Boron Isotopes in Antigorite from the Guatemala Suture Zone Serpentinite Mélanges, Guatemala: Origin of the Serpentinite.

The Guatemala Suture Zone (GSZ) features two serpentinite mélanges straddling the Motagua Fault system. In both North and South Motagua Mélanges (NMM and SMM, respectively), blocks of high-pressure, low-temperature metabasites (e.g., eclogites) are embedded into a serpentinite matrix, together with vein-related rocks (e.g., jadeitites). The origin of serpentinite forming the matrix is still unclear. It is commonly believed to represent either the seafloor / oceanic mantle hydrated by infiltration of seawater or the forearc mantle wedge hydrated by fluid released during subduction. Nowadays, the few data available on serpentinites from Mariana forearc, abyssal peridotite or ophiolitic massifs show positive or slightly negative δ¹¹B (-4.6 to +40‰), in support of the assumption of a shallow origin for the fluid. However, it has been suggested that serpentinites could form much deeper within a subduction channel (~ 30 km depth).

In situ boron analyses on antigorite from serpentinites of the GSZ have been carried out by LA-MC-ICPMS and SIMS. Serpentine matrices have low to moderate B concentrations, between 2 and 15µg/g. They display somewhat negative isotopic values, with δ¹¹B of -8.2 ± 2.8 and -11.9 ± 1.2‰ for samples from the NMM, and δ¹¹B of -4.1 ± 3.0 and -11.1 ± 1.8‰ for a sample from the SMM. In addition, this sample contains a late antigorite vein showing a δ¹¹B of -3.0 ± 3.5‰, and some veinlets with scattered values (δ¹¹B ranging from -12.5 ± 0.9 to -6.4 ± 1.4‰).

This first suite of measurements carried out directly on serpentinites from serpentinite mélanges displays very negative values, which do not overlap with previously published data. It indicates that the ultramafic precursors of the serpentine from the GSZ Serpentinite Mélanges were not hydrated by seawater, or by fluid released during the early stages of subduction. Rather, these negative values indicate that the fluid responsible for the hydration of ultramafic rocks likely comes from deep stages of slab devolatilization, in agreement with the hypothesis of deep serpentinite formation in the subduction channel. Thus, the serpentinite matrix of the mélanges probably represents hydrated mantle wedge.