Metalliferous sediments consisting dominantly of fine-grained iron and manganese oxides and hydroxyoxides have been widely recorded from the crests of sea-floor spreading ridges and as a basal facies of the sediment accumulations of the oceanic crust. Similar sedimentary rocks that, in Cyprus, for example, contain 10–44 wt.% Fe and 2–16 wt.% Mn, occur in association with ophiolitic rocks. These chemical precipitates are thought to have formed by oxidation of hydrothermal fluid released in submarine hot-spring areas in the discharge zones of ocean-floor geothermal systems that contained convectively circulating sea water.Lead isotope ratios of 18 samples associated with Upper Cretaceous ophiolitic rocks of the Troodos massif, Cyprus (6 samples), the Baër-Bassit area, Syria (6), and the Semail nappe in the Sultanate of Oman (6), indicate that the metalliferous sediments contain lead leached from the underlying basaltic oceanic crust during hot water – rock interaction. The amount of basaltic lead varies from comparatively little, in some samples from Syria, to essentially 100% in many of the samples from Oman. Linear mixing relationships characterized by correlation coefficients of 0.97 and 0.86 are defined on 208Pb/204Pb–206Pb/204Pb and 207Pb/204Pb–206Pb/204Pb diagrams. The mixing lines connect the less radiogenic mid-ocean ridge basalt field with the more radiogenic sea-water lead field of manganese nodules, which is also the average isotopic composition of continental crustal material. Negative covariations with Th, a trace element index of the detrital sedimentary component, and Pb/Fe, a monitor of diagenetic addition of Pb from pore waters, suggest that the main cause of the lead isotopic variation was initial adsorption of a variable ratio of leached basaltic lead to dissolved sea-water lead.The mean of 13 initial 87Sr/86Sr ratios (0.7079 ± 0.0013; 2 SD) is statistically indistinguishable from the estimated 87Sr/86Sr ratio for Late Cretaceous sea water at 0.7076 ± 0.0006 (25 values; Peterman et al.). Hence, strontium was largely derived by adsorption from sea water. However, three determinations are significantly more radiogenic than Late Cretaceous sea water. A statistically significant covariation with Rb (r = 0.78), one of the trace elements contained in the detrital, sedimentary component, suggests that the increase was caused by a variable admixture of terrigenous material.Neither lead nor strontium isotope ratios nor trace element concentrations suggest significant diagenetic modification of the isotopic compositions of the metalliferous sediments.
Thallium isotope ratios record a transition to melting of hydrated oceanic crust at the Hadean-Archean boundary
