Complex mineral zoning patterns caused by ultra-local equilibrium at reaction interfaces

Chemically zoned minerals are useful records of temporal variations in ambient conditions and bulk chemical composition of the fl uid from which the minerals precipitate. In fl buffered systems, zoning of mineral compositions is expected to refl ect directly the evolution of fl composition. Here we show that during rapid fl uid-rock reactions, ultra-local equilibrium can form complex mineral zoning patterns, even when the overall system is highly fl uid 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 fithat 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 fi nal reaction product. (1) On an hour time scale, calcite is dissolved and replaced by compositionally heterogeneous apatite. The thin reaction-interface fllayer becomes extremely enriched in arsenic at an ultra-local scale as the reaction removes phosphate faster than the interface fl uid can re-equilibrate with the bulk fl uid. (2) The heterogeneous apatite is replaced by homogeneous apatite that refl ects the bulk fl uid composition over a longer (days) time scale through interface-coupled dissolution-precipitation. This paper highlights the complexity that can arise from ultra-local fl uid composition variations due to rapid fl uid-rock interaction in a short-lived fl uid fl ow event, for example during a seismic cycle. Subsequent interpretation of complex zoning patterns as refl ecting the evolution of bulk fl uid would be erroneous.