits, showing a dependence of the spectrum structure upon the age of sample. The circled numbers denote the investigated conodont samples (see Table 14.1).
The electron spin resonance
236
Table 14.3 - Changes in the intensity of ESR signals present in the spectra of conodonts, resulting from the influence of temperature (data based on the experimental heating of two conodont samples (samples 1 and 4) to 100°C,IS0°C and 200°C for 8h, to 500°C for 3h, and to 1000°C for 6h. Change in intensity at the following temperatures Signal A B C D
100°C
150°C
No ~ change No No change change
t
0
No change
0
0
t
0
0
No change
t
t
t , increase;
14.6 COMPARISON WITH CONODONT
1000 °C
0
~
t
No No change change
~ ,decrease;
200 °C 500°C
technique
in conodont studies
[Ch. 14
a temperature of 500°C. Upon closer examination, signal B is shown to be a band consisting of two weakly separated signals, indicated B' and B" (cf. Fig. 14.6). With increasing CA! values, an increase in the intensity of signal B" is noted in relation to signal B'. Only one of the investigated samples, sample 9, departed from this trend, by exhibiting a signal B structure similar to that of conodonts with a CA! of 3.5 in spite of the sample colour corresponding to CA! 2.5. It appears that the absence of a precise quantitative correlation between the intensity of ESR signals and the conodont CAI value is a result of the thermal instability of the paramagnetic centres responsible for signals A, Band C (cf. Table 14.3), coupled with the fact that the intensity of the ESR signal is a combined function of both the radiation time and the thermal history of the conodonts.
Sec. 14.8]
the basal filling i c increase in the inten Therefore odont crowns copy. This is concentration
it is in alone of ad of tra
0, absent.
OF ESR SPECTRA CAI VALUES
Although a precise correlation between the intensity of ESR signals and conodont CAI values has not been recorded, nevertheless the ESR spectra of conodonts with an identical CAI value do show some similarities. These similarities are in accord with data which we obtained from the experimental heating of conodonts. The spectra of conodonts which have a CAI value of 1 show signals C and D to be always weaker than signal B, or to be absent, e.g. the Triassic samples. With higher CA! values, an increase in intensity of peaks C and D is initially observed. Signal C decays in conodonts with a CA! of 3.5 or higher, whereas both signals D and Bare strong (Fig. 14.5). The structure of signal B shows the strongest correlation with CAI value. The observation is surprising as experimental heating of conodonts does not indicate that signal B is responsible for conodont colour. This is why signal B decays at
14.7 EFFECT OF MICROMORPHOLOGICAL COMPOSITION Pietzner et al. (1968) showed that there are differences in the chemical compositions of the conodont crown and base. The basal filling is more fine grained and contains a higher concentration of trace elements; for example, yttrium can be enriched in the base by a factor of 20 greater than that in the crown. Only in one sample investigated (sample 7, from the Frasnian deposits of the Holy Cross Mountains) were a large number of conodonts observed in which the crown was attached to the base. The sample was divided into two portions, one with conodont crowns only, and the other in which crowns were attached to bases. Both portions were investigated by ESR spectroscopy. The portion containing crowns attached to the bases yielded spectra with signals which were all stronger than the portion comprising crowns alone; the intensities of signals A, Band C were twice as strong, and signal D was five times as strong. The greater organic content in
Fig.l-U-E
s
[Ch. 14
';pon closer examinbe a band consisting signals, indicated B' Ith increasing CAI uensity of signal B" is B'. Only one of the ple 9, departed from a signal B structure s with a CAI of 3.5 in orresponding to CAI
Application
Sec. 14.8]
237
the basal filling is one reason for the marked increase in the intensity of signal D.
most probably due to later (post-mortem?) inputs (Pietzner et al., 1968).
Therefore odont crowns copy. This is concentration
14.8 APPLICATION ESR spectroscopy was proposed as a dating technique for conodonts by Morency et at. (1970). The technique consists of subjecting the
it is important to investigate conalone when using ESR spectrosof additional importance, as the of trace elements in the bases is
D~
bsence of a precise erween the intensity odont CAI value is a dlity of the paramagJI signals A, Band C with the fact that the I is a combined functime and the thermal
Qc :AL
owed that there are 1compositions of the !. The basal filling is tains a higher conrents; for example, the base by a factor of crown. vestigated (sample 7, of the Holy Cross mmber of conodonts m was attached to the led into two portions, only, and the other in ihed to bases. Both :d by ESR spectrosling crowns attached ra with signals which e portion comprising es of signals A, Band md signal D was five :er organic content in
B~
/ ~
1 mT
'----'
Fig. 14.4 - ESR spectrum for Ordovician conodonts (sample 1) and its changes after heat treatment.
238
The electron spin resonance technique in conodont studies
conodonts to small doses of laboratory y-radiation in order to establish the dependence of signal growth upon the radiation dosage. If the signal growth is linear, then it is possible to
[Ch. 14
extrapolate to obtain the geological age of the sample if the total radiation dose is known (cf. Zeller et al., 1967). This investigation, however, has shown
Sec. 14.10]
@
B'-
G) @
®
@ CA/= 2 --------=--
-........J
@)
®
) ~
/ /
® Fig. 14.6 - Con signal B with IX numbers deno e samples
1 mT
Fig. 14.5 - ESR spectra of conodonts with different CAI values. Note the distinct increase in signals Band D for conodonts with a maximum thermal history. The circled numbers denote the investigated conodont samples (see Table 14.1).
that, for the 17 COD signal intensity is a most definitely, te event, even of short of signals A, B ar growth of signal :c from the ESR Sped misleading. In addn Palaeozoic conodo appeared. This diff intensity as a result
[Ch. 14 ~logical age of the dose is known (cf.
References
Sec. 14.10]
@~ S'_(\. ~ ~
'ever, has shown
CD
