Abstract A total dissolution technique has been developed and used to identify and quantify the incompatible element contents of fluids trapped in inclusions in minerals from peridotite xenoliths using “fluids” in the generic sense (i.e., COH fluids and melts). Fluids from lherzolites, a wehrlite, and a harzburgite host important quantities of alkalis, Ba, U, Th, Pb, and contain Sr and Nd as well. Quantitative application of the technique shows that the CO 2 -rich fluid in a Iherzolite from Nunivak Island, AK, USA and the CO 2 -poor melt in a Iherzolite from San Carlos, Arizona, USA, have incompatible element compositions similar to each other differing only in their K and Ba contents. With the exception of their K contents, the trace element compositions of these fluids resemble those of carbonatites and kimberlites. Major and radiogenic isotope data from the lherzolite and a phlogopite harzburgite from Nunivak suggest that the fluid trapped in the Iherzolite is associated with a carbonatite melt, linked to hydrous metasomatism in the region. A more dilute CO 2 -bearing melt was identified in a wehrlite from Salt Lake Crater, HI, USA resembling Hawaiian alkali basalt in its incompatible element composition. The strontium, neodymium, and lead isotope composition of the fluids resemble those of the surrounding mantle and do not reflect their parent/daughter ratios. Lead isotope data for fluid-bearing clinopyroxene in the wehrlite suggest fluid influx was recent. Conventional and laser oxygen isotope analyses show that most fluid inclusion-bearing xenoliths examined are out of oxygen isotope equilibrium. Diffusion-based arguments suggest that fluid infiltration in these xenoliths occurred over the last 1–10 My. The fluids identified in this study will dominate the incompatible element budget of typical mantle peridotite if present in greater than sub-weight percent quantities.