Abstract

We report high precision Sr isotopic data on carbonates from the Neoproterozoic Shaler Group, Victoria Island, Northwest Territories, Canada. Lithostratigraphic correlations with the relatively welldated Mackenzie Mountains Supergroup constrain Shaler deposition to ~770–880 Ma, a range corroborated by 723 ± 3 Ma lavas that disconformably overlie Shaler carbonates and by Late Riphean microfossils within the section. Samples with low 87Rb 86Sr ratios (< 0.01) were selected for Sr isotopic analysis. δ 18 O, Mn, Ca, Mg, and Sr data were used to recognize altered samples. The altered samples are characterized by high Mn Sr (≥2) and variable δ 18 O; most are dolomites. The data indicate that between ca. 790–850 Ma the 87Sr 86Sr ratio of seawater varied between 0.70676 and 0.70561. The samples show smooth and systematic variation, with the lowest 87Sr 86Sr value of 0.70561 at ca. 830 Ma. The low 87Sr 86Sr ratio of carbonates from the lower parts of our section is similar to a value reported for one sample from the Adrar of Mauritania (≈900 Ma), West African Craton. Isotopic ratios from the upper part of the Shaler section are identical to values from the lower part of the Neoproterozoic Akademikerbreen Group, Spitsbergen. Although a paucity of absolute age determinations hinders attempts at the precise correlation of Neoproterozoic successions, it is possible to draw a broad outline of the Sr isotopic composition of seawater for this period. Indeed, the Sr isotope data themselves provide a stratigraphic tool of considerable potential. Data from this study and the literature are used to construct a curve of the 87Sr 86Sr ratio of Neoproterozoic seawater. The new data reported in this study substantially improve the isotopic record of Sr in seawater for the period 790–850 Ma. The Sr isotope composition of seawater reflects primarily the balance between continental Sr input through river input and mantle input via hydrothermal circulation of seawater through mid-ocean ridges. Coupling of Nd and Sr isotopic systems allows us to model changes in seafloor spreading rates (or hydrothermal flux) and continental erosion. The Sr hydrothermal flux and the erosion rate (relative to present-day value) are modeled for the period 500–900 Ma. The results indicate that the hydrothermal flux reached a maximum value at ca. 830 Ma. In contrast, a large peak in erosion rate is indicated at ca. 570 Ma. The peaks in hydrothermal flux and erosion rate are most likely related to developments in the Pan-African and related orogenic events, whose initial development is characterized by production of juvenile crust during supercontinental break up and rifting. The time ca. 570 Ma is characterized by continent-continent collision and production of recycled crust. Sr isotope data from Proterozoic carbonates offer a valuable resource for understanding large-scale crust dynamics.

Highlights

  • OUR KNOWLEDGEOFcrustalevolution in the Neoproterozoic (540-loo0 Ma) is fragmentary at best, but the 87Sr/86Srrecord in marine carbonates offers a potentially powerful tool in tracing large-scale crustal processes during this period

  • The secular variation of seawater 87Sr/86Sr is a valuable tool in understanding geological processes through time

  • 1) While difficulties exist in establishing the seawater Sr isotope curve due to diagenetic alteration and stratigraphic resolution, it is possible to overcome these difficulties by careful screening of samples using petrography, elemental, and isotopic data such as Mn / Sr and Rbl Sr ratios, 6 180, and 613C.We have been able to achieve a reasonable degree of stratigraphic resolution by analyzing samples collected from measured sections in a single basin

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Summary

Introduction

OUR KNOWLEDGEOFcrustalevolution in the Neoproterozoic (540-loo0 Ma) is fragmentary at best, but the 87Sr/86Srrecord in marine carbonates offers a potentially powerful tool in tracing large-scale crustal processes during this period. Strontium has a long residence time in the ocean, about 4 Ma; the Sr isotope composition of seawater at any time is uniform to within present-day analytical precision. The Sr isotope composition of seawater reflects primarily the balance between continental river input and mantle input as a result of the interaction of seawater with ocean crust during hydrothermal circulation through mid-ocean ridges. The Sr isotope composition of marine carbonates, records the balance between these two contributions through time. JACOBSEN( 1988) suggested that this may be used to recognize three major episodes of high global erosion rates corresponding to major continental collisions during the past 600 Ma. Insofar as the relative contributions of the continental crust and the mantle are affected by tectonic processes and crustmantle dynamics, the carbonate g7Sr/s6Sr record provides a resource for studying these processes through Earth history

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