AmericanMineralogist, Volume 70, pages344-355, 1985 Genesisof diamond:a mantle saga HsNnv O. A. MeYEn Department of Geosciences Purdue Uniuersity West Lafayette, lndiana 47907 Abstract A model for the genesisof natural diamond is presentedbased on the physical,chemical and mineralogicalproperties and featuresof diamond. Optical studiessuggest that individual diamonds have had complex growth histories in which growth and dissolution may have occurred.Growth was not always continuousnor did diamonds grow in necessarilysimilar chemicalenvironments. Evidence for this is provided by variation in the nitrogen and trace elementcontents in diamondsas well as information from studiesof the mineralsincluded in diamond. Isotopic data suggestthat diamonds formed from carbon whose sourcesvaried isotopically.The possibilityexists that somediamonds may be productsof recycledsubducted carbon, whereasothers have formed from primordial material either through magmatic or metasomaticprocesses. It is also likely that most diamonds formed in the Archaean or Proterozoic.The cognate host rocks for diamond in the mantle were severalbut can be broadly grouped into eclogitic and ultramafic (peridotitic); however, in mineralogic and chemical detail these rocks are quite diverse.Although diamond is commonly found in kimberlite and in lamproite at the earth's surface,these two rocks are not geneticallyrelated to diamond formation. Instead they are the transporting vehiclesin which diamond ascended rapidly from mantledepths to the crust. Introduction initial evidencefor this latter idea was the discovery of Although diamond has beena sourceof fascination,the diamond in an eclogitexenolith from kimberlite (Bonney, origin of this mineral has for centuriesperplexed man. 1899).Du Toit (1906),Wagner (1914)and Sutton (1928) Greek philosophers and medieval alchemists ziscribed modified this idea and suggestedthat the eclogiteand peri- many mystical properties to diamond. When taken as a dotite xenoliths were cognate with the kimberlite. Dia- powder,voluntarily or involuntarily, it could, among other mond was thus genetically related to the early crys- things, cure diseases,poison ones enemiesor make the tallization of kimberlitemagma. honest strong and agile. An unusual belief, especially held The above two hypotheseshave lasted until the present in Greece and India was that diamond could procreate and have developed with some modifications into the itself-a boon to the owner of a diamond mine. phenocrystversus xenocryst schools(Dawson, 1980).For In 1772 Lavoisier demonstrated that diamond, like example,Gurney et al. (1979)and Harte et al. (1980)main- carbon, would burn in air. However, it was only later in tain that diamonds are geneticallyrelated to early crys- 1797 that Smithson Tennant proved that diamond consist- tallization products of kimberlite within the upper mantle ed of carbon. This led several gentlemen scientists of the and are thus phenocrysts.In contrast, Meyer and Tsai nineteenth century to suggest that diamond was formed (1976a),Robinson (1978),and Meyer (1982 a,b) have through the action of heat and pressure on plant remains arguedthat diamondsare accidentalinclusions in kimber- (Des Cloizeaux, 1855; Goppert, 1862). lite and thus are xenocrysts;the associationof diamond The discovery of diamonds in a volcanic rock (kimber- and kimberlite being one of passengerand transporting lite) at Kimberley, South Africa in 1871 led to more scien- vehicle. tific, and less philosophical studies. This did not, however, Most scientistsfamiliar with diamond concedethat dia- deter various authors from presenting opposing view- mond has grown stably within the upper mantle (Kennedy points, as summarized by Williams (1932). For example, and Nordlie, 1968;Meyer and Boyd, 1972;Orlov, L973; Lewis (1887) considered that diamond formed in the crust Sobolev,1974; Robinson, 1978). Omitted for purposesof as the kimberlite host rock solidifled-the carbon being this discussionare the polycrystalline aggregatesof dia- derived from coal and other carbonaceous material. mond (carbonado,framesite, boart) which have received In contrast others maintained that diamonds had orig- little scientificstudy (Trueb and DeWys,1969; 1971; Trueb inally formed in ultrabasic rocks at depths, and were subse- and Barrett, 1972; Gurney and Boyd, 1982) and whose quently released as the rocks fractured upon incorporation origin is even more uncertain than the single crystal dia- into the kimberlite melt (Harger, 1905; Holmes, 1936).The mond consideredhere. 0003-{04xl85/0304-{344$02.00 344 MEYER: GENES/S OF DIAMOND: A MANTLE SAGA 345 Suggestionsas to the sourceof carbon from which dia- mond forms have been diverse and range from coal and plant remains as favored in the 1800's,to carbon dioxide and methanetoday. However,whether or not the carbon is primitive or from recycledcrustal material is a necessary question.Current studieson carbon isotopes(Deines, 1980; 1982;Milledge et al., 1983),as well as on nitrogen (Becker, 1982)and rare gases(Ozima and Zashu, 1983)bear on this question. An important factor in understandingthe formation of natural diamond is afforded by detailed examination of minerals included in diamond. Thesestudies, mostly crys- tallographic and mineralogical,have beenreviewed by So- bolev (1974),Meyer and Tsai (1976a),Harris and Gurney (1979)and Meyer (1982a).Isotopic studies of these small inclusions in diamond are now possibleand future work should provide significantresults concerning diamond and the evolution of the upper mantle. This paper suggestsa model for the genesisof diamond, and its subsequentpassage to the earth's surface.Current interest in the evolution of the upper mantle and magma generationconsiders diamond to be an unreactivechemical probe from the depths. One aim of the discussionand model presentedherein is to place the genesisof diamond within the correct context of mantle processes.It is also hoped that the discussionwill remove various misunder- standingsthat are prevalent with respectto diamond and its relationshipto kimberlite and other rocks. A subsidiary aim of this paper is to bring to the attention of mineral- ogists the large amount of important information contrib- uted by physiciststo diamond research,and equally to exposephysicists to geologicalprocesses attendent on dia- mond formation and the subsequenthistory of diamond. Although the host rocks for diamond at the earth's sur- face are kimberlite and lamproite, it is believedthat these Fig. l. (a) A transparent,clear and well shapedoctahedron of rocks are not geneticallyrelated to diamond. Accordingly, diamond.Octahedral edge I mm. (b) A polishedand etchedsec- it is not the aim of this paper to dwell on the nature of the tion throughan octahedraldiamond (2.4 mm on edge),showing chemical and mineralogical differenc€sbetween various the internalstratigraphy of diamond.Various growth layers are kimberlites,and betweenkimberlites and lamproites.The eitherType I or TypeII diamond(Figure 3.1; Harrison and Tol- ansky,1964). interestedreader is referredto Dawson (1980)for kimber- lites, Mitchell (1984)for lamproites, and the proceedings volumesof the threeInternational Kimberlite Conferences. cuboid surfaces.More detailed descriptionsof this phe- Physical features of diamond nomenon are to be found in Suzuki and Lang (1976)and A considerableamount of detailedstudy into the physics Lang(1979).The stratigraphyof diamond can also be illus- of diamond has been undertaken over the past 35 years trated by cathodoluminescenceon polishedsurfaces ofdia- (Berman,1965; Field, 1979).Much of this researchhas mond (Moore, 1979) and by X-ray topography (Lang, significanceto mineralogy and bears on the formation of 1979).The growth stratigraphyis observedbecause various diamond. layers consist of either Type I or Type II diamond, and Figure la is a photographofa typical clear and colorless these two types have different chemical and physical diamond without any visible flaws. This clarity, shown by properties(Table 1). many diamonds,suggests to the observercrystallization in The presenceof Type I and II diamond was first demon- a singleuninterrupted event. This is not the case.In Figure strated by Robertsonet al. (1934)based on differencesin lb is shown a polished and etched surfaceof a diamond UV and IR absorption.Lonsdale (1942) showed that Type displayinga seriesof geometricallayers-referred to as the I diamond producesextra X-ray diflraction reflections,or stratigraphy of diamond (Harrison and Tolansky, 1964; spikes.These spikes were interpreted to be due to platelets Seal, 1965). These patterns were interpreted by Frank within the diamond structure (Frank, 1956).Kaiser and (1966)as being due to periodic growth on octahedraland Bond (1959),and later Lightowlersand Dean (1964)proved t46 MEYER: GENES/SOF DIAMOND: A MANTLE SAGA the presenceof nitrogen in Type I diamondsand showeda Table 1. Someproperties of diamond correlation betweennitrogen content and optical absorp- i ficat ion tion at 7.8 pm (1282 cm- r). In contrast Type II diamond CI ass Type la - Xosr comnon, appror. 981 of natural dianonds. Contains containsvery little nitrogen and no platelets. nitrogen up to 2500 ppn by uc. ss aggregares and platelets The aboveevidence suggests that the stratigraphyof dia- Type lb - Rare in naturc but mosc synthecic diamondE are of lhis Lype mond recordsperiodic growth in chemicallydifferent envi- Nitrogen < 20 pr by pt in drsperscd Eubstitutional fom. Type lla - Very ra.e. NitroAen. 20 pF by ur, Often the very ronments,at