Removal of oceanic REE by authigenic precipitation of phosphatic minerals

Abstract

Early-diagenetic phosphatic minerals containing rare earth elements (REE) are widespread in Australian shallow-marine sandstones of all ages. Most common is the aluminophosphate florencite (REE)Al 3(PO 4) 2(OH) 6, but the isomorphous and chemically intergrading phases crandallite, gorceixite and goyazite, all with up to several percent REE substituted for their major cation (Ca, Ba and Sr, respectively), may also be present. Together they comprise up to 0.144 wt% (average ∼0.0134 wt%) of their host rocks. The authigenic phosphates xenotime and apatite are also widespread but less common. They all apparently precipitated within the diagenetic zones of sulphate-reduction and methanogenesis as a result of `reverse weathering' reactions after the release of REE and PO 4 by reduction of Fe/Mn-oxyhydroxides and the decomposition of organic material coating detrital particles. As the aluminophosphates and apatite are enriched in light REE whereas xenotime concentrates heavy REE, they collectively deposit light REE at a much greater rate (La = 129×10 −9 g cm −2 yr −1) than heavy (Yb = 0.40×10 −9 g cm −2 yr −1). The total burial flux of light REE by shallow-marine phosphatic precipitation is about 50 times greater than previously determined for deep-sea sediments, whereas heavy REE are buried at comparable rates. This new sink produces a major imbalance in the marine REE budget, requiring the existence of a substantial, but currently unknown, source to restore balance. Its size has important implications for the use of REE abundances, distributions and isotopes as monitors of marine geochemical processes.