Neoproterozoic Fabric-retentive Dolomite at the Nanoscale
Abstract
Although dolomite is ubiquitous in the geologic record, its Precambrian and Phanerozoic forms are distinct. Most Phanerozoic dolomite is thought to be diagenetic, to have formed secondarily after initial calcitic or aragonitic phases were dissolved and replaced with dolomite crystals. Phanerozoic dolomitization, regardless of the diagenetic setting in which it took place, is often fabric-destructive and results in a coarsening of crystals. In contrast, Precambrian dolomites are almost always fabric-retentive and preserve a range of crystal sizes and morphologies, including micrite, ooids, and cements. The morphological differences between Phanerozoic and Precambrian dolomites have led to the suggestion that some Precambrian dolomites could have formed primarily, precipitating directly out of solution and not as a result of diagenetic processes. Our new compilation of Precambrian and Cambrian mineralogies shows the ratio of dolomite to calcite decreases spatially (with increasing water depth) and temporally (from the Neoproterozoic into the Cambrian). These morphological, spatial, and temporal observations of dolomite suggest changes in the precipitation of primary dolomite, in diagenetic processes, or both, from the Precambrian into the Phanerozoic. To determine whether some of the Precambrian dolomite record can be explained by primary precipitation processes, we study dolomitic ooid, stromatolite and cement samples from Neoproterozoic successions of Oman and Norway in a photoemission electron spectromicroscopy (PEEM) experiment. Polarization-dependent imaging contrast (PIC) mapping reveals spherulitic growth structures and suggests that dolomite crystals grew by particle attachment. Our results indicate that dolomite in these samples formed as the primary mineralogy and suggest that carbonate precipitation and diagenesis operated substantially differently during the Proterozoic than it did during the Phanerozoic. We suggest the observed differences in Precambrian and Phanerozoic dolomites were likely controlled by a combination of factors, including seawater chemistry, oxygenation, evolutionary advances, and water temperature, that affected both primary precipitation and secondary alteration of carbonate minerals.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP41E1898W
- Keywords:
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- 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 1051 Sedimentary geochemistry;
- GEOCHEMISTRYDE: 3675 Sedimentary petrology;
- MINERALOGY AND PETROLOGYDE: 4863 Sedimentation;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL