Lead and Strontium Isotopes in Ancient Carbonates
From the Urals and Siberia: Evolution of Seawater 87Sr/86Sr Over the Late Proterozoic

I. M. Gorokhov Inst. of Precambrian Geol. and Geochron., RAS, nab. Makarova 2, St. Petersburg 199034, Russia


M. A. Semikhatov Geological Institute, RAS, Pyzhevsky per. 7, Moscow 109017, Russia

G. V. Ovchinnikova Inst. of Precambrian Geol. and Geochron., RAS, nab. Makarova 2, St.Petersburg 199034, Russia

A. B. Kuznetsov Inst. of Precambrian Geol. and Geochron., RAS, nab. Makarova 2, St.Petersburg 199034, Russia

N. N. Melnikov Inst. of Precambrian Geol. and Geochron., RAS, nab. Makarova 2, St.Petersburg 199034, Russia

The Sr isotope composition of seawater reflects primarily the balance between continental river input and mantle input via hydrothermal circulation of seawater through mid-ocean ridges. The 87Sr/86Sr changes in Riphean ocean may be traced through analyses of marine carbonates. The preconditions are (1) the availability of unaltered samples and (2) a reliable calibrating of geochronological baseline. The former can be satisfied with geochemical screening through the use of Mn, Fe and Sr as indicators of post-depositional alteration of carbonates. The second precondition can be fulfilled by Pb-Pb dating the screened samples.

The carbonate samples from the Turukhansk Uplift, East Siberia, span the latest Middle Riphean (the Linok and Sukhaya Tunguska fms.) to the earlier Late Riphean (the Derevnya and Burovaya fms.). The samples from the Bashkirian Anticlinorium, the South Urals (the Katav, Inzer, Min'yar and Uk fms.), represent the bulk of the Late Riphean type.

Prior to dissolution in acetic acid, the samples of limestones and dolostones were treated by 1N ammonium acetate (NH4OAc) to remove loosely bound chemical elements from a silicate constituent of the rock and partly eliminate secondary carbonates. The low Mn/Sr (£0.7), Fe/Sr (£9), Rb/Sr (£0.005) ratios and only small difference (£2.10-3) between 87Sr/86Sr ratios in NH4OAc-soluble and major carbonate phases were used as the selection criteria of suitability for limestones. Dolostones were considered as suitable when above values were less than 1, 3, 0.005 and 3.10-3, respectively.

The Pb-Pb isochron dates of 1035 ± 60 Ma and 859 ± 27 Ma for the Sukhaya Tunguska and Inzer carbonates, respectively, are considered as the ages of early diagenesis and used for the control of geochronological baseline. The 87Sr/86Sr ratio in Proterozoic ocean (see Fig. 1: open circles - limestones, solid circles - dolostones) decreased from 0.7061 to 0.7052 for the period 1100-860 Ma, then it increased to 0.7060 between 860 and 650 Ma. There are several fluctuations against the general trend: the pronounced rises up to 0.7058 and 0.7055 at ca. 1010 Ma and 890 Ma respectively, and the drastic decrease to 0.7054 at ca. 660 Ma. On the whole, the 87Sr/86Sr ratios in Middle Riphean and particularly in Late Riphean seawater are well below those reported for the same period in the current literature. This fact points to a very high contribution of the mantle flux to the Sr isotope balance in Late Riphean ocean. It is suggested that the 87Sr/86Sr variations reflect a termination of the Grenville orogeny at ca. 1000 Ma, and the initiation of rift basins and a supercontinental breakup in Late Riphean time. The individual maximums on the curve are most likely related to local orogenic events. The pre-Vendian minimum may have resulted from extensive rifting giving rise to new oceans (e.g. the Central Asian and Iapetus). The high continental flux in the Vendian inferred by the dramatic increase of the 87Sr/86Sr ratio was due to production of recycled crust as result of collision events and the widespread Pan-African orogeny.


This work was supported by the RFBR Grants 93-05-09404 and 94-05-17214 and the ISF-RFG Grant MLO-300.