Rare earth element composition and mass accumulation rates at DSDP Hole 92-598

Site 598 sediments were analyzed to determine the factors controlling the rare earth element (REE) geochemistry of the hydrothermal component. Site 598 provides an ideal sample suite for this purpose. Samples are lithologically "simple," primarily consisting of a hydrothermal component and biogenous carbonates. Also, the composition of the hydrothermal component appears unchanged through time or space, and the site appears to have undergone minimal diagenetic alteration. The shale-normalized REE patterns are similar to the pattern of seawater, varying only in absolute REE content. The REE content increases with distance from the paleorise crest and exhibits a pronounced increase in sediments deposited below the paleolysocline. Results presented are consistent with the following model: the source mechanism for the REE content of hydrothermal sediments is scavenging by Fe oxyhydroxides from seawater. With prolonged exposure to seawater resulting from transport far from the injection point and/or long residence at the seawatersediment interface, the absolute REE content of hydrothermal sediments increases and becomes more like seawater.

Chemical composition of DSDP and ODP cherts

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 sum REE, approximations of excessive La (the amount of La in excess of that supplied by the detrital aluminosilicate fraction) indicate that Pacific chert contains the greatest excessive La (85% of total La) and SHL chert the least (38% of total La). As shown by interelement associations, this excessive La 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 normative La/Yb ~ 0.8-1, resulting from adsorption of local Ce-depleted seawater and preferential adsorption of LREEs from seawater (e.g., normative La/Yb ~0.4), which increases the normative La/Yb 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 normative La/Yb 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 normative La/Yb 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 P2O5 concentrations, even though phosphatic phases may be an important REE carrier.