187 Os/ 188 Os 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 . The calculated initial 187 Os/ 188 Os 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 87 Sr/ 86 Sr starting in early Ordovician [Qing et al., 1998].

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