Strontium isotopic variations of Neoproterozoic seawater: Implications for crustal evolution

Journal Article · Tue Oct 01 04:00:00 EDT 1991 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(91)90453-C· OSTI ID:7235859

Asmerom, Y; Jacobsen, S B; Knoll, A H; Butterfield, N J ^[1]; Swett, K ^[2]

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1. Harvard Univ., Cambridge, MA (United States)
2. Univ. of Iowa, Iowa City (United States)

The authors report high precision Sr isotopic data on carbonates from the Neoproterozoic Shaler Group, Victoria Island, Northwest Territories, Canada. Samples with low {sup 87}Rb/{sup 86}Sr ratios (<0.01) were selected for Sr isotopic analysis. {delta}{sup 18}O, Mn, Ca, Mg, and Sr data were used to recognize altered samples. The altered samples are characterized by high Mn/Sr ({ge}2) and variable {delta}{sup 18}O; most are dolomites. The data indicate that between ca. 790-850 Ma the {sup 87}Sr/{sup 86}Sr ratio of seawater varied between 0.70676 and 0.70561. The samples show smooth and systematic variation, with the lowest {sup 87}Sr/{sup 86}Sr value of 0.70561 at ca. 830 Ma. The low {sup 87}Sr/{sup 86}Sr ratio of carbonates from the lower parts of the section is similar to a value reported for one sample from the Adrar of Mauritania ({approx}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. Data from this study and the literature are used to construct a curve of the {sup 87}Sr/{sup 86}Sr ratio of Neoproterozoic seawater. 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 the authors 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.

• OSTI ID: 7235859
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 55:10; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
ALKALINE EARTH ISOTOPES
ALKALINE EARTH METALS
CANADA
CARBONATE ROCKS
CHEMICAL COMPOSITION
CHEMISTRY
DEVELOPED COUNTRIES
EARTH CRUST
ELEMENTS
GEOLOGIC AGES
GEOLOGIC FORMATIONS
GEOLOGIC HISTORY
GEOLOGIC MODELS
GEOLOGIC STRUCTURES
GEOTHERMAL SYSTEMS
HYDROGEN COMPOUNDS
HYDROTHERMAL SYSTEMS
ISOTOPE RATIO
ISOTOPES
METALS
NORTH AMERICA
NORTHWEST TERRITORIES
ORIGIN
OXYGEN COMPOUNDS
PLATE TECTONICS
PRECAMBRIAN ERA
RIFT ZONES
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STRONTIUM
STRONTIUM ISOTOPES
TECTONICS
WATER
WATER CHEMISTRY