Abstract
Dolomite (CaMg(CO3)2) forms in minor quantities in few modern
environments yet comprises most of the Precambrian carbonate rock
record. Precambrian dolomites are often fine-grained and
fabric-retentive and are interpreted to have precipitated as primary
cements or formed as early diagenetic replacements of CaCO3. Primary
dolomite precipitation from seawater in depositional environments has
not yet been described. Here, we use synchrotron radiation to produce a
nanoscale-resolution crystal orientation map of one exquisitely
preserved ooid deposited at the onset of the Shuram carbon isotope
excursion at ~574 Ma. The crystal orientation map
reveals small (~10 μm) acicular, radially-oriented
crystals grouped into bundles of similarly-oriented crystals with
varying optical properties. We interpret this dolomite formed via
primary, spherulitic precipitation during ooid growth in shallow marine
waters. This result provides additional evidence that the
physicochemical properties of late Precambrian oceans promoted dolomite
precipitation and supports a primary origin for the Shuram excursion.