Seismic Observations of Splitting of the Mid-transition Zone Discontinuity in Earth's Mantle

Seismic Observations of Splitting of the Mid-Transition Zone Discontinuity in Earth’s Mantle Data • Long period SS precursors • Seismograms are stacked in 10 degree bins A note about using SS precursors for topography studies: • Resolution extremely low (cannot resolve features less than about 1000 km) see insert reference here • Not ideal for mapping complicated topography, i.e. near subduction zones • Excellent global coverage Results • Globally seen arrivals from 410 and 660 discontinuities • Arrivals from the 520 discontinuity either: • Are absent • Show a single arrival • Show a double arrival • These observations (of the 520) correlate neither with surface tectonics, nor with global tomography maps Image removed due to copyright considerations. What’s the meaning of this? Image removed due to copyright considerations. Mineral physics explanation? • May be due to both the β – γ phase change and the gt – ca-pv phase change. • Both phase changes occur at the same depth at 1500 degrees • Gt – ca-pv has a clapeyron slope of 0 to -2 MPa/K, whereas the β – γ phase change has a positive clapeyron slope, thus… • Temperature variations would cause the pressures of these phase changes to be negatively correlated, causing two separate reflectors. However….there is no correlation between tomography (which they assume to reflect mostly temperature) and the distribution of the double and single reflections. Therefore… Perhaps the variations in transition depth of the two phase changes are caused by compositional variations. Compositional Effects? Images removed due to copyright considerations. • Iron partitioning between garnet and beta and gamma phases could reduce transition width, causing better observability of the discontinuity (Weidner and Wang, 2000). • If Mg # is too low, the beta – gamma transition does not occur– this would cause no, or one (from gt – ca-pv), discontinuity. • Low Ca content in gt would cause the transition to ca-pv not to occur. • Water and other trace elements may also change the sharpness of the discontinuity. ™ A single reflection could correspond to regions that are a) Ca-poor, b) Ferich, or c) low in water content. These factors may also explain the absence of an arrival. Splitting observations, revisited: Images removed due to copyright considerations. Synthetics • Created synthetic seismograms using 3 models: •(A) no mid-transition zone discontinuity •(B) one mid-transition zone discontinuity at depths of 525, 540, and 565 km •(C) split mid-transition zone discontinuity The authors say that the observations are best fit with the split discontinuity. From Figure 1, section a -- A Image removed due to copyright considerations. Questions • Do the synthetics really support their conclusions? • Splitting of arrivals may be caused by small scale topography that the SS precursors are unable to resolve insert reference • What are the implications of such compositional variation in the mantle? • Could these splitting observations be artifacts? From Figure 2, section a -- A Image removed due to copyright considerations. Conclusions z Mid-transition zone discontinuity arrivals are highly variable, showing single or double reflectors, or none at all z May be explained by dual phase change z Arrival variations may reflect compositional variations z Implies that mantle transition zone (TZ) is quite heterogeneous; if these observations are robust, mid-TZ arrivals may be used as a “probe for composition” in the TZ.