The effect of high-pressure metasomatism on the boron isotope signature of subducted oceanic crust in the Raspas Complex (Ecuador)

Abstract

Boron abundances and B isotopic compositions of well-characterized blueschists and eclogites from the Raspas Complex (Ecuador) were analyzed to improve the use of boron as a tracer for recycling at convergent margins. The MORB-type eclogite interacted with internally-derived fluids released from metabasalt during the transition from blueschist to eclogite, with input from sediments. During metasomatism, B was gradually leached from the MORB-type eclogites (decrease from 6\(\upmu \)g/g to 1.5\(\upmu \)g/g), and their B isotopic composition was driven to isotopically heavier values in the range of \(-\)7.4\(\permille \) to \(-\)3.4\(\permille \). The B isotopic composition of the metasomatic fluid is estimated between \(-3\) and +1\(\permille \). The isotopic composition of the least metasomatized MORB-type eclogite samples (\({-7.4\pm 0.7}{\permille }\)) is considered close to the B isotopic composition of the dehydrated AOC in the case of Raspas at the stage of deepest subduction and most extensive dehydration. This constitutes a decrease in \(\delta ^{11}\text {B}\) of approximately 10\(\permille \) from its likely pre-subduction AOC protolith. The blueschist experienced a type of high-pressure metasomatism that is distinct from the one that affected the MORB-type eclogites. The metasomatic fluids were internally-derived and released by metabasalt as well, but with more input from sediments. The metasomatic fluid had a B isotope signature of approximately \(-\)5.2\(\permille \). The zoisite eclogite samples show a very distinct mineralogical and geochemical composition that records the highest degree of high-pressure metasomatic overprint. Their elemental and isotopic composition was thereby set to \(\text {[B]}={2.1\pm 0.3}\upmu \hbox {g/g}\) and \(\delta ^{11}\text {B}={-5.8\pm 1.8}{\permille }\). As demonstrated in previous studies, the high-pressure metasomatic fluid that caused the metasomatic overprint was mainly derived from– or interacted with– serpentinite, but had admixed components from metabasalts and metasediments. The B isotopic composition of the respective fluid is estimated at \({-2.6} {\permille }\), which overlaps with the composition of most volcanic arc basalts. This study, therefore shows, that metasomatic fluids that migrated through the Raspas slab at a depth of 50–70km had a B isotopic composition between \(-5.2\) to +1\(\permille \) and were, thus, significantly heavier than that of the mantle.