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Mineral Sciences Investigations 1974-1975 Composition of Garnet Xenocrysts from Three Kimberlite Pipes in Arizona and New Mexico ABSTRACT Microprobe analyses were made of a large number of garnet xenocrysts from three kimberlite pipes in Arizona and New Mexico in a search for garnets having a composition similar to those found as inclusions in diamond. At none of the localities were any xenocrysts found having compositions lying within the diamond inclusion field in Ca-Cr, Ca- Mg-Fe, and Ca-Mg-Cr plots. From this one could conclude that none of the three kimberlite pipes examined is diamondiferous. If they were prospected on the basis of garnet composition, Garnet Ridge is the one most likely to contain diamond. Introduction Garnet xenocrysts in kimberlite may be derived from three types of xenoliths: from peridotites and eclogites of mantle origin, and disaggregated crustal xenoliths. Consequently, garnets from any one kimberlite pipe may vary widely in composition. This is reflected in the wide range of colors frequently seen in garnets from kimberlite. These colors may include red, orange, yellow, violet, purple, pink, brown, with all gradations between. George S. Switzer, Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Wash' ington, D.C. 20560. George S. Siuitzer Of particular interest are the garnets of high pressure type considered to be of mantle origin, and especially those having a composition similar to garnets found as inclusions in diamond. Gurney and Switzer (1973) have shown that there are small compositional differences between garnets found in common garnet peridotite xenoliths in kimberlite, and those found as inclusions in diamond. These differences are that the diamond inclusions have higher magnesium and lower calcium content associated with high chromium. In their study of garnets from the Finsch diamond mine, Gurney and Switzer found some Mg-rich, Cr-rich, Ca-poor garnets that fell within the composition field of garnets previously only reported as inclusions in diamond. These garnets are considered to have a deeper provenance than the Mg-garnets commonly found as xenocrysts in kimberlite and in garnet peridotite xenoliths. Their presence in a kimberlite should be diagnostic of the presence of diamond, although it was not demonstrated whether or not there was any quantitative relationship. This discovery showed that N. V. Sobolev, et al. (1969) were correct when they suggested that kimberlite heavy mineral concentrates had not been adequately studied to reveal their full compositional range. An extension of the work of Gurney and Switzer has been carried out by Lawless (1974). He found garnets with compositions similar to those found
Constituent SiO TiO Al 0 fl 2 3 Cr 0, 2 3 V 0 2 3 Fe 0^ 2 3 FeO MnO MgO CaO Na 0 Si .. Ti .. Al .. Cr .. V . . . Fe +3 Fe +2 Mn .. Mg .. Ca .. Na . . 1 42.7 0.02 18.0 6.08 6.02 a 0.21 23.4 1.33 97.8 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES TABLE 1.—Composition of ganlet inclusions in diamond used to delineate the "diamond inclusion field" 2 42.6 0.02 17.9 7.09 6.93 a 0.28 22.6 1.24 98.6 3.08 3.07 3 42.7 0.02 18.4 7.19 6.55 a 0.28 23.3 1.09 99.5 4 41.7 0.03 17.6 7.72 7.01 0.37 22.5 1.46 98.4 0.001 0.001 0.001 0.002 1.53 1.52 0.35 0.40 0.36 0.42 0.01 0.02 2.52 2.43 0.10 0.10 5 41.4 0.08 11.1 15.9 a 6.18 a 0.23 23.1 2.24 100.1 6 41.7 0.08 18.1 6.97 8.12 0.37 21.1 0.05 99.15 7 42.3 0.02 17.2 8.93 c c 1.45 i.oo a C C 4.06 3.85 0.21 25.3 1.09 100.56 8 42.8
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