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Age, Origin and Emplacement of Diamonds

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Age, Origin and Emplacement of Diamonds ScientiJic Advances in the Last Decade By Melissa B. Kirkley, John J. Gurney, and Alfred A. Levinson Cr;ontiGo nArmnrne ;n tho nnct AornAo he A~P~APn( the loanc ,,TAT mqinr ~ATF-~PPCin mrr UUIYIILIIIY CIU YUI.YD0 111 CllY YClLIC UYUUUY TT)lib UCbLIUb VI LIIb L/VVU OLIVV 1114)VL LIUILIIIUbO 111 VUL hove completely altered our understand- understanding of the age, origin, and emplacement of ing of certain concepts relating to the age iLiamonds. Much of the new data and their inter~reta- and origin of diamonds. As a generaliza- 1 %are found in highly technical scientific journais and tion, most diamonds formed more than conference proceedings that are rarely encountered by 990 million years ago, deep within the gemologists. Therefore, we have prepared this review earth, from either of two rock types, peri- dotite and eclogite. They were stored be- article to update gemologists on some of the latest facts low the base of cratons for varying periods and concepts with respect to the above topics. of time, some as long as 3,200 million The information contained in this article is applicable years, before being transported to lhe sur- to virtually all natural diamonds, both gem and industrial face. Kimberlite and lainproite, the two (figure I), except possibly for certain rare types of dia- rock types usucrlly associuted with dia- monds, such as those referred to as "fibrous" or "coated," monds, are only the mechanisms that and microdiamonds, as well as for diamonds related to brought diamonds to the surface and are meteorite impacts. In addition to our own experiences, we in no way related to the formation of have drawn from many volumes in the technical literature. most diamonds. Other topics that are For those interested in pursuing these topics further, we somewhat more speculative, for example, recommend the boolzs by Ross (1989), Nixon (1987), the source of carbon for ihe crystolliza- tion of diamonds and the mechanism of Mitchell (1986), Glover and Harris (1984), Kornprobst kimberlite and lamproite emplacement, (1984), and Dawson (1980), and the review articles by are also discussed and ihe latest concepts Gurney (1989) and Meyer (1985). presented. AGE OF DIAMONDS Until recently, one of the major unresolved problems in diamond research revolved around the age of diamonds. I Age dating of diamonds assists in understanding their ABOUT THE Atlrm.9 origin, which is a significant factor in diamond explora- Ds Kirk is post-doclmt r~& dficec end 01.f urmy Is press h h Depalnen ' tion. For many minerals, age can be determined directly a1 Gwhemistry, WwsiIy d Cape Town, using a number of well-established geochronological tech- Randebasch, Sooth Africa, DE Lwihscin k prc niques, such as the uranium-lead (U-Pb)method. However, fsssor h It)@ Deparfment of Cedogy and Geo physics, U~milyd Calgary, Alberfa. because diamond is essentially pure carbon, it does not ha&. contain any of the radiogenic elements on which such Achnowkdgnents: The authors sincere& than methods depend. Even the well-lznown carbon-14 (14C) 0s A. J A. Arise and Mr. F! d Darragh, lbolh method is useless for diamonds because it is restricted to al Wth, Auslralie, lor lhelr careful m&w d fhe nwivscripr. , organic carbon that has been involved in the earth's recent Gem & Gemology, 161.27,k. I, pp 2-25 near-surface carbon cycle. Although diamonds themselves cannot be dated, some O 1991 demological Institute d Amriia of their minute inclusions, such as pyroxene and garnet, 2 Origin of Diamonds GEMS & GEMOLOGY Spring 1991 Figrue I. Today, dia- monds are the most pop- ~~largemstone and a valuable industrial mate- rial. As a consequence, there has been consider- able research into the geologic origins of dia- monds to aid in explora- tion and mining. The last decade, in particu- lar, has prodczced some important advances in our rlnderstanding of the complex processes re- quired /or the formation and deposition of dia- monds. The ultimate re- sult of such research is evident in these superb earrings and necklace. The three large diamonds in the necltlace are (from the left) 9.81, 16.18, and 12.73 ct; they are sur- rounded by 254 dia- monds with o total weight of 67 ct; the ear- rings contain 84 dia- monds wit11 a total weight of 11 ci. {ewelry by Van Cleef and Arpels; photo courtesy of Sotheby's, New Yorl<. can, because these minerals contain measurable octahedron) of the diamond rather than that of quantities of the elements involved in radioactive their species (figure 3). decay systems. Some inclusions (e.g., garnet) were Several attempts were made prior to 1981 to formed at the same time, and in the same place, as date inclusions in diamonds; the study by Kramers their mineral host (e.g.,diamond), so that the age of (1979) is the most significant. Using lead (Pb) the inclusion is also the age of the host. Detailed isotopic compositions of sulphide inclusions in studies of olivine, garnet, pyroxene, chromite, and diamonds, he determined ages on the order of 2,000 other minerals in diamond have shown that these million years (My)for inclusions in diamonds from minerals were growing adjacent to the diamond, the Finsch and Kimberley pipes in South Africa; which then grew around and enclosed them (fig- those from the Premier mine appeared to be about ure 2). This physical relationship between dia- 1,200 My in age. However, Richardson et al. (1984) mond and its cogenic inclusions is sometimes were the first to date successfully a significant reflected by the crystal form of the silicate inclu- number of inclusions in diamonds, specifically sions, which take on the morphology (called cubo- inclusions of garnet in Finsch and Kimberley Origin of Diamonds GEMS & GEMOLOGY Spring 1991 3 Figure 3. nigons, which are typical of a dia- mond octahedron, can be seen on the face of this flattened inclusion of pyrope garnet, with a Figure 2. This garnet inclusion in a diamond colorless enstatite (orthopyroxene) or olivine (I mm across) from the Finsch mine, South crystal at the end. These features are taken as Africa, was dated by geochronological methods evidence that the diamond has forced its crys- to be about 3,300 million years old. Because tal habit on the guest mineral during the simul- such inclusions undoubtedly formed at the taneous crystallization of inclusion and host. same time as their hosts, they are the best Photomicrograph by Eduard 1. Giibelin; trans- means of age dating diamonds, which cannot mitted illumination, magnified 50 x . From Gii- be tested directly by standard dating methods. belin aid Koiv~ila(1986, p. 95). Photo courtesy of Dr. S. H. Richardson. berley pipe are as much as 3,200 My older than the age of lzimberlite emplacement (i.e., when diamonds, by means of the relatively new samar- the pipe reached the surface, about 100 My ium-neodymium (Sm-Nd) geochronological ago]. This example implies that: (a)diamonds method combined with the rubidium-strontium can be stored deep within the earth for an (Rb-Sr)technique. Table 1 summarizes these re- extended period of time before being carried to sults along with more recent data (Richardson, the surface by the lzimberlite; and (b) lzim- 1986; Richardson et al., 1990) on diamonds from berlite is merely the transporting medium for several other pipes in southern Africa and Austra- bringing diamonds (aswell as other materials) lia. (For a discussion of some earlier studies that to the surface. This process has been pictur- have a bearing on the dating of inclusions in esquely described by the analogy of an elevator diamonds, see Meyer, 1985, and Gurney, 1989.) or a bus (thelzimberlite] picking up passengers The results presented in table 1 may well be (diamonds) in the earth's mantle along its the most striking information about diamonds to route of ascent toward the surface. Note, emerge in the past decade, not only because they though, that there is no resolvable age differ- put accurate ages on diamonds in millions of years ence between the diamonds and lzimberlite (My) but also because of other implications, as emplacement at the Premier mine. This sug- discussed below: gests that, in this example, they may be contemporaneous. Whether the Premier mine 1. Diamonds are old and may have been forming is unusual awaits the determination of dia- continually, certainly intermittently, through- mond ages from additional localities. out most of earth's history. The 2,300 My period between -3,300 and 990 My represents 3. In samples from the Finsch mine, two ages- about half of the earth's 4,500 My existence, -3,300 and 1,580 My-have been obtained, and inclusions in diamond may yet be found the former for peridotitic, and the latter for that extend this range. eclogitic, inclusions. These two main types of inclusions in diamond are discussed in greater 2. Diamonds are usually very much older than detail below. The presence of diamonds of the lzimberlite that brought them to the sur- different ages within one pipe can, for the face. For example, diamonds from the Kim- present, most easily be explained by the fact 4 Origin of Diamonds GEMS & GEMOLOGY Spring 1991 that lzimberlites may obtain their diamonds mond at an extraneous source and its capture from more than one geologic environment by the Premier Izimberlite. (mantle source) during their rise toward the Although the concept that diamonds are surface. xenocrysts in lzimberlite was proposed as early 4. The data reported in table 1 have settled a long- as 1905, it was not until the Third ~nterna- standing debate in which scientists advocated tional Kimberlite Conference, held in France one of two hypotheses with respect to the in 1981, that "there was a realization that origin of diamonds.
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