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

Rhenium and osmium isotope and abundance data have been obtained on precise ly-located samples from three suites of immature, organic-rich mudrocks fro m Jurassic coastal outcrops in England, The data provide accurate whole-rod e ages of 207 +/- 12 Ma, 181 +/- 13 Ma and 155 +/- 4.3 Ma for suites of Het tangian, Toarcian (exaratum Subzone) and Kimmeridgian (sensu anglico, wheat leyensis Subzone) samples. These new Re-Os ages are indistinguishable, with in the assigned analytical uncertainties, from interpolated depositional ag es estimated from published geological timescales, and establish the import ance of the Re-Os dating technique for chronostratigraphic studies. Early-d iagenetic pyrite nodules possess levels of Re and Os which are similar to 1 -2 orders of magnitude lower than in the enclosing organic-rich mudrocks, i ndicating that these elements had already been removed from sediment pore w aters at the time of nodule formation. Thus the Re-Os isotope system in the se organic-rich mudrocks has been closed since, or from very soon after, th e time of sediment deposition. Because most of the Re (98+%) and Os (95-99. 8+%) in the mudrocks is shown to be hydrogenous, the Os-187/Os-188((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 Juras sic seawater. During the earliest Jurassic (Hettangian), the seawater Os-18 7/Os-188 ratio was extremely unradiogenic (similar to 0.15); it had increas ed to similar to 0.8 at the end of the Early Jurassic (Toarcian) similar to 20 Ma later, while in the Late Jurassic (Kimmeridgian) the seawater Os-187 /Os-188 ratio was similar to 0.59. The most likely explanation for the unra diogenic Os isotope composition of Hettangian seawater is that the contribu tion of unradiogenic Os to the oceans from the hydrothermal alteration of o ceanic crust greatly exceeded the input of radiogenic Os from the continent s at that time. This interpretation is in Line with observations suggesting that global weathering rates were low in the Hettangian, and that increase d hydrothermal and volcanic activity preceded the break-up of Pangea. The R e/Os ratios of Hettangian mudrocks (and by inference, of contemporaneous se awater) are similar to those of mudrocks deposited at later times during th e Jurassic, and argues against the unradiogenic Os in Hettangian seawater b eing derived from extraterrestrial meteoritic sources. (C) 1999 Elsevier Sc ience B.V. All rights reserved.