Fluid inclusions in jadeitite and jadeite-rich rock from serpentinite mélanges in northern Hispaniola: Trapped ambient fluids in a cold subduction channel

Abstract Freezing-point depressionwas measured in aqueous fluid inclusionsto determine salinities in six samples of jadeitite and jadeite-rich rock from the Jagua Clara serpentinite melange of the Rio San Juan Complex, Dominican Republic. The melange represents a fossil subduction-zone channel from a cold, mature subductionzone with a geothermal gradientof ~6 °C/km. One hundred and twenty-five determinations of salinity in primary inclusions hosted in jadeite, quartz, apatite and lawsoniterange between extremes of 1.2 and 8.7, but yield a well-defined mean of 4.5 ± 1.1 wt% (±1 s.d.) NaCl equiv, slightly higher than mean seawater (3.5 wt%). In one sample, eight additional fluid inclusionsin quartz aligned along grain boundaries yield slightly lower values of 2.7 ± 1.3 wt% NaCl equiv. Homogenization temperatures were also measured for 47 fluid inclusionsin two samples, but primary entrapment densities are not preserved. It is significant that the suite includes two types of samples: those precipitated directly from an aqueous fluid as well as examples of metasomaticreplacement of a pre-existing magmatic rock. Nevertheless, the results indicate identical salinity for both types and suggest a much stronger genetic link between the two types of jadeitite and jadeite-rich rock than has previously been assumed. Based on the results of conductivity measurements in modern subductionzones, we envision a pervasive fluid in the subductionchannel that evolved from salinity levels lower than those in sea-water up to the measured values due to on-going but largely completed serpentinization in the subductionchannel. The present data represent a reference marker for the subductionchannel of the Rio San Juan intra-oceanic subductionzone at 30–50 km depth and after 50–60 Myr of operation.