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

Abstract

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