Comparative compressibility of clinopyroxenes from mantle nodules.

The comparative compressibilities of two clinopyroxenes from mantle nodules were determined by single crystal X-ray diffraction with a Merrill-Bassett diamond anvil cell. The analysed crystals show different chemical compositions: PC 135, from a spinel lherzolitic nodule from Pico Cabugi, NE Brazil, has more R3+ (Fe3+ + VIA1 + Ti + Cr) in the Ml site, more Mg in the M2 site (i.e. lower Ca) and more Al in T compared with 3211, a crystal from a spinel harzburgitic nodule from Nemby, Paraguay. Lattice parameters were determined under different pressures between 1 bar and 46 kbar with both crystals mounted together in the diamond anvil cell. The different chemistries of the two samples produce opposite effects on the polyhedral bulk moduli. On the whole, the bulk modulus is the same in both samples: K is 1170(50) kbar in PC 135 and 1160(30) kbar in 3211. For both clinopyroxenes ß/> > ßc > ßαsinß. In particular, the following values were recorded: ß*> = 3.9(7) 10"4, ßc = 2.9(2) 10Λ ßαsin ß = 2.2(6) 10"4 kbar1 in PC135 and ft, = 3.9(8) 104, ßc = 2.7(2) 10'4, ßΩsin ß = 2.5(3) 10-4 kbar1 in 3211. Greater differences are seen in the 81/83 ratios between the semiaxes of the strain ellipsoid of the two clinopyroxenes: the higher content of R3+ in the Ml site and the lower Ca content in the M2 site of PC 135 compared to 3211 produces a more eccentric (010) section of the strain ellipsoid in PC 135 than in 3211. The pressure induced effects of composition on the geometrical deformations are in accordance with those simulated by theoretical calculations.