Precise Re–Os ages of organic-rich mudrocks and the Os isotope composition of Jurassic seawater

摘要: Abstract Rhenium and osmium isotope abundance data have been obtained on precisely-located samples from three suites of immature, organic-rich mudrocks Jurassic coastal outcrops in England. The provide accurate whole-rock ages 207±12 Ma, 181±13 Ma 155±4.3 for Hettangian, Toarcian (exaratum Subzone) Kimmeridgian (sensu anglico, wheatleyensis samples. These new Re–Os are indistinguishable, within the assigned analytical uncertainties, interpolated depositional estimated published geological timescales, establish importance dating technique chronostratigraphic studies. Early-diagenetic pyrite nodules possess levels Re Os which ∼1–2 orders magnitude lower than enclosing mudrocks, indicating that these elements had already removed sediment pore waters at time nodule formation. Thus system has closed since, or very soon after, deposition. Because most (98+%) (95–99.8+%) is shown to be hydrogenous, 187Os/188Os(i) interpreted contemporaneous seawater. thereby first estimates composition During earliest (Hettangian), seawater 187Os/188Os ratio was extremely unradiogenic (∼0.15); it increased ∼0.8 end Early (Toarcian) ∼20 later, while Late (Kimmeridgian) ∼0.59. likely explanation Hettangian contribution oceans hydrothermal alteration oceanic crust greatly exceeded input radiogenic continents time. This interpretation line with observations suggesting global weathering rates were low volcanic activity preceded break-up Pangea. Re/Os ratios (and by inference, seawater) similar those deposited later times during Jurassic, argues against being derived extraterrestrial meteoritic sources.