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 depression was measured in aqueous fluid inclusions to determine salinities in six samples of jadeitite and jadeite-rich rock from the Jagua Clara serpentinite mélange of the Rio San Juan Complex, Dominican Republic. The mélange represents a fossil subduction-zone channel from a cold, mature subduction zone with a geothermal gradient of ~6 °C/km. One hundred and twenty-five determinations of salinity in primary inclusions hosted in jadeite, quartz, apatite and lawsonite range 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 inclusions in 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 inclusions in 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 metasomatic replacement 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 subduction zones, we envision a pervasive fluid in the subduction channel 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 subduction channel. The present data represent a reference marker for the subduction channel of the Rio San Juan intra-oceanic subduction zone at 30–50 km depth and after 50–60 Myr of operation.