Frank Fueten

A computer-controlled rotating polarizer stage for the petrographic microscope

Abstract The computer-controlled polarizing stage replaces the polarizer and analyzer of the standard petrographic microscope for image-processing applications. The stage allows a thin section to remain fixed while the polarizing filters are rotated by stepper motors. This has the advantage that a point within a grain in the field-of-view remains registered to the same pixel at all positions of the polarizers, greatly simplifying the computational requirements. The independent rotations of the polarizer and analyzer are controlled by two stepper motors. The analyzer is mounted on an upper assembly which can be rotated in and out of the light path by a third stepper motor. The upper assembly also contains a neutral density filter for plane-polarized light observation and a fixed polarizing filter, used to set the rotating polarizing filters to a known orientation. All stepper motors, and hence all stage functions, are controlled by a simple command language through a microcontroller that communicates via a serial port to a computer with a video-capture board. The ability to keep the sample stationary while changing the polarization direction of the polarizing filters permits the collection of data which could not be obtained from any single image. The stage greatly improves the potential uses of the petrographic microscope in geological image-processing applications.

Sequence of infilling events in Gale Crater, Mars: Results from morphology, stratigraphy, and mineralogy

Gale Crater is filled by sedimentary deposits including a mound of layered deposits, Aeolis Mons. Using orbital data, we mapped the crater infillings and measured their geometry to determine their origin. The sediment of Aeolis Mons is interpreted to be primarily air fall material such as dust, volcanic ash, fine-grained impact products, and possibly snow deposited by settling from the atmosphere, as well as wind-blown sands cemented in the crater center. Unconformity surfaces between the geological units are evidence for depositional hiatuses. Crater floor material deposited around Aeolis Mons and on the crater wall is interpreted to be alluvial and colluvial deposits. Morphologic evidence suggests that a shallow lake existed after the formation of the lowermost part of Aeolis Mons (the Small yardangs unit and the mass-wasting deposits). A suite of several features including patterned ground and possible rock glaciers are suggestive of periglacial processes with a permafrost environment after the first hundreds of thousands of years following its formation, dated to ~3.61 Ga, in the Late Noachian/Early Hesperian. Episodic melting of snow in the crater could have caused the formation of sulfates and clays in Aeolis Mons, the formation of rock glaciers and the incision of deep canyons and valleys along its flanks as well as on the crater wall and rim, and the formation of a lake in the deepest portions of Gale.

Edge detection in petrographic images using the rotating polarizer stage

Abstract A new edge detection procedure which calculates closed edges (grain boundaries) for images of petrographic thin sections is presented. Edges are extracted from a gradient image obtained using the rotating polarizer stage microscope. The gradient image is calculated by passing a simple gradient operator over captured images during the sampling process. A series of image processing routines are passed over the gradient image to extract the edges. The individual routines are: Gaussian smoothing, localized enhancement, nondirectional nonmaxima suppression, double thresholding, dilation, skeletonization and the removal of undesirable boundary fragments. The output of the procedure contains only closed boundaries.

Mineral identification using artificial neural networks and the rotating polarizer stage

An artificial neural network is used for the classification of minerals. Optical data using thin sections is acquired using the rotating polarizing microscope stage, which extracts a basic set of seven primary images during each sampling. A selected set of parameters based on hue, saturation, intensity and texture measurements are extracted from the segmented minerals within each data set. Parameters such as pleochroism, plane light hue, and gradient homogeneity were a few that proved to yield class-discriminating properties. Texture parameters are shown to have the ability to classify colourless minerals. The neural network is trained on manually classified mineral samples. The most successful artificial network to date is a three-layer feed forward network using generalized delta error correction. The network uses 27 distinct input parameters to classify 10 different minerals. Testing the network on previously unseen mineral samples yielded successful results as high as 93%.

Documentation of a 1450 Ma contractional orogeny preserved between the 1850 Ma Sudbury impact structure and the 1 Ga Grenville orogenic front, Ontario, Canada

A 1453–1445 Ma contractional deformation event is documented in rocks of the Early Proterozoic Huron Supergroup at the southeastern boundary between the Hudsonian craton to the northwest and an Early to Middle Proterozoic granitoid suite that forms the northeasternmost recognized extension of the Transcontinental Proterozoic Provinces. Detailed structural mapping, quartz c-axis fabrics, and grain-shape analysis indicate that four major orogenic phases have affected the area: (1) the Penokean orogeny, which predated the 1850 Ma Sudbury impact event, (2) ca. 1750 Ma granite plutonism, (3) ca. 1450 Ma orogenic events, and (4) the ca. 1000 Ma Grenvillian orogeny. The Murray fault zone, which likely accommodated considerable ductile displacement during the Penokean orogeny, underwent only brittle deformation during the younger orogenies. The previously undocumented Long Lake fault represents the northern limit of significant ductile south-over-north reverse thrusting during 1453–1445 Ma deformation. The effect of the Grenvillian orogeny in this area appears to have been minor. No deformation features could be linked to the intrusion of the ca. 1750 Ma granitoid suite.

Interior layered deposits within a perched basin, southern Coprates Chasma, Mars: Evidence for their formation, alteration, and erosion

A basinA¢Â�Â�like area containing three interior layer deposits (ILDs) on the southern margin of Coprates Chasma was studied. We interpret the area as an ancestral basin and demonstrate that ILD deposition postdates the formation of the current wall rock slopes. The geometry of the ILD and the wall rock spurs form a catchment area between each ILD and the plateau to the south. Erosional remnants of extensive ash or dust layers deposited on the plateau south of Valles Marineris also crop out on the southern plateau of Coprates Chasma. A mass balance calculation suggests that the volume of each ILD is compatible with the volume of the ash or dust that would have been deposited within each catchment area. We propose that the ILDs likely formed by episodically washing such aerially deposited material down from chasma walls. Rifting of the IusA¢Â�Â�MelasA¢Â�Â�Coprates graben opened the enclosed basin and removed any standing water. Faults within the ILDs are compatible with this chasm opening. Sulfates are associated with the ILDs and lightA¢Â�Â�toned material on the basin floor. We suggest that they result from water alteration of preexisting deposits, though the timing of that alteration may predate or postdate the breaching of the basin. Scours within one ILD are similar to terrestrial glacial scours. During a period of high obliquity ice would accumulate in this region; hence we argue the scours are Martian glacial scours. A late deposited layer marks the end of the active local geological history between 100 My and 1 Gy.

Automatic mineral identification using genetic programming

Abstract. Automatic mineral identification using evolutionary computation technology is discussed. Thin sections of mineral samples are photographed digitally using a computer-controlled rotating polarizer stage on a petrographic microscope. A suite of image processing functions is applied to the images. Filtered image data for identified mineral grains is then selected for use as training data for a genetic programming system, which automatically synthesizes computer programs that identify these grains. The evolved programs use a decision-tree structure that compares the mineral image values with one other, resulting in a thresholding analysis of the multi-dimensional colour and textural space of the mineral images.

Quartz c-axes orientation determination using the rotating polarizer microscope

Abstract A fast and accurate optical method to calculate quartz c -axis orientations using the rotating polarizer stage and standard thin sections is presented. c -Axis orientations are calculated for each pixel and Achsenverteilungsanalyse (AVA) can easily be constructed to study problems that would normally require a prohibitive amount of tedious work. The computer controlled rotating polarizer stage replaces the polarizer and analyzer of the standard petrographic microscope and allows a thin section to remain fixed while the polarizing filters are rotated by stepper motors. Data are collected by stepping the polarizers through a 180° rotation, capturing a frame at each step and extracting information on the intensity of the pixel and position of the polarizers. The position data are used to calculate the trend or trend+180° of the quartz c -axes, while a simple mathematical relationship between intensity and plunge is derived which allows the plunges of c -axes to be calculated.

An artificial neural net assisted approach to editing edges in petrographic images collected with the rotating polarizer stage

A practical methodology to edit edges within petrographic images is presented. The procedure uses the existing output of a standard segmentation routine as input. Edges are skeletonized and converted into segments which separate two grains and join at nodal points. As series of colour and texture parameters are extracted from all grains. For each edge segment, an artificial neural net (ANN) evaluates differences in the parameters for the grains separated by the segment. ANN output is used to classify segments as true or false edges and can be thresholded at different levels. A manual procedure allows for the final classification of the segments. This method significantly improves the speed with which edges can be edited in preparation for other studies.

A model for the development of a domainal quartz c-axis fabric in a coarse-grained gneiss

Abstract Quartz c -axis fabrics in high-grade coarse-grained layered gneisses reveal a distinctive pattern, with three perpendicular maxima, which can be interpreted in terms of syntectonic recrystallization and growth. Mapping of the distribution of c -axes within a single thin section demonstrates that the fabric is domainal. Fabric domains are on the scale of millimetres and are associated with individual quartz-rich patches. No single domain exhibits the overall fabric of the complete thin section. Within each domain, weighting of the c -axis fabric by the area of each quartz grain demonstrates that the largest grains have their c -axes preferentially at the maxima. A model is presented to account for the development of this fabric. Grains which can keep their stored strain energy to a minimum by the use of a single glide system, oriented approximately parallel to the plane and direction of maximum resolved shear stress, grow at the expense of other grains. The largest, most successful grains deform by glide on only one of either the basal ( a ) or the prism ( a ) systems, depending on the crystallographic orientation of the grain. Each domain develops its own glide direction by co-operation of the glide systems of its constituent grains. The successful grains in a domain are those which can accommodate that direction. The bulk strain of the rock is achieved by the addition of the strains distinct to each domain.