Temperature and Water–Rock Ratio Controls on Boron Behavior in Serpentinized Peridotites

Serpentinites formed in abyssal settings show large variations in boron concentration and δ¹¹B, even within similar tectonic environments. To explore the processes controlling boron incorporation and isotopic fractionation during oceanic serpentinization, we developed a stepwise reaction-path model simulating progressive water–rock interaction, using experimentally derived data for B partioning and isotopic fractionation between fluid and rock. The model tracks the coupled evolution of B concentration and δ¹¹B in the solid through multiple reaction loops, characterized by evolving temperature and decreasing water–rock ratios.

Model results indicate that B concentration and δ¹¹B evolve asynchronously during serpentinization. However, at given B contents, serpentinites show a variety of δ¹¹B values, reflecting its strong sensitivity to reaction history rather than equilibrium with a single fluid reservoir. Progressive reaction loops produce divergent isotopic trajectories, in response to the degree of fluid renewal and cumulative fractionation during serpentinization.

Comparison with natural samples shows that B systematics of serpentinites from the Atlantis Massif are best explained by multi-stage serpentinization under relatively restricted fluid conditions, during which progressive fractionation drives δ¹¹B toward lower values despite significant B enrichment. In contrast, serpentinites from the 15°20′N transform fault, Mid-Atlantic Ridge, consistently exhibit seawater-like δ¹¹B, more resembling open-system behaviors involving repeated interactions between fresh fluids and new rock volumes.

These results demonstrate that reaction-path modeling provides a robust framework for interpreting boron isotope systematics in abyssal serpentinites and highlight the critical role of fluid–rock interaction history, along with temperature and bulk composition, in controlling δ¹¹B signatures.