UPPER-MANTLE MINERALOGY

The mineralogy of the upper mantle is reviewed in the context of the petrology of xenolith suites entrained in alkali basalts and kimberlites. Low pressure (spinel) and high pressure (garnet) Iherzolites (olivine + clinopyroxene + orthopyroxene) are the most abundant fertile (i.e. capable of producing basalt with high Ca, Al, Fe and Si) rock types. Xenoliths of high pressure partial melts (websterites, wehrlites, pyroxenites, but excluding eclogites) are more common in alkali basalts than kimberlites, depleted suites of harzburgite and dunite have the reverse distribution, whereas metasomatically-enriched xenoliths are equally abundant in both volcanic eruptives. Systematic mineralogy is discussed under primary minerals (olivine, pyroxene, garnet and spinel), metasomatic minerals (amphibole, phlogopite, apatite, sulfides, carbonates, ilmenite, rutile, exotic mineral oxides, armalcolite, baddelyite, zirconolite, and zircon), native elements (diamond, graphite, metals), and other minerals (carbide and magnesiowustite). Alkali basalts and kimberlites originate at similar to 60 and >250 km, respectively, and both rock types are related to, and are genetically dependent on, the long term infiltration of metasomatic, volatile-rich melts. Pristine xenoliths and extraordinarily rare. The increasing recognition of samples from the transition zone (410-660 km) and the lower mantle (>660 km) places new constraints on the geodynamics of the mantle and is an incentive to the possible reinterpretation of many xenoliths that were previously assumed to be of upper mantle origin.