Application of solid state nuclear track detection technique in the study of fossil bones

Vol. 28, Nos I-6. pp. 571-574. 1997 © 1997ElsevierScienceLtd Printed in GreatBritain. All rights reserved 1350-4487/97$17.00+ 0.00

Radiation Measurements.

Pergamon Pll: S1350-4457(97)00141-8

APPLICATION OF SOLID STATE NUCLEAR TRACK DETECTION TECHNIQUE IN THE STUDY OF FOSSIL BONES

A.A. QURESHI, N.U. KHATTAK, M. AKRAM, K. MEHMOOD, C.A. MAJID *, S.A. DURRANI**, A. HALEEM***, I.E. QURESHI AND H.A. KHAN Radiation Physics Division. PINSTECH, P.O. Nilore, lslamabad, Pakistan *Nuclear Materials Division, PINSTECH, P.O. Nilore, lslamabad, Pakistan **Department of Physics, Birmingham University, U.K ***Islamabad Museum, E-7, lslamabad, Pakistan ABSTRACT Mammalian fossil bones have been discovered in the rocks of 16 my old Siwalik System in Pakistan. These bones belonging to some animal similar to the rhinoceros of our present days, have been studied using petrography, X-ray diffraction, ~/-spectrometry,fission track dating, and a-autoradiography techniques. Presence of uranium in bones, now petrified to apatite, has been identified by fission track and alpha autoradiography methods. Interestingly, the petrified bones still contain well preserved bone cells and blood vessels in their original shape and red colour, perhaps due to the sudden burial of the animal in an anaerobic atmosphere. KEYWORDS

SSNTD; fission track dating; lexan; CN-85; mineralogy; fossil bones; X-ray diffraction; alpha autoradiography; gamma spectrometry.

INTRODUCTION Petrified fossil bones were discovered in a paleochannel in the Chingi Formation of the Lower Siwalik rocks exposed in Ban Amir Khatoon area near Chakwal, Pakistan. The paleochannel extends for about 1.5 km in E-W direction. The bone samples were collected for geological, paleontological and other studies. The area is situated 25 km in the south of Chakwal city, at a distance of about 6 krn on the west of the main Chakwal-Choa Saiden Shah road as shown in Fig. i. The Chingi Formation of Siwalik System is about 16 million years old and contains parts of skeletal remains of ancient mammals. The complete skeleton frame of any particular animal has not been so far found. The Siwalik rocks are widely distributed in Pakistan in the Indus basin, part of the Axial Belt, Waziristan, Parachinar area, Kohat-Potwar Province and Dera Ghazi Khan division. The Siwalik time has been the most remarkable one for the rapid evolution, development and differentiation of the mammals in India and Pakistan. This magnificent assemblage of mammals was not truly of Indian origin, but a large migration of herds of animals must have taken place from the other continents like Alaska, Siberia and Mongolia as reported by Pilgrim (1913). The area was first reported by the students of Archaeology Department, Punjab University as a reservoir of fossil bones. It was then investigated in more details by the archaeologists of Islamabad Museum. Later on, the locality of fossil bones was visited by scientists from Pakistan Institute of Nuclear Science and Technology (PINSTECH). A number of bone samples loosely lodged in a ravine were collected and the following studies were carried out on them.

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Fig. 1. Map of area showing location of Dhoke Ban Amir Khatoon where paleochannel containing fossil bones in Chingi Formation of Siwaliks is exposed.

MINERALOGICAL STUDIES The mineralogical studies on the petrified bone samples were carried out using petrographic and X-rays diffraction techniques. These studies indicate that bones consist of three distinct compositional layers as shown in Fig. 2. For the exact identification of minerals, a small amount of material from each zone was analyzed with the help of XRD facility. The XRD studies indicate that the outer portion of the fossil bone is composed of apatite with minor calcite and some other minerals which could not be identified. The middle zone is composed of apatite and calcite in about equal amounts. The inner zone is composed almost exclusively of the calcite as shown in Table 1. Table I. Results of XRD studies of the fossil bone samples Cross sectional Zone Outer

Middle Inner

Mineral constituents

Abundance (%)

1. Fluorapatite [Ca5 F(PO4)3 ] 2. Calcite [Ca CO3] 3. Miscellaneous

93%

1. Fluorapatite 2. Calcite

50% 50%

1. Calcite

5% 2%

95-100%

RADIONUCLIDES IN FOSSIL BONES The spectrometry of the fossil bones indicated the presence of radionuclides in these bones. The radionuclides found are of U 23s, U 23s and Ra 226. It has been found that the uranium and radium phases are in equilibrium with each other.

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Alpha A utoradiography A piece of fossil bone was mounted in eraldite, grounded, polished and placed in contact with CN-85 detector for 43 days. The detector was etched in 6N NaOH solution at 50 *C for 70 minutes to develop alpha tracks. It was found that the outer zone has alpha track density of the order of 5.1 x 104cm"2 while the middle zone has track density of2.6x104 cm 2 whereas no tracks were seen in the inner most calcite zone. The distribution of the alpha particle tracks indicates that the outer zone has more uranium as compared to the middle zone, while no uranium is present in the innermost calcite zone. The distribution of tracks corresponds to the amount of apatite present in the bone.

Uranium Content Determination Uranium content of the fossil bones was determined by fixing a piece of lexan detector on a polished bone section for irradiation in the reactor. Another piece of lexan was fixed on the standard reference glass (SRG No. 963(a) with uranium content = 0.083%). Both of these assemblies were irradiated in a reactor at 9 MW power for 200s under a neutron flux of 1.6x10 ~3 #. cm z. see q. The lexan fixed on SRG was used for fluence determination and other lexan was studied for uranium content determination in various parts of the bone sample as described by Durrani and Bull (1987). The fission track map is shown in Fig. 3. The uranium content was determined as 85 and 43 ppm in outer and middle zones respectively whereas the inner zone was almost devoid of any uranium content.

APATITE (IOO % )

CALCITE (60 %)

APATITE CIOO */.)

+ APATITE ( 6 o °/o )

Fig. 2. Cross section of bone showing various mineralogical zones.

CALCITE ( 0 0 "/o }

+ APATITE (~ %)

Fig. 3. Fission track map of bone section corresponding to various mineralogical zones

Fission Track Dating For the purpose of dating of petrified bones, the technique of fission track dating was applied. For this purpose a part of the fossil bone was mounted in eraldite and polished with diamond pastes of 6p.m, 3p.m and lp.m respectively. The polished mount was then etched in 1%, 2% and 5% HNO3 solutions at 23°C for various etching times, but no tracks could be developed. Etching of calcite was also tried with the olivine etch with NaOH added for raising the pH to 12, for 30 minutes as recommended by Fleischer et al., 1975, but no fossil tracks were seen. Thus it was concluded that bone pieces can not be used directly for the purpose of dating with fission track method. BONE CELLS AND BLOOD VESSELS A thin section of the outer portion of bone sample was prepared for study under a microscope. In plane light the thin section shows yellowish colour containing a large number of red circular spots and treelike branching structures which represent the bone cells and blood vessels as shown in Fig. 4. Interestingly it has been noted that bone cells and blood vessels have been preserved in their original

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shape and red coiour. The haversian canals, canaliculies and lacuna structures can be clearly seen in the bone sections.

Fig. 4. Photomicrographs showing bone cells (osteocytes), lacunae, haversian canals and blood vessels seen in the thin section of bone in original shape and red colour (not seen in this black and white picture).

CONCLUSIONS The mineralogical and x-ray diffraction studies indicate that the fossil bones are composed of three different concentric compositional zones. These zones are (1) the outermost Fluorapatite zone (2) the middle zone of Fluorapatite and calcite in 1:! ratio and (3) the innermost calcite zone. Fossil bones cannot be dated with fission track dating technique, due to their porous and fibrous nature. The occurrence of the fossil bones in the rocks of Middle Miocene indicates an age of about 16 my. The presence of uranium in the bone has been confirmed by fission track and autoradiographic studies. These studies also indicate that the Uranium contents of the bones is directly proportional to the apatite contents of the bones, being in maximum amount in the outermost zone and absent from the innermost calcitic zone. The high uranium content varies from 85 to 43 ppm from outer to middle zone. This indicates that the movement of uranium bearing solutions have been active during this period of geological history in this area. The high uranium in the bone samples indicates that the organic material provides a good nucleus for the uranium deposition. The bone samples may have been preserved soon after its formation under rapidly accumulating load of sediments in anaerobic (reducing) environment to preserve the original shape and blood red colour of the bone cells and blood vessels. REFERENCES Durrani S.A. and Bull R.K. (1987) Solid State Nuclear Track Detection." Principles and Applications. Pergamon Press, Oxford. Fleischer R L., Price P.B. and Walker R.M. (1975) Nuclear Tracks in Solids: Principles and Applications, University of California Press, Berkeley. Pilgrim G.E. (1913) The correlation of the Siwaliks with mammal horizons of Europe. India Geol. Surv., Recs., 43, 264-326.