Interoceanic variation in the rare earth, major, and trace element depositional chemistry of chert: Perspectives gained from the DSDP and ODP record

Abstract

Rare earth element (REE), major, and trace element abundances and relative fractionations in forty nodular cherts sampled by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) indicate that the REE composition of chert records the interplay between terrigenous sources and scavenging from the local seawater. Major and (non-REE) trace element ratios indicate that the aluminosilicate fraction within the chert is similar to NASC (North American Shale Composite), with average Pacific chert including ~7% NASC-like particles, Indian chert ~ 11% NASC, Atlantic chert ~ 17% NASC, and southern high latitude (SHL) chert 53% NASC. Using La as a proxy for ∑REE, approximations of La ex (the amount of La in excess of that supplied by the detrital aluminosilicate fraction) indicate that Pacific chert contains the greatest La ex (85% of La total) and SHL chert the least (38% of La total). As shown by interelement associations, this La ex is most likely an adsorbed component onto aluminosilicate and phosphatic phases. Accordingly, chert from the large Pacific Ocean, where deposition occurs relatively removed from significant terrigenous input, records a depositional REE signal dominated by adsorption of dissolved REEs from seawater. Pacific chert Ce Ce * ⪡ 1 and La n Yb n ~ 0.8–1 , resulting from adsorption of local Ce-depleted seawater and preferential adsorption of LREEs from seawater (e.g., La n Yb n ~ 0.4 ), which increases the La n Yb n ratio recorded in chert. Chert from the Atlantic basin, a moderately sized ocean basin lined by passive margins and with more terrigenous input than the Pacific, records a mix of adsorptive and terrigenous REE signals, with moderately negative Ce anomalies and La n Yb n ratios intermediate to those of the Pacific and those of terrigenous input. Chert from the SHL region is dominated by the large terrigenous input on the Antarctic passive margin, with inherited Ce Ce * ~1 and inherited La n Yb n values of ~1.2–1.4. Ce Ce * does not vary with age, either throughout the entire data base or within a particular basin. Overall, Ce Ce * does not correlate with P 2O 5 concentrations, even though phosphatic phases may be an important REE carrier. This and previous studies of the large-scale controlling parameters of sedimentary REEs across ocean basins collectively indicate that REE indices of depositional regime (e.g., Ce Ce * , La n Yb n , La ex) are reproducible in a variety of sediment and rock lithologies, ages, and ocean basins, and present a coherent tool for paleoceanographic and tectonic basin reconstructions.