BORON ISOTOPES DISCRIMINATION FOR SUBDUCTION-RELATED SERPENTINITES: EXAMPLE OF THE GUATEMALA SUTURE ZONE

Serpentinites are key features at subduction and suture zones. They originate by two different processes: 1) by reaction of abyssal peridotite or back-arc peridotite with seawater or 2) by reaction of mantle-wedge peridotite with fluid released from the slab during subduction. In many collisional zones, both types of serpentinite occur as paired belts, so that discriminating them can be difficult, without comprehensive field and laboratory studies.

The geology of Guatemala features a complex tectonic boundary between the North American and Caribbean plates, called the Guatemala Suture Zone (GSZ). Two serpentinite mélanges are found within the GSZ, the North Motagua Mélange (NMM) and South Motagua Mélange (SMM). A dismembered ophiolite sequence is found north of the GSZ. The mélanges contain blocks of serpentinites, HP–LT metabasites and vein rocks embedded in a serpentinite matrix. The HP-LT blocks (e.g., eclogite) are metamorphosed pieces of the subducted oceanic crust, while the vein rocks (e.g., jadeitite) represent fluid crystallization in the margin of the mantle wedge. Boron isotopes have been analyzed in situ by LA-MC-ICP-MS in 10 serpentinites (3 ophiolites, 7 mélanges) and 15 vein rocks and metabasites from Guatemala and provide important evidence about serpentine origin.

The serpentine from the ophiolites all have positive δ¹¹B, from +0.8 to +18.0 ‰, whereas all serpentine from the mélanges, both NMM and SMM, have comparable δ¹¹B, ranging from ‑14.4 to +9.7 ‰. The metabasites and vein rocks from both NMM and SMM have comparable negative to slightly positive δ¹¹B, which overlap mélange serpentine values; they range from ‑15.3 to +4.3 ‰. The overlapping values from metamorphic and vein rocks with those from serpentinite mélange indicate that the fluids released from the subducting slab that crystallized jadeitites also serpentinized the encapsulating mantle wedge. The negative B signature of the mélange serpentinites presented here represents mantle-wedge hydration at 30 - 70 km depth, whereas the ophiolites, which represent the oceanic crust, display only positive B signature.

The significant difference observed for B isotopes between the serpentinites from mélanges and from ophiolites makes B isotopes a key tool to discriminate the tectonic origin of serpentinites in subduction and suture zones.