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The Integrity of δ18O Records in Precambrian Carbonates: A Mesoproterozoic Case Study

By
Tracy D. Frank
Tracy D. Frank
Department of Earth Sciences, University of Queensland, Brisbane, Queensland 4072, Australia
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Timothy W. Lyons
Timothy W. Lyons
Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, U.S.A.
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Published:
January 01, 2000

Abstract

A high-resolution, integrated approach was used to assess the integrity of the oxygen-isotope record derived from carbonates of the Helena Formation of the Mesoproterozoic Belt Supergroup, western Montana. Paired stable-isotope and minor-element analyses were performed on multiple microsamples from the full range of petrograpic components, including ooids, intergranular cements, molar-tooth calcite, dolomitic raudstone, stromatolites, and late-stage diagenetic cements in crosscutting fractures. Data were examined within a context of sedimentologic and petrograpic constraints to assess temporal and spatial effects on isotopic compositions. petrograpic and isotopic constraints demonstrate that δ13O values that best approximate primary oxygen-isotope compositions occur in syndepositional and early diagenetic intergranular cements within oolitic grainstones that occur in the upper part of the Helena Formation. While cautious in our interpretation, we also recognize fundamental patterns in isotopic and geochemical data which indicate that oxygen-isotope compositions of other petrograpic components in the Helena Formation have not been completely homogenized and reset during diagenesis. These include: (1) stratigraphic and facies-specific trends in isotopic compositions that are consistent with inferred depositional conditions, (2) compositional heterogeneities within and among individual petrograpic components on small spatial scales, and (3) retention of the expected oxygen-isotope fractionation between early-formed calcite and associated dolomite pairs. Although diagenetic overprints are significant, these relationships argue against wholesale diagenetic resetting of primary compositions and suggest that with careful examination, general depositional controls can be discerned in oxygen-isotope data from the Helena Formation.

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SEPM Special Publication

Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World

John P. Grotzinger
John P. Grotzinger
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Noel P. James
Noel P. James
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SEPM Society for Sedimentary Geology
Volume
67
ISBN electronic:
9781565761896
Publication date:
January 01, 2000

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