Numerical modelling of geothermal and reflux circulation in Enewetak Atoll: implications for dolomitization

Two types of regional-scale seawater circulation have been proposed to expl ain the formation of Enewetak Atoll dolomites: geothermal and reflux circul ation. We have used a finite element groundwater flow model to examine the pattern, magnitude and dynamic interaction of these two different circulati on mechanisms in Enewetak Atoll. Geothermal circulation is concentrated aro und the atoll-margin whereas refluxing mesosaline brines flow from the atol l interior towards the margin to restrict and eventually shut off geotherma l circulation. Refluxing brines of 36-80 parts per thousand can account for the salinity signature recorded in dolomite fluid inclusions. Distribution s of fluid flux and Mg mass-balance calculations suggest that both geotherm al and reflux circulation mechanisms could account for the observed distrib ution of dolomite in Enewetak Atoll. Furthermore, the atoll interior may be extensively dolomitized as observed in other atolls. (C) 2000 Elsevier Sci ence B.V. All rights reserved.