Synthetic Carbonatite Magma 0
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770 NATURE March 14, 1959 voL. 1e3 unsuitable mineral to use for dating geological events, Subsequent studies on the syst,em Ca0-CO,.-H 20 since it is subject to recrystallization. We have no have shown that the minimum liquidus temperature knowledge of the geological or analytical details of at 1,000 bars pressure is 67 5° C. ± 5° C. At this the Leningrad sample, so it is difficult for us to decide temperature and pressure, a liquid with the com if it fulfils the stringent requirements of a calibration position calcium oxide 64, carbon dioxide 18, water sample. The problem of dl:l.ting the Ordovician and 18 (weight per cent) is in equilibrium with calcite, Cambrian remains. portlandite and vapour (almost pme water). At A close examination of the geology of the Shinko 680° C. and the same pressure, a liquid develops lobwe uranium deposit shows that it is far from being on the calcium oxide side of the Ca(OH) 2-CaC0 3 " one of the most certain points in the geologic time join ; this is in equilibrium with calcite, portlandite scale" (Prof. J. L. Kulp, in Robert, ref. 4, p. 17). and calcium oxide and has a composition calcium Thus, although the age of 630 ± 35 m.y. (as quoted oxide 68, carbon dioxide 19, water 13 (weight per by Robert) is generally agreed to be unusually good, cent). At 685° C. the two liquid fields coalesce the Shinkolobwe deposit is of no use in defining a and at still higher temperatures the range of com time-scale. The precise facts are that the uranium position of the liquid field increases. The minimum minerals are present in veins of mesothermal type liquidus temperature in the system has been determ occurring within folded dolomites of the Systeme ined for a range of pressures. At 500 bars the tem Schisto-dolomitique of Katanga. These dolomites perature is 680° C., and at 4,000 bars it is lowered and other similarly folded dolomites overlying them to 640° C. At pressures lower than 50 bars, the in nearby areas contain Collenia and Stromatolithes, minimum liquidus temperature decreases, and at fossils lmown to exist from the early pre-Cambrian 27 bars pressure the appropriate composition is com to the Cambro-Ordovician inclusive. The strati pletely liquid at 665° C. The high melting tempera graphic definition of the uranium deposits is therefore ture of calcite is therefore lowered markedly by the of the poorest. The absence of ordinary marine lower addition of calcium oxide and water under pressure. palreozoic faunas is of no weight in this argument, as The low-temperature liquid may be regarded as a they are generally unknown from South and Central simplified carbonatite magma. Although calcite iR Africa_ We have discussed these remarks on the generally the dominant mineral in carbonatites, African samples with Prof. J. L. Kulp, and we learn dolomite is frequently an important constituent, that his original statement did not refer to the while apatite and mica are constant accessories. stratigraphy, but to tho measured age. Experiments are under way to find the effect of additional components (such as magnesium oxide, K. I. MAYNE phosphorus pentoxide, silicon dioxide and alkalis) Clarendon Laboratory, Oxford. on the minimum liquidus temperature in the ternary system. Solubility determinations are also being R. ST. J. LAMBERT made to see how the solubility of calcite varies with D. YORK increasing content of carbon dioxide in the aqueous vapour phase. Many geologists believe that a high Department of Geology and Mineralogy, tomperature vapour, consisting of water and carbon Oxford. dioxide, is able to dissolve large amounts of calcium 1 Ahrens, L. H., Geochim. Cosmochim. Acta, 8, 1 (1955). oxide, magnesilllll oxide, ferrous oxide, etc. 'Inghram, M. G., et al., Phys. llev., 80, 916 (1950). Detailed results of these studies will be published • Backus, M. M., Ph.D. thesis, Mass. Inst. Tech. (October, 1955). in due course, but we take this opportunity of present 'Robert, M., "Geo!. et Geog. du Katanga" (Brussels, 1956). ing these facts now because they will be of interest to geologists working on carbonatites. P. J. WYLLIE Synthetic Carbonatite Magma 0. F. TUTTLE Department of Geophysics and Geochemistry, JUDGING from the literature during the past decade, College of Mineral Industries, there is a growing interest in problems related to the Pennsylvania State University, genesis of carbonatites and associated alkaline igneous University Park, Pa. rocks. The interest has tmdoubtedly been fostered Jan. 27. by the discovery of large reserves of rare earths in 1 Tuttle, 0. F., and Wyllie, P. J . , Bull. Geo/. Soc. Amer., 69, No. 12. these rocks. Many field geologists believe that Part 2, 1655 (1958). carbonatites were emplaced as liquid magmas ; but hitherto there has been no conclusive experimental evidence to support the existence of carbonatite Manganese in Modern and Fossil liquids. In fact, from the available experimental evidence it seemed unlikely that such liquids could Gastropod Shells exist at the low temperatures indicated by field VERY little work has been done on manganese studies. We have recently obtained experimental concentrations in shell material and particularly in evidence that simplified carbonatite magmas can gast,ropod shells, although Doring in Vinogradov's exist at moderate temperatures through a wide range book1 reported several analyses and found from O ·l to of pressures. 0 ·0001 per cent manganese by weight in gastropod Results1 from a study of the join calcite-water in shells. Krinsley and Bieri2 have determined mangan the ternary system CaO-C0 2- H 20 show that calcite ese in the shells of modern and fossil pteropods. begins to melt at a temperature of 740° C. at 1,000 This trace element was examined in modern and bars pressure of water vapour. Only a trace of liquid fossil shells to determine whether burial and sub was developed at this temperature, but the amount sequent diagenesis might have altered the original increased with increasing water content and with material. increasing temperature. This evidence implied, but Eighty modern and fossil shells of Olivella biplicata did not prove, the existence of carbonatito magmas. (Sowerby), 89 of Tegula .fnnebralis (A. Adams) and © 1959 Nature Publishing Group.