22 Precambrian II: Meso- and Neoproterozoic Evolution of Western Laurentia I: In Honor of Don Winston

Tuesday, 20 May 2014
1:00 PM - 5:00 PM
Gaines Hall 101

MICRO-CT ANALYSIS OF ANOMALOUS TUBE STRUCTURES FROM THE MESOPROTEROZOIC NEWLAND FORMATION, MEAGHER COUNTY, MONTANA

Paper number: 22-9
Program: Theme Sessions
Time: 3:30 PM – 3:50 PM

Authors

Abstract

Unusual decimeter-scale structures of irregular, variably oriented, 1-3 mm wide tubes have been observed in close association with SEDEX deposits in the Mesoproterozoic Newland Formation of the Helena Embayment. These structures have been previously interpreted as mineralized microbial sheaths (McGoldrick and Zieg, 2004), but may alternatively represent fluid flow structures similar to those seen in modern submarine hydrothermal vent systems. Two end-member chimney types both preserve intricate porosity patterns, common sulfide walled chimneys and more rare carbonate-brucite walled chimneys like those from the alkaline Lost City hydrothermal field. Mineralized sheaths should yield a continuous and oriented group of tubes with relatively consistent diameters. In contrast, an inorganic origin related to hydrothermal fluid flow should be reflected in fully enclosed, discontinuous and randomly oriented tube structures of varying diameter.

We used X-ray micro computed tomography (MicroCT) to quantify the three dimensional structure, orientation and size of Mesoproterozoic tube structures. Tomographic volumes were generated for two subsurface samples (one cemented and one uncemented), as well as one sample from a nearby outcrop. A modern hydrothermal vent sample with abundant inorganic pores was also analyzed for comparison. Three-dimensional porosity was reconstructed based on density variations using the defect analysis tool in VGStudio Max 2.2.

The Mesoproterozoic tube morphologies range from smooth-walled with a moderate length to width ratio, to textured and amorphous. Tubes within the subsurface samples display no preferential orientation, although the sample from outcrop has flattened tubes oriented parallel to bedding. Tube volumes range from less than 0.5 mm3 to ~40 mm3. The modern vent sample displays a similar range of tube and void morphologies and orientations, although the voids are generally larger in size and more likely to be oriented vertically. Similarities in tube structure between the modern and Mesoproterozoic samples suggest concentrated fluid flow may be a component of tube formation. This novel application of MicroCT to the rock record proved valuable for quantifying the spatial variation within ancient three-dimensional structures.