SUBDUCTION ZONE FLUID COMPOSITION ESTIMATED FROM FLUID INCLUSIONS IN GUATEMALAN JADEITITE

Jadeitites (jade) precipitate directly from hydrous fluids in subduction zone complexes and record parental fluid compositions. Fluid inclusions in jadeitites from slices of serpentinite-matrix mélange along the Motagua Fault zone (MFZ) in Guatemala should therefore provide a direct estimate of the fluid composition(s). The inclusions contain low salinity brines trapped in several different P-T regimes at low to moderate subduction-zone geothermal gradients.

Jadeitite occurrences along the MFZ extend for >150 km on the north and 25 km on the south. In jadeitites north of the MFZ from Pachalum (Granados) to Panaluya (Río Hondo), initial fluid inclusion results testify to salinities that range from 3 to 7 wt% NaCl equivalent. However, these are not pure NaCl fluids, as initial melting temperatures are depressed. Some samples show salinity variations linked to changes in cathodoluminescence color. There is also a range in homogenization temperatures from low in early jadeite to high in secondary jadeite, which may reflect changes either in T or P during jadeitite formation. Northern jadeitites probably all formed in the same P-T regime of 6 to10 kb and 300 to 400°C, based on their similar mineral assemblages.

In contrast, petrologic data suggest that south of the MFZ jadeitites represent 3 distinct P-T regimes. The one sample analyzed so far for fluid inclusions (from La Ceiba) has a mineral assemblage preliminarily interpreted as P ~ 10 to 14 kb and T <300 to 400°C. It contains lower salinity fluid inclusions (0 to 2 wt% NaCl equivalent), and lower homogenization temperatures than counterparts from north of the MFZ. Jadeitite samples from the other two P-T regimes south of the MFZ await analysis.

Overall, our results show a range in low salinity fluids that implies the subduction fluid was dominated by one fluid that underwent minor changes in chlorinity and ionic strength during deposition of jadeitite, while facilitating the striking contrasts in abundances of LIL and LREE recorded by these rocks.

Having lived in Guatemala, Maria Luisa Crawford wanted to study these rocks but political and exposure complexities prevented it. We are most fortunate to have Weecha's insight into aqueous fluid inclusions to guide our work. At this Theme Session we congratulate her upon her retirement from “active” teaching.