BORON ISOTOPES AS TRACERS OF THE TECTONIC ORIGIN AND GEOLOGICAL HISTORY OF SERPENTINITES IN SUBDUCTION AND SUTURE ZONES

Serpentinites are known to play a key role in subduction, because they contain significant water content (up to 14 wt% H₂O) and can be enriched in elements such as As, B, Li, Sb, Sr, Cs, U, and halogens. They most commonly originate by hydration of peridotite by two different processes: (i) by a seawater source reacting with peridotite beneath the ocean crust and (ii) by reaction of peridotite at the base of the mantle-wedge with fluids released from the slab during subduction. In suture zones, it is relatively common to find serpentinite from both exhumed subduction channel mélange (likely originating from the mantle wedge) and ophiolite (originating from the oceanic crust), but recognizing them and their tectonic origin can be difficult.

A recent study based on samples from the Guatemala Suture Zone (Guatemala) demonstrated that boron (B) isotopes could be used as a probe of the fluid from which serpentinites form. Serpentinites from an ophiolite complex have positive δ¹¹B, as expected for peridotites hydrated by seawater-derived fluid, whereas serpentinite samples from the matrix of the mélange (expected to come from the roof of the subducting channel) have negative δ¹¹B, in agreement with hydration of mantellic peridotites by fluids released at 30-70 km depth from metamorphic rocks.

Serpentinites from tectonically well-constrained locations were selected to verify this hypothesis. They include samples from the oceanic crust (ophiolite = Guatemala, Iran, Cuba), the subduction forearc (Nicaragua), and the mantle wedge (Guatemala, Iran, Japan). The trace-element contents and B isotopes were measured in situ, respectively by LA-ICP-MS and LA-MC-ICP-MS. The spider diagrams and REE patterns, as well as a B/La vs. As/La diagram do not show any reliable difference to distinguish the tectonic origin of the serpentinite. However, in a δ¹¹B vs. B content diagram, the serpentinites plot along different trends whether they form from seawater (δ¹¹B = 40‰, [B] = 5ppm) or from subduction-related metamorphic fluids (δ¹¹B varies with temperature from +19 to – 15‰, [B] is poorly constrained but likely varies with depth (i.e., T) from hundreds to a few ppm). Some samples might express signatures of both fluids. This study confirms that the tectonic origin of serpentinites encountered in suture areas can be defined by a δ¹¹B vs. B diagram.