Boron isotope fractionation in subducting oceanic crust

The boron stable isotope ratio is employed to investigate fluid-rock interaction processes in subducting slabs and the slab–mantle interface. Two groups of subduction-related rocks were collected from Syros (Greece): the first group includes high-pressure metamorphic (HPM) rocks, which are interpreted as metamorphosed oceanic crust. They are characterized by prograde mineral assemblages; the second group of samples were collected from reaction zones, which formed in a high-pressure mélange zone and are characterized by the metasomatic formation of hydrous minerals (tourmaline, white mica, glaucophane) and secondary omphacite during exhumation.

Tourmaline and mica show the highest B contents, but in mica-free samples, glaucophane and omphacite control the boron budget. Boron isotope ratios of whole rocks, white mica, glaucophane and omphacite were determined at FIERCE by LA-MC-ICPMS. Published data for tourmaline in these samples are also available.

Group–1 HPM rocks show an average δ11B value of 0 ±5 ‰ (4 samples), which is higher than predicted from oceanic-crust dehydration modelling. This may suggest that published models strongly overestimate boron isotope fractionation in subduction zones, likely caused by erroneous assumptions on the B speciation in dehydration fluids. For the boron isotope fractionation among coexisting minerals we found the following: tourmaline–white mica +13 4 ‰ (3 samples); tourmaline–glaucophane: +11 ±4 ‰ (2); tourmaline–omphacite: +15 ±5 ‰ (5). These values suggest that boron shows the same coordination to oxygen (i.e. tetrahedral) in white mica, glaucophane and omphacite, which contrasts with the dominantly trigonal coordination of B in tourmaline.