Most mineral assemblages in depleted mantle xenoliths are garnet-free, and not amendable to available geothermobarometers. Forthis reason, the provenance and origin of depleted mantle xenoliths are still uncer tain. A suite of depleted harzburgite xenoliths from the Kimberley pipe in South Africa contains symplectites of spinel-clinopyroxene, which also are common in depleted peridotites worldwide and interpreted as the breakdown products of pre-existing garnet. Recalculated garnet compositions were used to estimate the P-T conditions of the harzburgites; these fall in the lower-P end of the range for xenoliths from cratonic mantle, but on a very high geotherm (P = 3-5 GPa and T = 1450-1600 C). These parameters correspond to the garnet-spinel transition zone, where garnet begins breaking down to spinel at about 3 GPa. This garnet-spinel transition zone is one of major phase boundaries in the upper mantle, but is geophysically invisible. MgO-FeO modelling and the Mgf residual olivine suggest that these peridotites can be residues after 30 to 40% partial melting of the upper mantle. Re-Os model ages suggest these melting events occurred from 3.6 to 2.7 Ga, corresponding to the major crust-extraction events in the Kaapval Craton. These data confirm the emerging consensus that the most primitive Archaean SCLM was a depleted harzburgite-dunite, representing the residues of high-degree melt extraction at pressures of a few GPa.
|Number of pages||1|
|Publication status||Published - 2012|
|Event||International Geological Congress (34th : 2012) - Brisbane, Australia|
Duration: 5 Aug 2012 → 10 Aug 2012
|Conference||International Geological Congress (34th : 2012)|
|Period||5/08/12 → 10/08/12|