Abstract

Dolomites are more common in Precambrian than in Phanerozoic rocks and, from a petrographic-isotopic study of the middle to upper Proterozoic Beck Spring Dolomite of eastern California, it is suggested that Precambrian dolomites are different from those of the Phanerozoic. Petrographically, all fabric details of Precambrian dolomites studied are preserved, just as if they were limestones, and isotopically, they show the same trend as limestones: increasingly negative δI8O and to a lesser extent increasingly light δI8C, from depositional components, through early fibrous dolomite cements to later dolomite spar cements. The data suggest that in Precambrian time dolomite was the principal carbonate mineral precipitated from seawater and during diagenesis, and this implies that Precambrian seawater was different from that of the Phanerozoic.

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