Relationship Between Differential Travel Time of SS Precursors and Mantle Temperature and Composition Based on Mineral Physics Modeling

Differential travel times of SS precursors are widely used to study upper mantle discontinuity depths, and upper mantle discontinuity depths (or relative depths) are often used as a thermometer in the mantle assuming an endothermic phase transformation at the 660-km discontinuity and an exothermic phase transformation at the 410-km discontinuity. However, the relationship between the differential travel times of SS precursors and mantle temperature and composition is complex. Differential travel time of SS precursors is affected by the absolute S velocity in the upper mantle, the upper mantle discontinuity depths, and the detailed characteristics of the upper mantle discontinuities. All these seismic characteristics are controlled by mantle temperature and composition in a complex way. For example, the 660-km discontinuity consists of two phase transformations with an endothermic phase transformation from ringwoodite to perovskite plus magnesiowustite and an exothermic phase transformation from garnet to perovskite, and these two phase transformations interact each other chemically. At a low mantle temperature or a low Al content, double discontinuities may appear near the depth of 660 km. The composition effects on mantle velocity are also complex because of its influence on phase transformations. For example, an increasing Al content would cause an increasing S velocity in the shallow part of the upper mantle, but a decreasing S velocity in the bottom of the transition zone and the top of the lower mantle. In this study, we use a mineral physics program we developed in our recent studies to study the relationship between differential travel time of SS precursors and mantle temperature and composition. We use the mineral physics modeling method to predict mantle velocity structures based on a series of mantle temperatures and compositions (Al, Fe, Ca contents), theoretically calculate the seismograms of SS precursors at the epicentral distances between 90° and 140° based on the predicted velocity structures, and measure their differential travel times. We present the predicted differential travel times of the SS precursors for all range of possible mantle temperatures and compositions, and discuss strategies for quantitatively linking the observed SS precursors times to mantle composition and temperature.