The Congo deep-sea fan: Mineralogical, REE, and Nd-isotope variability in quartzose passive-margin sand

Abstract

ABSTRACT The Congo deep-sea fan, the largest on Earth fed entirely with anorogenic detritus, is characterized by quartzose to pure quartzose sand, reflecting multiple recycling coupled with extreme chemical weathering in cratonic equatorial Africa. The very youthful lower course of the Congo River connects directly to a steep canyon, where detritus including quartz grains up to a few millimeters in diameter is funneled towards Atlantic Ocean floors and deposited at abyssal depths more than a thousand kilometers away from shore. This article illustrates for the first time in detail the mineralogical and geochemical signatures of Congo Fan sands and discusses the factors controlling their intersample and intrasample variability as a key to understand how sediment is generated, recycled, and finally transferred to the deep sea. Compositional variability is largely grain-size-dependent. Combined petrographic and Raman spectroscopy analyses demonstrate that quartz increases in coarser samples and size classes, whereas feldspars are concentrated in finer sizes, plagioclase relative to K-feldspar and orthoclase relative to microcline, defining an order of mechanical and chemical durability among detrital tectosilicates. Because of overwhelming quartz abundance and very low heavy-mineral concentration, quartz contributes significantly to the REE budget and up to 40–50% of Nd in coarser samples, characterized by εNd values as low as –21. The strong grain-size-dependent variability of εNd suggests that quartz carries a markedly more negative εNd signature than monazite and other detrital components. This is chiefly ascribed to the durability of quartz grains, able to survive repeated cycles of weathering and diagenesis through Proterozoic and Phanerozoic time better than all other minerals. Neodymium model ages are influenced less by grain size and quartz abundance but more by the Sm/Nd ratio of different detrital components, and samples hydrodynamically enriched in LREE-rich minerals display TNd,CHUR and TNd,DM ages 1.2–1.4 Ga younger than samples enriched in HREE minerals. Not all detritus in the Congo Fan is supplied transversally by the Congo River. Forward-mixing calculations based on mineralogical data indicate that sand entrained northward by longshore currents mixes progressively with Congo River sand along the northernmost Angola coast, penetrates in the Soyo estuary, and is eventually captured in the canyon and transferred to the deep-sea fan, where it is estimated to represents 7 ± 2% of turbidite deposits.