2005 Sal Abstract

Bina, C. R., E. A. Okal, and S. van der Lee, Pyroxenites and low shear velocities beneath the Ontong-Java Plateau, Abstracts of the SAL2005 International Workshop on Ocean Island Volcanism, Sal, Cape Verde Islands, 87, 2005.

Rayleigh-wave tomography of the Ontong-Java Plateau (OJP) reveals a root extending as deep as 300 km, in which shear velocities are depressed by up to 5% (Richardson et al., 2000). Analyses of multiple-ScS phases confirm this slow root and indicate unusually high Q_ScS, a combination of low velocity and high strength suggestive of compositional rather than thermal origins (Gomer and Okal, 2003). While recently ascribed to a bolide impact (Ingle and Coffin, 2004), the OIB-like isotopic compositions (Tejada et al., 2003), evidence of discrete episodes of activity every ~30 My since 122 Ma (Chambers et al., 2003), and recent measurements of xenolith ages (Ishikawa et al., 2005) are consistent with deep plume origins (Larson, 1991). Melt extraction from mantle lherzolite would leave a strong, low-H₂O residue (Hirth and Kohlstedt, 1996), consistent with the observed high Q values, but could it give rise to low shear velocities?

A recent petrological and thermobarometic study of xenolith suites from beneath the OJP suggests a layered lithospheric mantle featuring deepseated pyroxenites (Ishikawa et al., 2004). We numerically (Hacker and Abers, 2004) estimated shear velocities for several xenolith compositions. While some of the compositions (mainly harzburgites and lherzolites) do not exhibit low shear velocitites, others (mainly pyroxenites) do. Garnet websterites at ~60 km and both garnet orthopyroxenites and garnet clinopyroxenites at ~100 km depth exhibit estimated shear velocities even slower than those in the model JV30 (Richardson et al., 2000) determined for the OJP. We suggest that low shear velocities beneath the OJP may arise from deep-seated pyroxenitic melt residues.

Some xenolithic pyroxenites contain subcalcic diopside phases, and we speculate that shear velocities potentially may further be depressed by shear softening associated with displacive transitions in clinopyroxenes that contain a significant clinoenstatite component (Angel et al., 1992; Mendelssohn and Price, 1997; Tribaudino et al., 1999; Arlt et al., 2000; Jackson et al., 2004; Kung et al., 2004; J. Kung, pers. comm.). Ca-poor clinopyroxene compositions may arise from significant degrees of partial melting at high pressures (Walter, 2004; M. Walter, pers. comm.).