COMPARATIVE COMPRESSIBILITY OF CLINOPYROXENES FROM MANTLE NODULES

The comparative compressibilities of two clinopyroxenes from mantle no dules were determined by single crystal X-ray diffraction with a Merri ll-Bassett diamond anvil cell. The analysed crystals show different ch emical compositions: PC135, from a spinel lherzolitic nodule from Pico Cabugi, NE Brazil, has more R(3+) (Fe3+ + Al-VI + Ti + Cr) in the M1 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, Pa raguay. Lattice parameters were determined under different pressures b etween 1 bar and 46 kbar with both crystals mounted together in the di amond anvil cell. The different chemistries of the two samples produce opposite effects on the polyhedral bulk moduli. On the whole, the bul k modulus is the same in both samples: K is 1170(50) kbar in PC135 and 1160(30) kbar in 3211. For both clinopyroxenes beta(b) > beta(c) > be ta(asin beta). In particular, the following values were recorded: beta (b) = 3.9(7) 10(-4), beta(c) = 2.9(2) 10(-4), beta(asin beta) = 2.2(6) 10(-4) kbar(-1) in PC135 and beta(b) = 3.9(8) 10(-4), beta(c) = 2.7(2 ) 10(-4), beta(asin beta) = 2.5(3) 10(-4) kbar(-1) in 3211. Greater di fferences are seen in the epsilon(1)/epsilon(3) ratios between the sem iaxes of the strain ellipsoid of the two clinopyroxenes: the higher co ntent of R(3+) in the M1 site and the lower Ca content in the M2 site of PC135 compared to 3211 produces a more eccentric (010) section of t he strain ellipsoid in PC135 than in 3211. The pressure induced effect s of composition on the geometrical deformations are in accordance wit h those simulated by theoretical calculations.