Mafic and ultramafic clasts (mostly ~1-5 cm in size) were recovered from three different serpentinite mud volcano seamounts in the Mariana forearc during Integrated Ocean Drilling Program (IODP) Expedition 366. Mafic clasts from drill sites distant from the trench bear lawsonite, Al-rich riebeckite, jadeitic pyroxene (~80 mol% jadeite), and aragonite as metamorphic minerals, whereas mafic clasts from drill sites closer to the trench are characterized by prehnite-pumpellyite-facies mineral associations and/or the presence of analcime and natrolite. An occurrence of antigorite-bearing ultramafic clasts becomes progressively more frequent with distance from the trench. One amphibolite clast from a mud volcano, near the trench also has prehnite filling veins, and it also occurs as pseudomorphs after plagioclase. Amphibolite clasts at mud volcanoes distant from the trench are partially overprinted by blueschist-facies minerals. The apparent metamorphic grades increase with distance from the trench; these metamorphic conditions represent the increasing depth from zeolite- to lawsonite-blueschist-facies conditions in a subduction zone. Considering the consistency of the grade of the low-temperature metamorphism of mafic clasts and serpentinite mineralogy in each mud volcano, they likely reflect the thermal structure of the slab-mantle interface before ascent. As a result, these clasts were brought up to the seafloor en masse by the serpentinite mudflow. The polymetamorphosed amphibolite clasts suggest cooling of the hot forearc-mantle at the initiation of Mariana subduction in the Eocene. The ultramafic clasts in the mud volcanoes distant from the trench frequently contain clinoamphibole and talc, which indicates hot mantle.