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An updated classification scheme for mantle-derived garnet, for use by diamond explorers |
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| Authors: | Herman S Grütter John J Gurney Andrew H Menzies Ferdi Winter |
| Institution: | a De Beers GeoScience Centre, PO Box 82232, Southdale, Gauteng 2135, South Africa b Mineral Services Canada, 205-930 Harbourside Drive, North, Vancouver, BC, Canada V7P 3S7 c Mineral Services South Africa, PO Box 38668, Pinelands 7430, South Africa |
| Abstract: | Mantle-derived garnets recovered in diamond exploration programs show compositional variations in Cr, Ca, Mg, Fe and Ti that reflect the chemical, physical and lithological environments in which they occur, occasionally together with diamond. The association of diamond with mantle garnet has progressed through a number of geochemical advances, most notably those of Dawson and Stephens (1975) and Gurney (1984), which are integrated in this work with less well known petrological advances made primarily in xenolith and experimental petrology. A simple, robust garnet classification scheme is formulated which accommodates empirical garnet–diamond relationships for peridotitic (G10, G9, G12), megacrystic (G1), Ti-metasomatised (G11), pyroxenitic (G4, G5) and eclogitic (G3) lithologies in eight distinct garnet classes. The calcium-saturation characteristics of harzburgitic (G10), lherzolitic (G9) and wehrlitic (G12) garnets are described by a Ca-intercept projection that also shows promise as a relative barometer for garnet lherzolite (Grütter and Winter, 1997). Thermobarometric aspects of garnet–diamond associations are highlighted in the scheme through the use of the minor elements Mn and Na, though analysis by anything other than an electron microprobe is not required for classification. A “D” suffix is added to the G10, G4, G5 or G3 categories to indicate a strong compositional and pressure–temperature association with diamond. The scheme remains open to improvement, particularly with regard to delineation of pyroxenitic (or websteritic) diamond associations and to advances in Ca-in-garnet and Na-in-garnet thermobarometry. |
| Keywords: | Diamond exploration Pyrope Ca-intercept Peridotite Megacryst Eclogite Pyroxenite G10 G9 G1 G3 |
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