SPONTANEOUS MOVEMENT OF IONS THROUGH CALCITE AT STANDARD TEMPERATURE AND PRESSURE

At the resolution Limits of traditional geochemical techniques, there is Little evidence to challenge the common assumptions that, under the Earth's ambient surface conditions, dry calcite is static and that th e bulk mineral behaves as a dosed system. Solid-state diffusion has be en recognized at elevated temperatures(1-3), but it has always been as sumed that diffusion in carbonate minerals is negligible under standar d conditions(4,5). There is, however some evidence to the contrary, Mo re than 30 years ago, the Ca-45 diffusion coefficient was estimated to be similar to 8 x 10(-20) cm(2) (ref. 6) and, more recently, we have demonstrated movement of adsorbed Cd2+ and Zn2+ into bulk calcite at r ates of tens of nanometres over weeks to months (refs 7, 8). Here we p resent evidence that monovalent ions, Na+, K+ and Cl-, originating fro m fluid inclusions, accumulate in crystallites on the surface of calci te. This process is spontaneous at the Earth's surface conditions, in air. The results show that calcite under standard conditions does not always behave as a dosed system, which is a critical assumption in the use of isotope ratios, trace-element distribution and fluid-inclusion composition for interpretations of palaeoclimate, geochronology or pe trogenesis. Moreover, calcite's uptake capacity for contaminants in en vironmental systems is probably higher than current models predict, be cause surface sites are constantly renewed by ionic mobility.