Nanoscale crystal fabric of primary Ediacaran dolomite

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 similarlyoriented 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.

indicators, and their tendency to preserve mineralogy-dependent crystal fabric makes them 84 useful samples for petrographic analysis (Sandberg, 1975). The upper Khufai oolite is a 85 stromatolite bioherm-bearing, 1-30m thick, cross-stratified ooid and intraclast grainstone 86 that directly overlies mudstones containing evaporite pseudomorphs and is interpreted to 87 have been deposited in a high-energy, shallow marine setting with elevated seawater sulfate nanocrystals and displays them in 3D, at 20nm resolution, using color: hue and bright-117 ness represent in-and out-of-plane angles, respectively (see Appendix A2 and Gilbert et 118 al. (2011)). Since PIC mapping is based on x-ray linear dichroism -a physical effect that 119 depends on bond angle orientation and crystal structure -its prior observation and use in 120 aragonite, calcite, and vaterite (DeVol et al., 2014) suggests that it also works for dolomite, 121 as demonstrated here for the first time. carbonate. Most studies of mimetic dolomites invoke an "early diagenetic" or synsedimen-154 tary origin for the secondary dolomite (e.g., Hood & Wallace, 2018;Corsetti et al., 2006;155 Osburn et al., 2014;Zempolich & Baker, 1993), an interpretation derived from comparison 156 to fabric-destructive (interpreted as late diagenetic) dolomites (e.g., Corsetti et al., 2006) 157 and experimental results (Zempolich & Baker, 1993).  in SEM images (Zempolich & Baker, 1993). In another mimetic dolomitization experiment,

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Bullen and Sibley (1984) dolomitized a range of skeletal materials and produced fine, but 168 coarser than primary CaCO 3 , crystal sizes in echinoids and preserved radial extinction in 169 forams, but found similar coarse dolomite rhombs as Zempolich and Baker (1993) on the 170 exterior of the fossils that protruded into pore space.

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Our results highlight key differences with these dolomitization experiments (Zempolich 172 & Baker, 1993;Bullen & Sibley, 1984). We see no evidence for an outer rind of euhedral 173 dolomite crystals, which would be visible as a thin ring around the PIC mapped ooid ( Figure   174 3a), nor do we observe replacement rhombs protruding into pore space. Dolomite crystals 175 within the cortex of Khufai ooids are predominately acicular, not euhedral or rhombic, 176 and are coherently organized into bundles of similarly-oriented crystals, preserving primary 177 optical characteristics. There is also no evidence that replacement occurred from the outer 178 edge of the ooid inwards. If dolomite did replace primary CaCO 3 in the Khufai, it did so via 179 a different, and possibly novel mechanism capable of precipitating non-euhedral dolomite 180 pseudomorphically, to preserve primary crystal orientations and thus optical properties.

Implications for the Shuram excursion and Ediacaran environments 286
Our results support a primary origin for the Shuram excursion. We interpret the upper 287 Khufai dolomitic ooids to have formed as primary precipitates from seawater and do not 288 observe evidence for the precipitation of late diagenetic minerals, meaning most geochemi-289 cal signals hosted by these ooids should reflect the seawater from which they precipitated.

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As has been discussed recently by carbonate sedimentologists working in modern and an- Range province (Bergmann et al., 2011;Corsetti et al., 2006).
(1) We collected the data displayed in the PIC map in Fig. 3 on beamline 11.0.1.1 at the 319 Advanced Light Source, Lawrence Berkeley National Laboratory, using X-ray PhotoEmis-320 sion Electron spectroMicroscopy (X-PEEM). The polished sample was mounted such that 321 incident X-ray beams hit the sample at a 60 • incident angle. Working at a fixed photon 322 energy of 534 eV (π * peak in carbonate oxygen K-edge spectra), the linear polarization was 323 rotated at the undulator source from 0 • to 90 • in 5 • increments. The resulting 19 images 324 were stacked and analyzed using GG Macros (see Sun et al. (2017)). In each pixel of the 325 stack, the intensity (I) vs. polarization angle (χ) curve was fit using a cosine-squared func- and Si in sample MD6 258.6 B3 (see Table S1), respectively. b and c show Mn and Si in sample MD6 258.6 e1 (Table S1).  Figure S2. Histograms of crystal orientations within three bundles highlighted in Figure 3e of the main text. Like the two bundles displayed in Figure 3b,c of the main text, crystals within these bundles are co-oriented within ∼ 60 • within in bundle. Each bundle is plotted in its original orientation.