Diamonds from the asthenosphere and the transition zone

Over the last two decades inclusions in diamonds have been recognised from a number of mines which represent chemically pristine samples of the sublithospheric mantle. X-ray diffraction studies show that isochemical phase transitions occurring in the plastic deformation field of diamond have completely eliminated crystal structures which are stable at transition zone and lower mantle pressures only. In the case of lower mantle inclusions ultra-deep parageneses may nevertheless be readily recognised by their unique chemical composition. For the chemically much less distinct asthenospheric and transition zone parageneses, majorite garnet is the only reliable indicator. From the majorite garnet record, the deeper upper mantle and transition zone appear to be dominated by basaltic material. Apart from elevated Si contents, the major element composition of majorite garnets is not significantly different from lithospheric eclogitic garnets, although there is a bias to higher Na and Ti and lower Fe contents. Coexisting clinopyroxene completely falls within the compositional ranges of the normal eclogitic suite. Trace element studies indicate that the majoritic source rocks have REE_N patterns which range from MORB type to strongly LREE enriched. As petrological data are not consistent with a basaltic asthenosphere and transition zone, deep diamonds may represent either an exceptional sampling bias or, more likely, preferential diamond formation in eclogitic environments. This may be due to the pyrolitic parts of the transition zone being too reducing for diamond formation. It is speculated that sublithospheric eclogitic diamonds originated in subducted oceanic crust, which is relatively oxidised compared to pyrolite mantle due to sea water alteration. In such a scenario diamonds may originate either from redox reactions involving reduced mantle fluids and slab material or from reduction of carbonates which occurs invariably with increasing depth as pressure promotes the formation of ferrous iron rich minerals and therefore decreasing fO₂. The latter process may explain the isotopically heavy composition (δ¹³C up to +0.9 ‰) of some asthenospheric and transition zone diamonds. Isotopically light diamonds (δ¹³C down to -24.4 ‰) are probably related to chemical fractionation processes since subducted organic matter which is frequently invoked to cause such signatures should convert to diamond at much shallower depth.