The petrology and geochemistry of ultramafic nodules from Pipe 200, northern Lesotho

The Pipe 200 ultramafic nodule suite is dominated by relatively depleted garnet- and/or chromite-bearing lherzolites (whole rock 100 Mg/ (Mg+) 91.0–93.5). These contain olivines (Fa 7.0–8.6), orthopyroxenes (Fs 6.3–7.7) with 0.54–1.04 wt.% Al2O3, clinopyroxenes with consistent 100 Ca/(Ca+Mg) (46.0–48.4) but variable Al2O3, (1.64–3.99 wt.%), Cr2)3(1.59–4.25 wt.%) and Na2O (0.81–3.14 wt.%), chrome pyrope garnets and/or primary magnesiochromites. By contrast four ultradepleted chromite harzburgites/lherzolite (100 Mg/(Mg+Fe) 95.0–95.4) contain more magnesian olivines (Fa 4.9–5.0) and orthopyroxenes (Fs 4.1–4.3). Provisionally preferred P/T estimates for the garnet–bearing lherzolites are 906–949°C and 24.5–30.8 kb, the temperature range being somewhat wider (873–962°C) if one also includes lherzolites in which garnet is absent but is considered likely to have been originally present. An ultradepleted chromite lherzolite nodule, which is unlikely to have ever contained garnet, yields an equivalent equilibration temperature of 950°C. It is concluded that these various ultramafic nodules may all have been derived from much the same depth zone in the mantle (roughly 70–100 km below surface on preferred estimates), the presence or absence of primary chromite relative to garnet being related to shifts in the position of the spinel lherzolite/garnet lherzolite transition boundary for compositions with various Cr2O3/(Cr203+Al2O3) ratios. The observed nodule suite does, however, contain one harzburgite nodule with more aluminous primary spinel and orthopyroxene which seems likely to have equilibrated under significantly lower P/T conditions and hence have been derived from depths < 50 km. Mineralogical and chemical comparison between the Pipe 200 ultramafic nodules and those from other pipes in northern Lesotho and in the Kimberley area, as well as in the Lashaine volcano, indicates that the mantle beneath Pipe 200 and much of northern Lesotho may be extensively depleted down to greater depths, perhaps reflecting the intensive Karroo volcanism in that area.