Elisabeth Widom

Eleven core samples of Ordovician carbonates and shale from within and nearby the Serpent Mound (Ohio) cryptoexplosion structure were analyzed for Re‐Os isotopes. Whole‐rock samples span a large range in measured ¹⁸⁷Os/¹⁸⁸Os (0.714 to 6.083), and exhibit a linear correlation of ¹⁸⁷Os/¹⁸⁸Os with ¹⁸⁷Re/¹⁸⁸Os. The linear relationship is inconsistent with a meteorite‐crust mixing trend, but could represent an errorchron with an age of 485 ± 35 Ma, and an initial ¹⁸⁷Os/¹⁸⁸Os of 0.54 ± 0.15. In order to assess whether the errorchron slope and initial ¹⁸⁷Os/¹⁸⁸Os ratio may provide constraints on depositional age and the ¹⁸⁷Os/¹⁸⁸Os ratio of Ordovician seawater, acetic acid and HCl leaching studies were done on selected samples. The ⁸⁷Sr/⁸⁶Sr ratios of the leachates vary from 0.7081 to 0.7091, within the range found in Middle‐Upper Ordovician brachiopods [Shields et al., 2003], and consistent with the leachable fraction of the carbonates representing Ordovician seawater compositions. ¹⁸⁷Os/¹⁸⁸Os signatures for the leached samples are generally lower than those of respective whole rocks. However, five of six leachate samples produce a strong linear correlation on a Re‐Os isochron diagram, the slope of which reflects an “age” of 443 ± 34 Ma, consistent with biostratigraphic constraints on the depositional ages of the sampled rock units (∼445–472 Ma). The calculated initial ¹⁸⁷Os/¹⁸⁸Os of 0.53 ± 0.04 is distinctly lower than present‐day seawater, and, if representative of Ordovician seawater, may reflect relatively low continental weathering rates due to waning Pan African orogenies as proposed to explain the slow decrease in seawater ⁸⁷Sr/⁸⁶Sr starting in early Ordovician [Qing et al., 1998].

Osmium concentrations and isotopic signatures were measured in 28 primarily Holocene basalts (22 of which have been analyzed for Sr–Nd–Pb isotope composition), two carbonatites and two mantle xenoliths from the Canary Islands, Selvagen Grande and Madeira in the eastern North Atlantic. ¹⁸⁷Os/¹⁸⁸Os ratios in the basalts range from 0.129 to 0.183. The Os isotope systematics indicate that the basalts fall into three petrogenetic groups: (1) a ‘radiogenic’ group with high ¹⁸⁷Os/¹⁸⁸Os from 0.152 to 0.183; (2) an ‘unradiogenic’ group with low ¹⁸⁷Os/¹⁸⁸Os from 0.129 to 0.138; (3) an ‘intermediate’ group with ¹⁸⁷Os/¹⁸⁸Os between 0.139 and 0.151. The Os isotope systematics of the radiogenic group samples are consistent with minor contamination of the basalts by marine sediment. All samples in the unradiogenic group contain mantle xenoliths, and the unradiogenic Os can be explained by bulk assimilation of ≤ 5% mantle peridotite in the form of disaggregated xenoliths. The radiogenic and unradiogenic groups are also characterized by higher ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb but lower ¹⁴³Nd/¹⁴⁴Nd than samples with similar ²⁰⁶Pb/²⁰⁴Pb from the intermediate group, which is interpreted to reflect interaction of plume magmas with the lithospheric mantle. The intermediate group samples are believed to represent the isotopic signature of the mantle plume. The Os isotopic composition of the Canary plume is among the most radiogenic found in ocean island basalts, comparable with the endmember HIMU islands Mangaia and Tubuaii, but at significantly lower ²⁰⁶Pb/²⁰⁴Pb. The radiogenic Os and moderate ²⁰⁶Pb/²⁰⁴Pb signature of the Canary plume is consistent with a plume which contains 25–35% of relatively young (∼1.2 Ga) recycled oceanic crust. Variable degree of mixing of the Canary Island plume source with shallow depleted asthenosphere containing a component of Paleozoic oceanic crust produces the limited range in Os isotopic signatures observed in the Madeira and Canary Island basalts despite a large range in ²⁰⁶Pb/²⁰⁴Pb isotopic composition.