[ Petrography ] [ Microbiology ] [ Neutron Tomography ] [ Home ]

Seven polished thin sections of travertine samples from the McLaughlin Reserve were examined with a petrographic microscope under polarized and reflected light. Samples were also viewed under the Cold Cathode Luminescence Model 8200MKII microscope at 10-20 kv and 500-700 µA. The samples were stained with Feigl's solution to differentiate aragonite from calcite and Alazarin Red S to differentiate calcite and aragonite from dolomite.

The samples fall into two general categories. The older, oxidized samples are composed mainly of detrital serpentine with minor amounts of calcite and aragonite cement coatings. The younger samples are composed primarily of bladed aragonite crystals in horizontal to subhorizontal layers, lesser amounts of euhedral and blocky calcite cements, primary dolomite, and weathered grains of serpentinite which were incorporated into the sample as water flowed over the mound. All of the younger samples have a prominent bacterial population associated with cement precipitation.

photo left: A common sequence of precipitation for these travertines is visible in this sample. Precipitation of aragonite (long, bladed crystals) and high Mg calcite (blocky crystals above aragonite) are followed by precipitation of primary dolomite (white bands) and a 2 µm layer of smectite.

photo right: Low temperature primary dolomites (white crystals) are interbedded with high Mg calcite.

photo left: The subhorizontal cement coatings may reflect precipitation on an irregular topography or precipitation around organic material.

photo right: The aragonite crystals have precipitated from a bacterial substrate (black knob) and are long and bladed with sharp terminations. Botryoids of acicular aragonite growing in a radial pattern away from the bacterial nucleus are present throughout the samples.

 

photo left: Blocky calcite cements often fill elongated pore spaces. The dark matrix is a combination of organic material, aragonite, and clay.

photo right: A CL image of the same pore space from the image on the left reflects distinct periods of high Mn waters flowing through the pore space.

Cocoidal cyanobacteria are present on the surface of the active travertine mounds. The type of cyanobacteria associated with travertines is a function of the lithography, the climate (number of rain days and sun days) and the water stress. The organism identified on these travertines is a Gloeocapsa sp. and has a thick sheath to protect it from desiccation during times of high water stress and high light irradiance (a common condition on these travertines at the McLaughlin preserve). The common presence of cyanobacteria on the surface suggests that the organic material identified in the petrography of the travertines may have resulted from cyanobacterial activity.

Note the different morphologies of the cyanobacteria in these two photographs.

Neutron Computed Tomography (CT ) imaging is a technique being developed to view internal structures in rocks. It is a non-destructive technique that uses the interactions between neutrons and various elements to map the interior of samples. In the past, to get a 3-D image of a carbonate rock, such as travertine, it would have to be cut into many thin slices and then imaged individually. That technique is not only extremely time consuming, it also destroys the sample. With very old or rare samples the neutron technique is valuable because the sample is preserved. The elements carbon and hydrogen have strong signatures with this technique, while the heavy elements often interact less, or not at all, with the neutrons. This makes nuetron tomography very useful for looking at microbial structures.

A large piece of travertine was imaged. The sample was placed on a rotating stage and was shot with a neutron beam 360 times, once per every half of a degree of rotation. These 360 images were then combined to form a 3-D image. Using a 3-D software imaging program, the data were sliced into 279 individual images which can be stacked to make movies moving through the sample. It is also possible to cut slices perpendicularly through the stack. The gray areas are most likely aragonite, dolomite, or calcite (the mineralogy obtained from the XRD). The very dark gray/black areas are most likely areas of organics (material with high concentrations of hydrogen and carbon), which would have the darkest signatures. Layering in travertines is common and layered structures in the carbonates are very apparent in this sample. The ripple-like marks in the slice images are caused by the rotation of the stage during the imaging process.

photo left: This is a cut perpendicular to the 3-D slices. It shows the layering in the travertine.

photo right: This is another perpendicular slice. The darkest areas indicate where the organics are located.

Neutron class: http://www-geology.ucdavis.edu/~inaa/
MNRC home page: http://mnrc.ucdavis.edu/

[ Field Images ] [ Mineralogy and Magnetics ]
[ Isotopes and Major & Minor Elements ]

[ GEL 281 Home Page ]
schiffman@geology.ucdavis.edu