The geochemical behavior of F and Cl during the weathering–diagenesis–metamorphism–anatexis cycle. Insights from the clay fraction of fine sands from the Amazon River mouth and metapelititic rocks from the Seridó Belt, Brazil

Abstract

The average contents F = 788 ppm and Cl < 45 ppm of the clay fraction from Amazon River mouth sediments represent the composition of a large portion of weathered South American continental crust prior to any contact with high salinity formation waters. The data show a large fractionation between F and Cl with the latter being strongly leached during weathering along with Na and Ca. The one-order-of-magnitude fractionation is comparable to Cl fractionation from degassing during early Earth accretion. In contrast, F is strongly retained in the hydroxyl sites of neoformed clay minerals, mainly illite, as well as in detrital apatite and muscovite. The high average ratio F/Cl >10 of the clay fraction from Amazon River sediments are comparable only to Cl-poor MORB and evolved granite and rhyolite. The high content Cl = 180 ppm and low F/Cl = 3 of average shale is due to interaction of the sediments with high salinity diagenetic fluids. The Seridó schist belt is used to assess the behavior of F and Cl during shale metamorphism and anatexis. Its ratio F/Cl >10 ppm is high compared to average shale and is related to the strong partitioning of Cl into metamorphic fluids. The obtained data suggest that the Cl partitioned into shales during diageneses is released back into fluids produced during metamorphic conditions. The experimental data indicate that anatexis of mica schist significantly fractionates F/Cl ratios producing schist residue with F/Cl > 38 and peraluminous melts with F/Cl = 6.