THE high-pressure, low-temperature metamorphic rocks known as blueschi
sts have long been considered to form in subduction zones, where the d
escent of a relatively cold slab leads to the occurrence of unusually
low temperatures at mantle pressures. Until now, however, the link bet
ween blueschist-facies rocks and subduction zones has been indirect, r
elying on a spatial association of blueschists with old subduction com
plexes, and estimates of the geothermal gradients likely to exist in s
ubduction zones. Here we strengthen this link, by reporting the discov
ery of blueschist-facies minerals (lawsonite, aragonite, sodic pyroxen
e and blue amphibole) in clasts from a serpentinite seamount in the fo
rearc of the active Mariana subduction zone. The metamorphic condition
s estimated from the mineral compositions are 150-250-degrees-C and 5-
6 kbar (16-20 km depth). The rocks must have been entrained in rising
serpentine mud diapirs, and extruded from mud volcanoes onto the sea f
loor. Further study of these rocks may provide new insight into the te
ctonics of trench-forearc systems, and in particular, the processes by
which blueschist-facies clasts come to be associated with forearc sed
iments in ancient subduction complexes.