Rare-earth elements in fossil bones
The increased radioactivity met in dinosaur and other animal bones from the Nemegt Valley, brought by the Polish Palaeontological Expedition of Polish Academy of Sciences from Mongolia, made us examine content of rare-earth elements which are often accompanied by actinides (thorium, uranium). Preliminary determination of lanthanides in the Mongolian dinosaur bones showed their comparatively considerable abundance. Dinosaur bones, found in the Nemegt Valley (the Gobi Desert), and bones from Bajn Dzak in Mongolia were the basic material under examination. In addition to the Mongolian bone material, there were examined samples of Ichtyosauria bones (Trias), Devonian fish carapaces, mammoth ribs (Pleistocene) and recent animal bones. To make a thorough study of the process of bone fossilization and of the enrichment of fossil bones in rare-earth elements, the X-ray diffractometric investigations (the Rigaku-Denki diffractometer, radiation Cu-Ka filter Ni), as well as, full chemical analyses, isolation of rare-earth elements and their separation by means of the ion-exchange chromatography method (Moret, Brunisholz) were accomplished. Powder diffractograms of the bone material including recent animal bones, as well as, fossil ones allow an observation of structural change occuring during the process of fossilization. Values of unit cell parameters of the phosphate mineral, calculated from the recent bone diffractograms (table 1), correspond to dahllite (carbohydroxyapatite). In minerals of the apatite group in mammoth bones a certain decrease of parameter a can be observed, as well as simultaneous occurrence of two kinds of apatite (of the dahllite and francolite types), shown by double reflections on the diffractograms (table 1, K-17). Apatite of the francolite type occurs in the dinosaurs bones from Mongolia. The same type of apatite can be found in the Triassic bone fragments and Devonian fish carapaces (tables 2 and 3). Also calcite, detritic quartz, and synchisite (met in bones for the first time) occasionally occur. They were all shown by diffractograms of samples from dinosaur ribs. Preliminary chemical analyses of bones (table 4) show, besides the main constituents of apatite, also Mg, Na: K, Sio(2), Fe, A1 and rare-earth elements. Comparing the chemical constitution of the fossil and recent bones a variability of CO(2), P(2)O(5) and F contents is well discernable (table 4). A secondarily crystallized calcite occurs in bones containing above 4 per cent of CO(2); it is connected with a decrease of the P20 5 content. Considerable amounts of an organic substance are characteristic of recent and Pleistocene bones. To separate minerals of the apatite group, powdered recent and Pleistocene bones had undergone an extraction with ethylene-diamine and then they were treated with the Silvermann solution to remove calcite (table 5). The analysed phosphate material of bones contains small amounts of such elements as Mg, Na, K, TR, Si, Fe, Al. No account has been taken of Si, Fe, Al, while evaluating the chemical constitution of bone apatite minerals, since they probably come from the goethite and clay minerals solved by hydrochloric acid. The amounts of the nine constituents of apatite were adjusted by a common factor to total 100 per cent, after allowance for the oxygen equivalent of fluorine (table 6). The occurrence of rare-earth elements in the Pleistocene bones amounts from several thousandths to several hundredths per cent, while in older bones these elements are contained in amounts from several tenths up to almost two per cent (table 6). Hence, the enrichment of Pleistocene bones in rare-earth elements reaches the values far below their average content in the earth crust (Goldschmidt, 1935) or it is equal to it, while the enrichment of older bones considerably exceeds the mentioned average (table 7). The chromatographic separation of rare-earth elements (table 8) shows three compositional types in respect of the ratio of cerium earths to yttrium ones. Cerium earths, mainly Ce, Nd, La, prevail in the constitution of rare earth in the Pleistocene and Triassic bones which come from some regions of Poland. The dinosaur bones from Mongolia (Cretaceous) show the yttrium constitution of rare-earth element or, sometimes, an inconsiderable predominance of cerium earths. The enrichment in lanthanium and neodymium and also sometimes in europium, together with a strongly decreased cerium content is stated in dinosaur bones. Mainly epidote and titanite occur in the heavy minerals assemblage present in clastic deposits in the Nemegt Valley. Since both these minerals are carriers of rare-earth elements and are not the most durable ones, they should be regarded as the source of rare-earth elements, apart from volcanic glass. On the other hand, the presence of europium in some dinosaur bones can be connected with basic plagioclases able to selective concentration of this element and which are present in arkose-type sands of the Nemegt Valley. Microscopic examination shows that, the fossilized bone tissue forms homogeneous, microcrystalline, white or pale-yellow substance of low birefringence which is characteristic for francolite. The marrow cavities are filled by the secondarily crystallized calcite with admixture of clay minerals and scattered concentrations of goethite, and often with clastic material composed mainly of quartz and feldspar. Synchisite occurs in Haversian canals in form of aggregates composed of fine, yellow-brownish, anhediral and isometric crystals (PI. XLI, Fig. 2) in some ribs of the Mongolian Dinosaurs. Transparent crystals of the 'secondary fluoroapatite with tabular habit according to (001) (PI. XLI, Fig. 1) are also occasionally found. Diffractometric investigations of the recent animal bones show that inorganic substance of the bone tissue has the structure of á mineral of the apatite group. On diffractograms there appear reflections characteristic of carbohydroxyapatite; it should be pointed on that no reflection indicating the presence of calcium carbonate (calcite, aragonite) has been found there. Parameters of the apatite unit cell of recent bones and the chemical constitution show that living organisms produce carbohydroxyapatite resembling dahllite in the bone tissue. The process of fossilization is already marked in the Pleistocene mammoth bones by lowering the parameter a of the apatite unit cell. Simultaneous occurrence of apatite of the dahllite and francolite types, giving double reflections on diffractograms, shows that transformation of the phosphate substance to francolite proceeds gradually from the surface parts of bones which are in close ccntact with circulating mineralizing solutions (table 1, K-17). The presence of fluorine up to 1,5 per cent also indicates that the transformation of carbohydroxyapatite into francolite is a partial one. Dinosaur bones from Mongolia contain phosphate minerals altered to francolite, which is shown by the unit cell parameters and by fluorine and carbon dioxide contents. Triassic bones and fragments of Devonian fish carapaces display similar characteristics. Having compared the change of unit cell parameters and the change of fluorine, carbon dioxide and water contents in recent animal bones, in fossil bones and in the francolite from Staffel (table 10), a decrease of the parameter a value, according to the geological age, can be observed. This is connected with simultaneous increase of the fluorine content and slight lowering of the CO(2) content. Equal fluorine content in the Cretaceous and Triassic bones, as well as in fragments of the Devonian fish carapaces can indicate that the process of transformation of the phosphate mineral has reached an optimum. The mineral formed during the process of fossilization resembles francolite in its chemical constitution; it contains about 0,5 per cent less of fluorine and about 1 per cent more of CO(2) than a typical francolite from Staffel. The water content, characteristic of the apatite mineral of recent and fossil bones, does not undergo any essential changes in the process of fossilization. The increase of the rare earth content in fossil bones takes place simultaneously with the fluorization, while the ratio of the atom quantity of rare earth elements and of fluorine per an unit cell increases almost ten times from the Pleistocene to the Cretaceous (table 11). It has been stated recently that the participation of rare earth elements in the process of bone fossilization can also lead to crystallization of the synchisite mineral which is the fluorocarbonate of rare earth and of calcium. Since the diffractogram shows reflections characteristic of synchisite s. stricto and not of doverite (yttrium synchisite), it should be assumed that a part of the rare earth elements, especially of the cerium group, has formed synchisite in Haversian canals of bones, while yttrium and yttrium-earth elements have entered into the composition of francolite as isomorphous substitutions of calcium. Applying Mac Connell's method (I960), who presented the best structural model for carbonate apatites, a calculation for bone analyses (table 6) has been done, based on the unit cell with the cation charge = — 53 (table 12). It should be noted (as it was observed by Brophy and Nash, 1968) that the whole H20 cannot be included in the structural positions. Quantities of H(2)O which cannot be contained in the structural positions correspond approximately to H(2)O —300 (table 6). The ascertainment of the fact that samples of bones desiccated in a temperature of 300°C regain their original weight not only in normal conditions but also in the conditions without the access of CO(2) suggest that H(2)O - 300 may be of the zeolite water type. Increase of value of the unit cell parameter a in the apatite mineral of bones which contains rare earth elements in a quantity above a few hundredths per cent shows that their influence on the size of unit cell cannot be ignored. The calculation on the ground of the equation: a(p)=a(k)+x(H)-y(C)-z(F)-n(S)+q(TR) (MacConnell, 1970) supplemented with an expression g(TR) where
a(p) — the predicted dimension a in A
a(k) — the constant, inherent to the structure,
H, C, F, U, TR — numbers of atoms of elements x = 0,0075, y = 0,070, z = 0,015 coefficients related to the sizes of the particular atoms adopted after MacConnell in A per atom per unit cell, a(k) = 9,404 A, allowed an estimation of the coefficient q = 0,4. A precise examination of the dependence of the parameter numerical values on the TR content will be possible when synthetic TR-Ca apatites have been obtained, as it was already done by Collin (1960) for apatites containing strontium. The process of bone fossilization is a complex one; it includes the removal of the organic substance (collagen), the alteration of the bone phosphate mineral from carbohydroxyapatite to francolite and the formation new minerals. Methods of dating the relative age of bones which, however, applicable in a limited way only, are based on the two formerly mentioned phenomena. Rare-earth elements, together with fluorine, not only take part in the transformation of carbohydroxyapatite to francolite but also form fluorocarbonate of rare earths and of calcium in Haversian canals of bones. Despite the hitherto existing views, it proves the possibility of a low-temperature formation of synchisite. Although the determination of the rare earth element content cannot serve to date the relative age of bones, the composition of oxides may indicate the environment in which they were burried and subjected to fossilization. Calcium in minerals of the apatite group can be equally substituted by elements of both cerium and yttrium groups. Therefore, in consequence of the lack of selection in these substitutions, the rare earth elements reflect their proportions in the mineralizing solutions, dependent both on the source of the rare earth elements and on the environmental conditions. It has been shown that: 1° a low ratio of cerium to the remaining rare earth elements is connected with oxidizing conditions found in the bone depositional environment, 2° the enrichment in europium is connected with the occurrence of basic plagioclases in the bone-bearing deposits.
