Trace element concentrations in conodonts measured by the Bochum proton microprobe

. li!iB

cQk!A SW

__

ErSEYVIER

Beam Interactions with Materials&Atoms

Nuclear Instruments

and Methods in Physics Research B 130 ( 1997) 636-640

Trace element concentrations in conodonts measured by the ~ochum proton microprobe Frank Bruhn ‘v*, Christoph Korte ‘, Jan Meijer b, Andreas Stephan b, JQn Veizer a*c ’ ~~lst~t~t~~r Ceolttgie,

R~~r-U~~~fr.~it~t

h institut,ftir Physik mit Ionenstrahlen. ’ Derry/Rust

Research Unit. Ottuwa-Curleton

3~~~urn, 44780 3~~~~~rn~ Germany

Ruhr-Unir
Geoscience

Bochum, 44780 Bochum,

Centre, Unioersity

of Ottawa,

Ottuwu,

Germany Ontario

KIN

6N.5, Cunada

Abstract The Proton Microprobe at Bochum has been used to study trace element contents and elemental distributions in a suite of Devonian and Triassic conodonts, skeletal parts of marine invertebrates composed of the CO,- and F-rich m~ification of apatite (francolite). The conodonts contain a plethora of substituting elements (Mn, Fe, Sr, Y, light REE) at high concentrations, and linescans and elemental maps reveal preferential enrichments of many of these elements around skeletai rims. This observation tends to support the concept of post-depositional chemical aiteration of the conodonts, but within an environment that still reflects the ambient marine conditions. ~ons~uently, despite the high degree of substitution in the apatite structure, the Sr isotope signals of ancient seawater may still be retained by the samples. 0 1997 Elsevier Science B.V.

1. Introduction The proton microprobe has proved to be a valuable tool for studies of trace element patterns in brachiopod shells, the latter serving as a carrier-phase for isotopes in paleoceanographic studies [ i,2]. Unfortunately, the how-Mg-calcitic shell material of desirable quality and stratigraphic resolution is not available for the entire Phanerozoic. Other fossil skeletal materials must therefore be employed for such purposes. Conodonts, index fossils utilized mainly for biostratigraphic studies, are now being increasingly employed as suitable carriers that may preserve strontium isotopic composition of the co-

* Corresponding author. Present address: CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113, Australia. Fax: + 61 2 9490 892 1;e-mail: f.b~n~dem.csiro.au

eval ambient seawater. Conodonts are minute (5 2 mm> skeletal parts of a group of marine invertebrates that became extinct at the Triassic/Jurassic boundary. They are composed of the CO,- and F-rich modification of apatite (francolite: Ca,Na, ,4 (PO,),,,(GO,),,,F,,,,(H,O), 85 [3I). They are conducive to (isotope-~st~tigraphic studies because they (a> experienced rapid evolutionary and morphological changes throughout geologic history, (b) were abundant on a world-wide scale, and (c) have incorporated sufficient strontium into the crystal structure (several thousand ppm) to be analyzed for the “Sr/ 86Sr ratio. Nevertheless, and despite the claims that apatite is more resistant to post-depositional alteration than calcite 143, conodonts are prone to chemical changes in the course of diagenesis. The conodont colour alteration index (‘CAY [S]), may serve as one measure of the degree of alteration of

0168-583X/97/$17.00 0 1997 Elsevier Science B.V. Ail rights reserved F’II SO168-583X(97)00261-9

637

F. Bruhn et al. / Nucl. insrr. and Meth. in Phys. Res. B 130 f 1997) 636-640

was derived from analysis of single-element metal foils (Ti, Fe, Ag) of 250 pm thickness. Repeated measurements of standard reference material AGV- 1 yielded external accuracies and internal precisions of - 10% for a number of elements [S]. Beam current was in the range of a few nA, resulting in a typical accumulated beam charge of 1 to 10 p.C for every measurement. In view of the high penetration depth of the proton beam, additional care had to be taken to probe areas of sufficient thickness, or, if not feasible, to obtain accurate info~ation on the thickness of the analyzed samples. In few cases, thickness was below 30-50 km, resulting in Ca concentrations of less than 40%. In order to correct for this, sample thickness entered into the GUPIX software was varied until the Ca reached the required stoichiometric concentration of 40%. The detection limits CLOD’s), depending on the element of interest and beam charge, were usually in the range of lo-30 ppm, lo-30 ppm, and lo-20 ppm for Mn, Fe, and Sr, respectively. However, mainly due to peak overlaps (91, they were often considerably higher (x . 10 to x . 100 ppm) for the REE. As a result, the accuracy for REE data may be in the range of +20% when the measured concentrations are well above the detection limits, but can reach values in excess of 5.50% when near the detection limit. For this reason, only REE concentra-

the skeletai material, with a range from 0 (minimal thermal influence) to 8 (metamorphic conditions at temperatures above 600°C). In this study, we utilize trace element microanalysis as another tool that enables us to interpret the quality of Sr isotope data derived from conodonts.

2. Methods For the isolation of single conodonts, masses as high as 3 kg of limestone samples were crushed to small chips and reacted with 6% acetic acid. The conodonts were then hand-picked from the insoluble residue under a binocular microscope, mounted on a suprapure (< 1 ppm contamination) silica glass backing and subsequently polished. In addition to point analyses, using a 3 MeV proton beam with a diameter of - IO pm, linescans and elemental maps have been performed in order to visualize spatial distribution of trace elements within samples. In order to attenuate the dominant Ca peak, a 54 pm Al filter and a 50 pm Mylar filter were mounted between the samples and the Si(Lil detector. Absolute concentrations from the measured PIXE spectra were calculated by the GUPIX software package [6,7]. The instrumental constant H that comprises the remaining unknown parameters (i.e. the detector solid angle and the calibration of the beam dose monitor)

Table I Bulk elemental

Sample

concentrations

of some Triassic conodonts Element

Age

Ca % K61A

Triassic

7375-

Triassic

1

7375-2

Palazl7B

Devonian

Devonian

Triassic

Zn

Rb

Sr

Y

La

Ce

Pr

Nd

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

18

129 12

50 6

24

2195 12

34 8

302

239 201

168 150

114

LOD

41 0.03

9

622 5

16 2

5

2639 5

109 5

120

355 78

89 65

196 55

cont. LOD

36 0.01

33 4

1012 3

12 1

4

3295 2

51 2

91 48

277 31

147 26

133 25

cow. LOD

38 0.03

54 9

1877 5

18 2

14

3033 3

53 4

209 100

428 64

251 54

171 49

COIIC.

41 0.1

115 34

7146 24

76 9

76

1795 13

188 8

948 420

1469 247

559 218

248 196

COEK.

LOD cont. = concentration:

Fe

ppm

36 0.07

COIIC.

LOD K67

Mn

LOD = limit of detection;

void spaces mean that the concentration

is below the LOD.

XII. GEOLOGY

AND PLANETARY

SCIENCE

638

F. Brtrhn ef a/. / Nucl. bsfr. cmd Meth. in Ph_w. Res. 3 130 f f Y971 636-640 Mn

CZt

Fe

Fig. 1. Elemental map of B Triassic conodont, as outlined in the lower left corner of the figure. Ca and Sr reveal homogeneous dist~butions, whereas Mn, Fe, Y and Nd show signi~cant enrichment at the conodont rims.

tions above 3 LOD for the given element ingful.

3. Geochemical

Fig. 2. Thin section photomicrographs of several Triassic conodonts, with points indicating the PIXE micro~~yscs in Table 2. Point 2 of sample Paiazl and point 1 of sample Palazl7 are within the basal body, while all other measurements were cited in the crown material of the conodonts.

are mcan-

observations

element patterns. Bulk analyses (Table 11, perfo~ed with the proton beam scanned over the entire samples, show significant amounts of Mn (up to Il.5 ppm), Fe (129-7146 ppm), Zn (up to 76 ppm), Sr

A suite of Devonian and Triassic c~n~onts with low (I 1.5) CAI has been analyzed for their trace

Table 2 Elemental

concentmt~ons

of basal bodies and crown material in some Trimsic conodonts Element

Sample

KolPl

K61P2

KalP3

K61P4

Palaz2PI

Pak’P2

Pdaz2P3

Palazl7PI

(cf. Fig. 2)

crown material

crown material

crown material

crown material

crown material

basal body

crown material

basal body

cont. = concentration;

Ca %

Mn

Fe

Zn

Rb

Sr

Y

La

Ce

Pr

Nd

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

COW. LOD

40 0.06

45

228 18

27 9

33

2874 2s

80 25

1196 241

768 211

337 215

184

cone. LOD

42

24 IO

34

1880 44

61 28

1416 477

646 442

473

434 362

4.3

58

285 36

31

0.01

cont.

37 0.06

3717

161

21.4

IS

58 64

38

LOT) cont. LOD

37 0.02

II

77 7

cont. LOD

40 0.03

185 13

cont. LOD

38 0.05

cont. LOD cont. LOD

SC/Y

Sr/Nd

36

8

5

23 9

235 150

231 78

131 64

174 53

59 4

16

2961 8

20 12

as9 126

334 92

153 78

70

1085 8

37 5

3154 6

90 10

480 126

288 107

202 92

267 69

35

!I

287 22

390 17

34 8

3473 11

224 13

546 205

769 143

469 134

498 116

16

7.0

26

38 0.01

288 13

309 11

53 4

3861 5

192 9

283 125

519 89

252 78

380 67

20

10.2

13

38 0.02

40 8

111 64

74

3254 4

29 9

177 IO0

168 74

108 59

90 50

112

36.2

g

LOD = limit of detection;

void spaces mean that the concentration

is below the LOD.

14s

Il.8

F. Bruhn et at. /iVucl.

lnstr. and Meth. in Phys. Rex 3 130 f1997f 636-640

(1795-3295 ppm), and Y (34-l 88 ppm), as well as incorporation of light REE with concentrations of up to > 1000 ppm. Rubidium, potentially influencing Sr isotope results, was always below the detection limits of usually I 30 ppm. Point analyses in areas of special interest (i.e. crown material vs. basal body) reveal, in several cases, concentrations of light REE in excess of 1000 ppm. In contrast, incorpora&ion of heavy REE (> Nd) appears to be less pronounced, suggesting REE fractionation in the course of skeletal formation. Line scans and elemental maps (Fig. I> reveal homogeneous distributions of Ca and Sr, but Mn, Fe, Y, and Nd (light REE) are restricted to specific domains, particularly to the rims of the skeletons. The Sr/Y ratio displays no significant difference between the crown material and the basal body of the conodonts (cf. Fig. 21, in contrast to Cambrian and Ordovician conodonts [3]. In our samples, the ratio varies widely from 16 to 161, with Sr concentrations exceeding by far the Y contents (Table 2). Likewise, in contrast to observation made on Ordovician samples [lo], antithetic distributions of Sr and Nd (where LOD’s for Nd were sufficiently low for accurate determinations) within crown material and basal body were not observed.

4. Discussion

and implications

639

alteration, should be potentially utilized for Sr isotopic studies. The present results do not yield any unequivocal answer. The observed enrichments of many elements at conodont rims tend to support the concept of post-depositional incorporation of trace elements, either by precipitation or by ion diffusion mechanisms. This alternative is further supported by the frequent appearance of brightly luminescing rims under the cathodoluminescence microscope. This luminescence can perhaps be interpreted as being activated by REE incorporated into the francolite structure [14], probably in Ca*+ positions [15]. If so, because of the low CA1 of the analyzed samples, such chemical and isotopic alteration must have occurred at low temperatures at a very early stage of diagenesis. In this case, the Sr isotopic composition in the samples could indeed still reflect the 87Sr/86Sr ratio of the contemporaneous sea water. In contrast to previous observations on early Paleozoic conodonts [3,10], our studies do not show any significant differences in Sr/Y and Sr/Nd ratios for different parts of the skeletons. This may possibly reflect an evolutionary history of the conodont group, with the younger, late Paleozoic and Mesozoic ones having more uniform trace element distribution. Additional PIXE microanalyses of Cambrian conodonts are in progress in order to test this alternative.

to isotope geology

The observed trace element patterns support the previously reported concentrations and diversity of trace elements, even at per mil levels, in the francolite structure [ 111. One of the fundamental issues for paleoceanographic studies based on conodonts is whether the trace element patterns are a consequence of biological ’ vita1 effects’ by living organisms or of their post-mortal incorporation. The latter may have taken place during either a very early diagenetic stage 112,131, or in the course of deep burial. If original or even early diagenetic, the Sr isotopic signal may still reflect the ancient marine conditions, providing early diagenetic alteration that happened at a time when pore waters were still similar to the overlying sea water. If, however, the trace element patterns are a consequence of burial diagenesis, then only the samples with low concentrations of REE, Mn and Fe, that suffered only minor diagenetic

5. Conclusions Despite the fact that the results of this study are still somewhat equivocal, the observed trace element distributions in skeletons of late Paleozoic and Mesozoic conodonts support the hypothesis that the patterns are a result of post-depositional incorporation at a very early stage of diagenesis. At this stage, the chemical properties of pore waters may still have reflected the coeval marine conditions.

Acknowledgements This study was financially supported by the Leibniz Prize of the Deutsche Forschungsgemeinschaft to J. Veizer. Travel funds for F. Bruhn to attend this conference were also provided by the Deutsche XII. GEOLOGY AND PLANETARY

SClENCE

640

F. Bruhn et al. / Nucl. Instr. and Meth. in Phy.

Forschungsgemeinschaft. The authors wish to thank F. Eickhoff for preparation of thin sections.

[lOI [Ill

[l] F. Bruhn, P. Bruckschen, J. Veizer, Min. Mag. 58A (1994) 128. [2] P. Bruckschen. F. Bruhn, J. Meijer, A. Stephan, J. Veizer. Nucl. Inst. and Meth. B 104 (1995) 427. [3] H. Pietzner, J. Vahl, H. Werner, W. Ziegler, Paleontographica 128 (1968) 115. [4] J. Karhu. S. Epstein, Geochim. Cosmochim. Acta 50 (1987) 1745. [5] A.G. Epstein, J.B. Epstein, L.D. Harris, U.S. Geol. Surv. Prof. Pap. 995 (1977) 1. [6] J.A. Maxwell, J.L. Campbell, W.J. Teesdale, Nucl. Inst. and Meth. B 43 (1989) 218. [7] J.A. Maxwell. W.J. Teesdale, J.L. Campbell, Nucl. Inst. and Meth. B 95 (1995) 407.

(I 997)

636-640

[81 F. Bruhn, P. Bruckschen, [91

References

Rex B 130

[121 [I31 [I41

[I51

D.K. Richter, J. Meijer, A. Stephan. J. Veizer. Nucl. Inst. and Meth. B 104 (1995) 409. H.J. Annegam, S. Bauman. Nucl. Inst. and Meth. B 49 (1990) 264. C. Holmden, Ph.D. Thesis, University of Edmonton, Alberta, Canada (1995). P.L. Roeder. D. MacArthur, X.-P. Ma, G.R. Palmer, A.N. Mariano, Amer. Miner. 72 (1987) 801. H. Eldertield, E.R. Sholkovitz, Earth Planet. Sci. Lett. 82 (1987) 280. P. Grandjean, H. Cappetta. A. Michard, F. Albarede, Earth Planet. Sci. Lett. 84 (1987) 181. S. Noth, D. Habermann, R.D. Neuser and D.K. Richter, Abstracts, International Conference on Cathodoluminescence and Related Techniques in Geosciences and Geomaterials (1996) p. 103. J. Wright, H. Schrader. W.T. Holser, Geochim. Cosmochim. Acta 51 (1987) 631.