The mobility of the rare earth elements: Evidence and implications from selected terrains affected by burial metamorphism

Abstract

The mobility of the rare earth element (REE) during hydrous burial metamorphism is described from three localities. Comparison of REE abundances in relict domains and metadomains from flood lavas in the Maddina Volcanics, Fortescue Group, Western Australia shows that, relative to the relict domains, REE may be strongly depleted in certain metadomains. Strong variations in La/Yb, La/Sm and Eu may also occur due to postcrystallization secondary processes. Similar comparisons in flood lavas from Mamainse Point in the Canadian Keweenawan Series show that REE are mobile and increase in abundance in metadomains. Spilites from U.S. Virgin Islands also show evidence for REE mobility during low grade burial metamorphism. In this case light REE (LREE) have been preferentially mobilized with heavy REE (HREE) remaining parallel or sub-parallel on chondrite normalized plots. REE analyses from these locations together with an evaluation of published work suggests that the mobility of REE can be described by: 1. gross REE and selective LREE enrichment; 2. REE movement around a primary mean; 3. gross REE depletion; 4. selective REE mobility. Failure to take into account REE mobility may lead to incorrect conclusions concerning petrogenetic models based on altered basic sequences. The coherence of REE with other key trace elements during alteration is also discussed.