Tectonophysics, 173 (1990) 207-210 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands 207 E_~te~ded abstract Measured and calculated elastic wave velocities for xenoliths from the lower crust and upper mantle zyxwvutsrqponmlkjihgfe IAN JACKSON I, ROBERTA L. RUDNICK ‘, S.Y. O’REILLY 2 and C. BEZANT ’ ’ Research School of Earth Sciences, Australian h’arional University, G. P.O. Box 4, Canberra. A.C. T. 2601 (Australia) ’ School of Earth Sciences, Macquarie Ciniversity, Sydney, N.S. W. 2109 fAu.s?ralia) (Received April 20,1989; accepted May 30,1989) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR Abstract Jackson, I., Rudnick, R.L., O’Reilly, S.Y. and Bezant, C., 1990. Measured and calculated elastic wave velocities for xenoliths from the lower crust and upper mantle. In: J.H. Leven, D.M. Finlayson, C. Wright, J.C. Doohey and B.L.N. Kennett (Editors), Seismic Probing of Continents and their Margins. Tectonophysics, 173: 207-210. Compressional wave velocities have been mea- sured by ultrasonic pulse transmission as func- tions of pressure (generally to 1 GPa) on jacketed specimens prepared from a variety of mafic and ultramafic xenoliths derived from the lower crust and upper mantle beneath eastern Australia. Three broadly representative suites have been studied: (1) mafic, pyroxene-rich, garnet granulites and eclogites from the Calcutteroo kimberlite pipe in South Australia (Jackson and Arculus, 1984), (2) mafic, plagioclase-rich garnet granulites from the Chudleigh volcanic province of north Queensland (Rudnick and Jackson, in prep.), and (3) ultra- mafic rocks (spine1 lherzolites and a garnet pyrox- enite) from the adjacent Bullenmerri and Gnotuk maars and from Mt. Porndon in Victoria (C)‘Reilly et al., in prep.). The measured velocities typically increase sharply with increasing pressure below - 400 MPa as a result of crack closure, but be- come much less pressure sensitive at higher pres- sures. Specimens prepared from pervasively cracked ultramafic xenoliths (as surface-epoxy-im- pregnated rectangular prisms rather than the usual cylindrical cores) continue to increase substan- tially in velocity to 800 or even 1000 MPa (1 GPa). In either case, the velocity measured at the highest pressure (typically 1 GPa) is considered to be approximately representative of the intrinsic properties of the untracked rock. The fidelity with which the mean velocities measured at high pressure (averaged over three mutually orthogonal propagation directions) re- flect the primary mineralogy has been assessed by comparison of the measured velocities with those calculated for the modal mineralogy from ap- propriate single-crystal elasticity data. The strategy outlined in Table 1 has been employed in the calculation of density and both compressional and shear-wave velocities for composites of the miner- als quartz, plagioclase, clinopyroxene, orthopyrox- ene, olivine, spine1 and garnet. The relevant elas- ticity data for the mineral end members are as- sembled in Table 2. There exists an encouraging degree of overall consistency between calculated and measured wave velocities for ‘the freshest xenoliths. Any discrepancies are therefore di- agnostic of departures of the actual mineralogy of the measured specimen from the nominal modal mineralogy often determined on a separate sample of the same xenolith. Such departures may be 0040-l 951,/90/$03.50 0 1990 Elsevier Science Publishers B.V.