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.本数据集涵盖了深海钻探计划（Deep Sea Drilling Project, DSDP）与大洋钻探计划（Ocean Drilling Program, ODP）采集的40块结核状燧石的稀土元素（Rare Earth Element, REE）、主量元素及微量元素丰度与相对分馏特征，研究结果表明，燧石的稀土元素组成记录了陆源物质来源与当地海水吸附作用之间的相互作用。主量元素与（非稀土）微量元素比值显示，燧石内的铝硅酸盐组分与北美页岩组合（North American Shale Composite, NASC）相似；其中太平洋燧石平均包含约7%的类NASC颗粒，印度洋燧石约含11%的NASC组分，大西洋燧石约含17%的NASC组分，南极高纬度（southern high latitude, SHL）燧石则含有53%的NASC组分。以镧（La）作为总稀土元素的替代指标，对过剩La（即碎屑铝硅酸盐组分供给之外的La含量）的估算结果显示，太平洋燧石的过剩La占比最高（达总La的85%），南极高纬度燧石的占比最低（仅为总La的38%）。通过元素间关联分析可知，该过剩La极有可能是吸附于铝硅酸盐与磷酸盐相中的组分。 因此，沉积作用受显著陆源输入影响相对较弱的大型太平洋海域所产出的燧石，其沉积型稀土元素信号以海水中溶解态稀土元素的吸附作用为主导。太平洋燧石的Ce/Ce*值远小于1，标准矿物计算的La/Yb比值约为0.8~1，这源于当地贫铈海水的吸附作用以及海水中轻稀土元素（Light Rare Earth Elements, LREEs）的优先吸附（例如标准矿物La/Yb比值约为0.4），该过程提升了燧石中记录的标准La/Yb比值。大西洋盆地作为以被动大陆边缘为界的中等规模洋盆，其陆源输入量高于太平洋，该海域产出的燧石记录了吸附型与陆源型稀土元素信号的混合特征，表现为中等程度的负铈异常，且标准La/Yb比值介于太平洋燧石与陆源输入物的比值之间。南极高纬度海域的燧石则受南极被动大陆边缘的强陆源输入主导，其继承的Ce/Ce*值约为1，标准矿物计算的La/Yb比值约为1.2~1.4。无论是在整个数据集范围内，还是在单个洋盆内部，Ce/Ce*值均不随年代发生变化。总体而言，尽管磷酸盐相可能是重要的稀土元素载体，但Ce/Ce*值与五氧化二磷（P₂O₅）浓度并无相关性。