Mid-mantle heterogeneities in the southern Pacific observed from SS and PP Precursors

Mid-mantle heterogeneities are often associated with stagnant slabs or deflected mantle plumes as a result of phase, compositional changes or viscosity jumps at ~1000 km depth. Thus, understanding the origins of these heterogeneities shed light on mantle composition, rheology as well as dynamics. Here, we used underside reflected waves, SS and PP precursors, combined with mineral physics models, to constrain the depth and sharpness of the mid-mantle reflectors in the southern Pacific region. We binned and stacked the SS and PP dataset to create linear and 2nd root vespagrams which allow us to identify the SS and PP precursors based on their travel-times and slowness. Our stacking results of SS and PP precursors reveal two groups of mid-mantle discontinuities: one at ~900 km, and the other at ~1,200 km depth. These mid-mantle reflectors are less commonly detected from PP precursors compared to SS precursors, partly due to the poorer data coverage of our PP data set in this region. Despite the difficulties, we have found some bins where both PP and SS precursors yield consistent depths of mid-mantle reflectors, which further supports the existence of these reflectors. In addition, we also detected the P520P and P660P phases in some of the bins, although these phases were rarely reported in previous global studies of mantle transition zone discontinuities. To interpret these mid-mantle reflectors, we computed mineral physics models based on the mechanical mixtures of harzburgite and basalt end-member models for a range of mantle geotherms. Our modeling suggests that a hot mantle (TP=2000 K) with a high basalt fraction (>0.6) can produce a seismic discontinuity at ~900 km depth due to the garnet phase transition, which provides a good explanation for the 900-km reflectors. This indicates that the hot mantle plumes in the southern Pacific region can potentially entrain basalts from the lower mantle, thereby causing the garnet phase transition in the mid-mantle. Instead, the deeper reflectors at ~1200 km depth could be associated with a compositional change across the top boundary of the Pacific LLSVPs.