Cr-rich spinel and garnet in two peridotite xenoliths from the Frank Smith mine, South Africa: significance of Al and Cr distribution between spinel and garnet.

Peridotite xenoliths 73-106 and 73-109 have coarse textures. All minerals in 73-106 are very Ti-rich, and include olivine (Fo 87.6), orthopyroxene (En 90.0), garnet (core TiO2 0.8, Cr203 7; rim 1.4, 4 wt. ~o), spinel (TiO 2 5, Cr203 38), clinopyroxene (A1203 1 8) secondary mica (TiO2 6, Cr203 2.5 near garnet 0.5 away from garnet), and serpentine (FeO 5-12) enclosing perovskiterimmed ilmenite and minute apatites. All minerals in peridotite 73-109 are Ti-poor, and include olivine (Fo 92.6), orthopyroxene (En 93.6), garnet (Cr203 3.1-3.8), spinel (55), ureyitic diopside, primary mica (TiO 2 0.06, CrzO 3 1.0, BaO 0.5, C1 0.04), and serpentine (FeO 2-23). Various thermometers indicate ~ 1350K (73-106) and ll00K (73-109). The low A1203 in the orthopyroxenes gives 39-47 kb for 73-109 from the Wood-Banno barometer. In 73-106, the spinel lies in secondary mica next to the garnet rim, whereas the spinel of 73-109 occurs in grains enclosed by garnet. The former assemblage indicates diffusion-dependent disequilibrium, whereas the latter is attributed to simultaneous growth of spinel and garnet, perhaps consequent upon exsolution from orthopyroxene. Complex behaviour was found for the Cr/AI distribution in published analyses of garnet and spinel. The 73-109 pair lies near the Thaba Putsoa trend, and the 73-106 assemblages are displaced towards the kelyphite region in which garnet and spinel have similar Cr/AI. Spinel and garnet may coexist over a 30 kb pressure interval ranging from ~ 16 kb for low bulk Cr to ~ 40 kb for high Cr. DETAILED study of the texture and mineral chemistry of peridotite xenoliths can provide information on the physical and chemical nature of the upper mantle. Complex textural relationships between garnet, A1,Cr-spinel, and silicates testify to incomplete accommodation to changing physical conditions (Aoki and Prinz, 1974; Basu and MacGregor, 1975; Dawson et al., 1978; Reid and Dawson, 1972; Reid et al., 1975). Occurrence * Present address: Department of Chemistry, Arizona State University, Tempe, Arizona 85281 USA. t~ Copyright the Mineralogical Society of spinel in kelyphitic coronas around garnet is particularly common, but only a few specimens (Lock and Dawson, 1980) have escaped metasomatism which has removed or obscured important textural information. The transition from the spinelto the garnet-peridotite facies is an important control in the upper mantle (O'Hara et al., 1971), but its position depends greatly on the bulk Cr/A1 ratio. Whereas the transition from spinelto garnet-peridotite occurs in the CaOMgO-A1203-SiO 2 system near 15 kb at 1200 K and about 20 kb for 1600 K (data collected in Jenkins and Newton, 1979), replacement of half the A120 3 by Cr20 3 raises the pressure by about 16 kb (MacGregor, 1970, fig. 4; see also Wood, 1977), and of all the A120 3 by ~ 28 kb (O'Neill, 1981). Although coarse grains of garnet and spinel tend to be mutually exclusive in peridotite xenoliths, the following papers report coexisting assemblages: Boyd and Nixon (1978), Kimberley, S. Africa; Carswell et al. (1979) and Mitchell et al. (1980), Pipe 100, Lesotho; Danchin (1979), Premier, S. Africa; Ferguson et al. (1977), NSW, Australia; MacGregor (1979), Kao, Lesotho; Nixon and Boyd (1973), Thaba Putsoa and Mothae, Lesotho; Smith and Dawson (1975), Matsoku; Nixon and Boyd (1979), Malaita, Solomon Islands: Sobolev (1977), occurrences in Russia. Spinel and garnet pairs in alkremites (Nixon et al., 1978), and in a spinelgarnet websterite (Irving, 1974) were not considered because their bulk compositions are much richer in A1 and Ca, and lower in Cr, than typical ultramafic peridotites, and because they lack the olivine and pyroxene of lherzolite assemblages. We describe coexisting spinel and garnet in two peridotite xenoliths from the Frank Smith mine, Bellsbank, S. Africa. Associated micas are significant for storage of K in the mantle. The xenoliths 73-106 and 73-109 were collected by JVS from a 130 R. A. E X L E Y E T AL. T~LE I. Electron microprobe analyses of 73-I06 I 2 3 4 5 6 7 8 9 I0 I ] 12 ]3 P205 O. 06 0.03 O. Ol nd nd nd nd nd nd SiO 2 41.2 42.2 39.6 57.1 54.5 48.1 37.5 37.8 34.3 40.5 0.16 0.6 0.5 TiO 2 0.84 1.36 0.03 0.19 O.l? 1.7 5.7 5.6 5.14 51.0 54.5 Al203 18.1 19.6 0.02 0.76 1.7 7.8 14.4 14.7 0.6 0.4 16.2 0.4 0.7 Cr203 6.7 3.6 0.04 0.34 1.3 2.2 2.5 0.5 38.2 0.8 nd