17:15 〜 19:15
[SIT17-P04] Effect of basaltic melt on the sound velocities of Ringwoodite in the lowermost mantle transition zone
キーワード：High-pressure, Elastic wave velocity, Partial melt, Mantle Transition Zone
Recycling of subducted oceanic lithosphere at subduction zones has been argued as the main source of long-term heterogeneities in the Earth’s mantle, as supported by samples returned from the mantle and seismic scatterers widespread at multiple depths across the globe [1]. The mantle transition zone (MTZ) appears to be a very complex layer with seismic velocity increments at the main discontinuities (410’ and 660’) whereas velocities decrements, in addition to high attenuation, are observed in adjacent areas at 350-410 km depths atop 410’ and beneath 660’ [2,3]. Incidentally, the composition of the MTZ is still not well constrained with models ranging from a fully equilibrated pyrolitic composition to a mechanically mixed MTZ enriched in basalt. While melting temperature of MORB in the MTZ is too high compared to that of the mantle geotherm [4], presence of volatiles or increasing temperature by upwelling plumes underneath the MTZ may partially melt basalt in the lowermost MTZ, for which the effect on the sound velocities of mantle rocks is still unknown.
In this study, we investigated P- and S-wave velocities (VP and VS) of Ringwoodite (Rw) + 1–5 wt.% Mid-Ocean Ridge Basalt (MORB) and MORB aggregates, at a fixed pressure of 20 GPa, and temperatures up to 2000 ºC, using ultrasonic interferometry combined with synchrotron X-ray techniques in the multianvil press, at the beamline BL04B1 in SPring-8. Analyses of X-ray diffraction spectra of Rw + MORB and MORB samples indicated the coexistence of Rw + melt and Majorite + melt at 2000 ºC [4], respectively. Our data showed VP and VS decreased monotonically with increasing temperature up to 2000 ºC, suggesting melt distribution at the Rw and Mj grain boundaries did not disrupt the propagation of elastic waves, particularly S-wave velocities. These results suggest the presence of partially molten basalt in the lowermost MTZ is unlikely to cause large low shear velocity anomalies at the interfaces between the subducted oceanic crust and pyrolitic mantle.
References
[1] Ballmer et al., Science Adv. 1, 11 (2015)
[2] Revenaugh & Sipkin, Nature 369, 474 (1994)
[3] Schmandt et al., Science 344, 6189 (2014)
[4] Litasov & Ohtani, Adv. in High-Press. Mineralogy. Geological Society of America (2007).
In this study, we investigated P- and S-wave velocities (VP and VS) of Ringwoodite (Rw) + 1–5 wt.% Mid-Ocean Ridge Basalt (MORB) and MORB aggregates, at a fixed pressure of 20 GPa, and temperatures up to 2000 ºC, using ultrasonic interferometry combined with synchrotron X-ray techniques in the multianvil press, at the beamline BL04B1 in SPring-8. Analyses of X-ray diffraction spectra of Rw + MORB and MORB samples indicated the coexistence of Rw + melt and Majorite + melt at 2000 ºC [4], respectively. Our data showed VP and VS decreased monotonically with increasing temperature up to 2000 ºC, suggesting melt distribution at the Rw and Mj grain boundaries did not disrupt the propagation of elastic waves, particularly S-wave velocities. These results suggest the presence of partially molten basalt in the lowermost MTZ is unlikely to cause large low shear velocity anomalies at the interfaces between the subducted oceanic crust and pyrolitic mantle.
References
[1] Ballmer et al., Science Adv. 1, 11 (2015)
[2] Revenaugh & Sipkin, Nature 369, 474 (1994)
[3] Schmandt et al., Science 344, 6189 (2014)
[4] Litasov & Ohtani, Adv. in High-Press. Mineralogy. Geological Society of America (2007).