Silicate-oxide symplectites in complex mineral intergrowths are relatively common in upper mantle xenoliths and in xenoliths in the Jagersfontein Kimberlite, South Africa. Harzburgites of olivine and high-Al (1.9-3.6 wt%), Ca (0.6-0.9 wt%) and Cr (0.3-0.9 wt%) enstatite contain symplectites of spinel and diopside, or spinel, diopside and lower-Al (0.8-2.2 wt%), Ca (0.1-0.4 wt%) and Cr (0.02-0.8 wt%) enstatite. From textures and mineral chemistries these symplectites are interpreted to have formed by mineral unmixing and migration from Al-Ca-Cr discrete enstatite to adjoining mineral interfaces. Garnet harzburgites are composed of large (0.5-1 cm) olivine, equally large discrete low-Al (0.6-1.1 wt%), Ca (0.1-0.5 wt%), and Cr (0.1-0.3 wt%) enstatite and smaller interstitial garnet, diopside, and high-Cr and low-Al spinel. Symplectites are composed of either spinel + diopside + garnet, or garnet + spinel. Spinel diopside garnet symplectites have cores of spinel + diopside, resembling symplectites in harzburgites, but surrounded by rims of garnet or garnet + undigested globular spinel. From textures and chemistries we suggest that the spinel + diopside cores formed from Ca-Al-Cr-rich orthopyroxene initially as a nonstoichiometric homogeneous single phase clinopyroxene enriched in Fe, Cr and Al. This was followed by decomposition of the clinopyroxene to diopside + spinel, and subsequent garnet formation in a prograde reaction with olivine or enstatite. In both harzburgites and garnet harzburgites the metastable cellular structures may also have formed by the simultaneous precipitation of pyroxene and spinel. In all cases there is a strongly preferred embayment of symplectite bodies into olivine. Olivine appears to have activated adjacent orthopyroxene stimulating exsolution and mineral migration. We attribute these reactions to stress-assisted precipitation (SAP) during transient events of disruption and decompression of the subcontinental lithosphere, prior to and during protokimberlite genesis and sampling. The SAP mechanism may have been aided by redox-assisted-inter-diffusion (RAID) of oxygen and hydrogen from within minerals and along mineral interfaces. The implications of the symplectite assemblages are that two-phase olivine and high-Al enstatite harzburgites are possible precursors to some lherzolites, garnet lherzolites, and garnet harzburgites in depleted subcratonic lithospheres. The formation of embryonic symplectites, with subsequent growth and equilibration: (1) will increase bulk densities with the formation of spinel and garnet, (2) will enhance the potential of partial melting, (3) may be progenitors to the extraction of small pockets of melts of eclogitic or pyroxenitic composition, and (4) have bulk compositions that approach green, uvarovite-rich garnets.
