Processes controlling Rare Earth Element distribution in sedimentary apatite: Insights from spectroscopy, in-situ geochemistry and O and Sr isotope composition

Abstract

In phosphorites, the content and distribution of rare earth elements are linked to the environment of phosphogenesis. This paper focuses on the question of sources and processes controlling the rare earth element content of apatite from Belgian phosphorites formed during three major phosphogenic events in the Lower Palaeozoic, Late Cretaceous and Cenozoic. To constrain sources and processes, new data include petrological, mineralogical (including cathodoluminescence and Raman spectroscopy) and in situ trace element and Sr and O isotope analyses of apatite. Fluorapatite from Lower Palaeozoic P-rich conglomerates has the greatest rare earth element enrichment. It is affected by metamorphism that led to deformation of apatite nodules and formation of garnet porphyroblast inclusions. The role of Fe-oxyhydroxides in element scavenging is highlighted by some apatite nodules that maintain their primary middle rare earth element enrichment, while others are characterized by altered rare earth element patterns resulting from competition for these elements between co-crystallizing minerals during deformation. A systematic shift towards lower δ¹⁸O and radiogenic Sr isotopic composition compared to contemporaneous seawater indicate interaction with ¹⁸O-depleted meteoric fluids and a crustal component. By contrast, carbonate-rich fluorapatite from the Late Cretaceous phosphatic chalk mostly keeps its primary trace element and isotopic signatures (close to seawater), although an external rare earth element addition is noted as well as rare earth element redistribution induced by diagenetic alteration. Cenozoic carbonate fluorapatite nodules mostly present flat rare earth element patterns that are indicative of a detrital influence. Slight changes in rare earth element distribution are assigned to post-depositional alteration, which also led to an increase in radiogenic Sr, with unchanged δ¹⁸O compared to seawater. The methodology followed here efficiently helps in deciphering the processes that modified the chemistry of apatite in the frame of major phosphogenic events.