Complex mineral zoning patterns caused by ultra-local equilibrium atreaction interfaces

Abstract

Chemically zoned minerals are useful records of temporal variations in ambient conditions and bulk chemical composition of the fluid from which the minerals precipitate. In fluidbuffered systems, zoning of mineral compositions is expected to reflect directly the evolution of fluid composition. Here we show that during rapid fluid-rock reactions, ultra-local equilibrium can form complex mineral zoning patterns, even when the overall system is highly fluid buffered. We reacted cleaved calcite single crystals with aqueous arsenate-phosphate solutions with molar ratios of As/(As + P) between 0.01 and 0.15 at 250 °C and water-saturated pressure. We find that complex zoning patterns and solid solution between hydroxylapatiteand arsenate-bearing hydroxylapatite that pseudomorphically replaced calcite formed within hours, and these zoning patterns were destroyed within days during secondary reactions. We propose a two-stage reaction process in the formation of the final reaction product. (1) On an hour time scale, calcite is dissolved and replaced by compositionally heterogeneous apatite. The thin reaction-interface fluid layer becomes extremely enriched in arsenic at an ultra-local scale as the reaction removes phosphate faster than the interface fluid can re-equilibrate with the bulk fluid. (2) The heterogeneous apatite is replaced by homogeneous apatite that reflects the bulk fluid composition over a longer (days) time scale through interface-coupled dissolution-precipitation. This paper highlights the complexity that can arise from ultra-local fluid composition variations due to rapid fluid-rock interaction in a short-lived fluid flow event, for example during a seismic cycle. Subsequent interpretation of complex zoning patterns as reflecting the evolution of bulk fluid would be erroneous. © 2014 Geological Society of America.