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

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.

The Zambezi deep-sea fan: mineralogical, REE, Zr/Hf, Nd-isotope, and zircon-age variability in feldspar-rich passive-margin turbidites

“…8) are confirmed by combined mineralogical and grain-size information obtained by semiautomated Raman analysis (r ≥ 0.96, considering only samples in which enough grains were identified in four phi classes at least). The tendency of the Q/F ratio to increase with grain size, long reported from continental to shallow-marine sandstone suites (Graham 1930;Odom et al 1976), is thus firmly documented to occur in deep-sea turbidites as well (Marsaglia et al 1996;Garzanti et al 2021b).…”

The Segmented Zambezi Sedimentary System from Source to Sink: 1. Sand Petrology and Heavy Minerals

“…River segmentation was far less pronounced before man's intervention, as indicated by the poor compositional match between the present Lower Zambezi sand and upper Quaternary offshore sediments, which have notably higher Q/F ratio and a little more clinopyroxene. Such differences cannot be dismissed as a grain-size effect, because the Q/F ratio typically increases with increasing grain size (e.g., Garzanti et al 2021c) and our river samples are very fine to fine sands whereas offshore samples are sandy silts. Rather than additional contribution by longshore-drifting sediment from outside the Zambezi delta, the plausible explanation is that a larger amount of detritus generated in the upper and middle catchment reached the ocean before closure of Kariba and Cahora Bassa dams.…”

“…Which is patently wrong. Beside being subject to marked grain-size control (Garzanti et al 2021c and references therein), the Q/(Q+F) ratio reaches 100% in sand of both hyper-humid equatorial Congo and hyper-arid tropical Arabian or Sahara sand seas (Garzanti et al 2013a(Garzanti et al , 2019cPastore et al 2021), making it evident that climatic conditions cannot be naïvely inferred by mineralogical parameters such as the MIA.…”

The Segmented Zambezi Sedimentary System from Source to Sink: 2. Geochemistry, Clay Minerals, and Detrital Geochronology

“…Strong grain-size control on εNd has been recently documented in Congo Fan sediments, where it was ascribed to the more negative εNdcarried by multiply recycled quartz grains originally derived from older terrains on average (figs. 6 and 8 in Garzanti et al 2021b). In recycled Congo Fan sediments, however, quartz and monazite account for similar amounts of Nd, quartz being 10 5 times more abundant in weight but monazite containing 10 5 times more Nd.…”