Distinct REY sources recorded in the Doushantuo phosphorite

Abstract

During the Ediacaran period, massive marine phosphorites were deposited worldwide, which could represent a global sink of rare earth elements and yttrium (REY) in oceans. In the Doushantuo Formation of South China, the REY source has been preliminarily constrained as the mixed sources of seawater and terrigenous matter in these phosphorites. However, the REY composition of both seawater and terrigenous endmembers is still unclear; therefore, it’s difficult to identify the contributions of seawater and terrigenous sourced REY. In this work, REY and other trace elements of bulk phosphorites, in situ carbonates, and apatite grains were analyzed from a drill core (Fangmashan) of the Doushantuo Formation in South China. Combined with mineralogical observations, the REY composition of seawater and terrigenous endmembers is well-constrained. Along the investigated Fangmashan drill core, all in situ carbonates (calcites and dolomites) show a modern seawater-like REY pattern, indicating that the REY characteristics of the Doushantuo seawater could be similar to those of modern oxic seawater without significant changes. Alternatively, apatites from the lower and upper phosphorites showed distinct REY patterns. One type of apatite in the upper phosphorites showed a modern seawater-like REY pattern, indicating authigenic REE enrichment from the seawater column. However, another type of apatite in the upper phosphorites with an REE pattern that deviated from seawater showed high ΣREY (∼691.16 ± 309.78) and low Y/Ho (∼40.52 ± 4.05), probably reflecting mixed REY sources and/or the effects of early diagenesis. It is noted that most apatites in the lower phosphorites are obviously enriched in light REY (LREY: La - Nd) compared to heavy REY (HREY: Ho - Lu and Y), which may represent a significant contribution of the terrestrial REY source. This study reveals that the REY composition of paleo-seawater could be similar to modern seawater as early as the Neoproterozoic and supports a significant contribution to understanding the difference in the REY composition of phosphorites in geological history.