The Integrity of δ18O Records in Precambrian Carbonates: A Mesoproterozoic Case Study
Tracy D. Frank, Timothy W. Lyons, 2000. "The Integrity of δ18O Records in Precambrian Carbonates: A Mesoproterozoic Case Study", Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, John P. Grotzinger, Noel P. James
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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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Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World - Precambrian carbonates are usually regarded at the simple cousins of the sedimentary realm, composed of stromatolites and dolostones, texturally not challenging and commonly altered beyond recognition by the vagaries of time, diagenesis and metamorphism. However, these carbonates that formed deep in time are commonly exquisitely preserved and contain within them a record of the evolving young earth. SEPM Special Publication 67 explores these aspects. Resulting from a 1997 SEPM/CSPG symposium entitled? Precambrian Carbonates,? these 18 papers demonstrate the importance of understanding these rocks, since within them is contained a record of the early ocean, atmosphere, and biosphere.