MANTLE FLUIDS - EVIDENCE FROM FLUID INCLUSIONS

A total dissolution technique has been developed and used to identify and quantify the incompatible element contents of fluids trapped in in clusions 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 CO2-rich fluid in a Iherzolite from N univak Island, AK, USA and the CO2-poor melt in a lherzolite from San Carlos, Arizona, USA, have incompatible element compositions similar t o each other differing only in their K and Ba contents. With the excep tion of their K contents, the trace element compositions of these flui ds resemble those of carbonatites and kimberlites. Major and radiogeni c isotope data from the lherzolite and a phlogopite harzburgite from N univak suggest that the fluid trapped in the lherzolite is associated with a carbonatite melt, linked to hydrous metasomatism in the region. A more dilute CO2-bearing melt was identified in a wehrlite from Salt Lake Crater, HI, USA resembling Hawaiian alkali basalt in its incompa tible 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 fl uid-bearing clinopyroxene in the wehrlite suggest fluid influx was rec ent. Conventional and laser oxygen isotope analyses show that most flu id inclusion-bearing xenoliths examined are out of oxygen isotope equi librium. 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 quant ities.