Various changes in seismic structures across the mantle transition zone (MTZ) indicate that it may hamper thermal and chemical circulation. Here we show how thermal elevation of the postspinel phase transition at 660 km depth plus harzburgite segregation below this depth can project as narrow high-velocity anomalies in tomographic images of continuous thermochemical mantle upwellings. Model S40RTS features a narrow high-velocity anomaly of +0.8% near 660 km depth within the broad low-velocity structure beneath the Samoa hot spot. Our analyses indicate that elevation of the 660 phase boundary in a hot pyrolitic plume alone is insufficient to explain this anomaly. An additional effect of harzburgite enrichment is required and consistent with geodynamic simulations that predict compositional segregation in the MTZ, especially within thermochemical upwellings. The Samoa anomaly can be modeled with a 125–175°C excess temperature and a harzburgite enrichment below 660 of least 60% compared to a pyrolitic mantle.