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Although minor constituents of the average mantle, presence of these oxides change the mineralogy of mantle :

  • A.\(\mathrm{Al_{2}O_{3}}\), CaO and \(\mathrm{Na_{2}O}\)
  • B.\(\mathrm{K_{2}O}\), \(\mathrm{P_{2}O_{5}}\) and CaO
  • C.\(\mathrm{Pb_{3}O_{4}}\), \(\mathrm{Fe_{2}O_{3}}\) and \(\mathrm{TiO_{2}}\)
  • D.Al(OH), \(\mathrm{H_{2}O}\) and \(\mathrm{Na_{2}O_{3}}\)

Answer: A

The oxides are Al2O3, CaO and Na2O.

The mantle is dominated by MgO, SiO2 and FeO, which make olivine and pyroxene. The few percent of alumina, lime and soda decide which aluminous phase is present: plagioclase at shallow depth, spinel deeper, and garnet below about 80 km, and they also control the amount of clinopyroxene. They are minor constituents but they set the mineralogy of mantle peridotite.

K2O and P2O5 are trace components; Pb3O4 (red lead) is not a mantle oxide; and option D lists formulas such as Al(OH) and Na2O3 that are not the mantle's oxides.

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