Abstract

An upper-mantle nodule found in kimberlite of Cenozoic age in southeastern Australia contains the assemblage aluminous spinel + garnet + orthopyroxene + clinopyroxene + olivine. This inclusion is the first recorded occurrence of an equilibrium mineral assemblage lying on the quasi-univariant boundary separating the spinel and garnet lherzolite fields inferred in the upper mantle. The temperature and pressure estimate of 1240 ± 20 °C and 22 ± 1 kb for the source of the inclusion indicates an abnormally high geothermal gradient, actually exceeding die oceanic geotherm. Heat-flow measurements also indicate a similarly high geothermal gradient for this part of Australia. It is inferred that this nodule is derived from a region of perturbed steady-state geotherm, generated during Tertiary volcanism and continuing to the present day.

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