- Email: [email protected]
Three-dimensional distribution of minerals in diamondiferous eclogites, obtained by the method of high-resolution X-ray computed tomography
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 575 (2007) 255–258 www.elsevier.com/locate/nima
Three-dimensional distribution of minerals in diamondiferous eclogites, obtained by the method of high-resolution X-ray computed tomography K.E. Kupera,, D.A. Zedgenizovb, A.L. Ragozinb, V.S. Shatskyb, V.V. Poroseva, K.V. Zolotareva, E.A. Babicheva, S.A. Ivanova a
Budker Institute of Nuclear Physics of SB RAS, Lavrentiev prospect 11, Novosibirsk 630090, Russia b Institute of Geology and Mineralogy of SB RAS, Novosibirsk 630090, Russia Available online 16 January 2007
Abstract High-resolution X-ray computed tomography (HRXCT) is a technology ideally applicable to a wide range of geological investigations. It is an express non-destructive method to produce images corresponding to series of slice projections through a sample. In the present study, HRXCT was applied to rock samples with the use of synchrotron radiation from the VEPP-3 storage ring, at the ‘‘X-ray microscopy and tomography’’ station. The method was calibrated and preliminary measurements were carried out on the Low-Dose Digital Radiographic Device ‘‘Siberia’’. The data obtained have determined the internal structure of rock samples with a spatial resolution of 100 mm. HRXCT has been applied to the xenoliths of diamondiferous eclogites from the Udachnaya kimberlitic pipe located in Yakutia, Russia. It has allowed determination of the distribution of rock-forming (garnet and clinopyroxene) and accessory (diamond, rutile, and sulfide) minerals of different X-ray absorption. This is important to find out the genetic relationship of diamonds with associated minerals and the sequence of crystallization. r 2007 Elsevier B.V. All rights reserved. PACS: 41.50.+h; 07.85.Tt; 91.65. n Keywords: X-ray tomography; Petrography; Diamond; Eclogite
1. Introduction High-resolution X-ray computed tomography (HRXCT) gives a detailed three-dimensional (3D) imagery of the interiors of rocks, realized in a non-destructive manner. The essence of the method is scanning a sample with X-ray beams that are differently absorbed in its various areas. The attenuation of X-rays along the beam is an integrated characteristic of the density of the object to investigate. The degree of distinguishing different components of rock volume in HRXCT data depends on the difference in their linear attenuation coefficients (Fig. 1). The resulting data are reconstructed from the projections obtained during rotation of a sample as series two-dimennsional (2D) slices. Collecting a sequence of contiguous slices allows reconCorresponding author. Tel.: +7 3832 394 259.
E-mail address: [email protected] (K.E. Kuper). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.01.053
struction of a full 3D volume. The HRXCT method can be applied ideally to a wide range of geological investigations. Geological applications include interior examination of textural analysis of igneous and metamorphic rocks and other tasks demanding 3D data that previously required serial sectioning. Some HRXCT studies of the internal structure of geological samples were previously reported (e.g., [1,2]). These data revealed general relationships among rock components in the 3D space. It is important that this non-destructive technique maps and locates minerals within rocks, thereby enabling one to investigate the nature of their formation in their original setting. The rare specimens of diamondiferous eclogite xenoliths make them invaluable samples of the Earth’s mantle. As part of comprehensive study of diamondiferous xenoliths, we have conducted HRCXT studies of several xenoliths of diamondiferous eclogites from the Udachnaya kimberlitic pipe (Yakutia).
ARTICLE IN PRESS K.E. Kuper et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 255–258
256
Linear attenuation coefficient, cm-1
6 5 Garnet Mica Clinopyroxene Sulfide Diamond
4 3 2 1 0 50
60
70 80 X-ray energy, keV
90
100
Fig. 1. Linear X-ray attenuation coefficients for different minerals in a diamondiferous sample.
2. Experiment and setup Two subsystems were used to image geological specimens across a range of size classes. The high-resolution subsystem uses an X-ray source and linear array detectors to image samples of 50–80 mm in diameter with a slice thickness of 0.2 mm. The second subsystem uses synchrotron radiation (SR) from the VEPP-3 storage ring and 2D detector to image samples of 10–40 mm in diameter at slice thickness down to 100 mm. Both systems used polychromatic sources. The first system was mounted on the base of the LowDose Digital Radiographic Device (LDRD) ‘‘Siberia’’. We used an X-ray tube with a tungsten target with a focal spot size of 0.6 mm and a typical voltage of 110 kVp. An additional filter of 0.6 mm of Cu was installed on the output window of the X-ray tube. A slit collimator formed a thin fanlike beam. A detector measured the distribution of radiation intensity in the horizontal direction. It is possible to consider the detector as 1024 independent channels with a strip of 0.4 mm [3]. The detector was filled with Kr gas under a pressure of 40 bar. The vertical beam size was defined by the collimator, and the X-ray focal spot size equaled 0.2 mm in the object area. During scanning, the object was rotated with a constant angular speed and data were registered line by line every 5 ms. For the next slice projection, the sample was vertically scanned with a step of 200 mm. A layout of the scanning technique is presented in Fig. 2. An example of the data obtained (sinogram) is also shown in the figure. The second part of experiment was carried out at the station ‘‘X-ray microscopy and tomography’’. SR is a very strong source of radiation in the X-ray range that is four times brighter than a photon flux from an X-ray tube. It enables us to use a detector with higher spatial resolution than in the previous experiment. In these measurements, the size of samples did not exceed 40 mm, and the
resolution was about 100 mm. The 2D RadIcon image sensor with gadolinium oxysulfide scintillator is a 1024 1000 CMOS matrix with a 50 mm pixel. Fig. 3 demonstrates the installation for sample scanning and an example of projection of a diamondiferous rock on a logarithmic scale. Each scan consists of 360 projections with an angular step of 0.51 (from 01 to 1801). Small-angle SR deviation of about 0.2 mrad makes it possible to use an algorithm for a parallel beam geometry, which simplifies the process of 3D reconstruction of the object and significantly improves the quality of the image. Five millimeter copper filter was used to cut off soft components in the photon spectrum, (Fig. 4). In both cases, the effects caused by heterogeneities in the incident X-ray beams and non-uniformity in the response of the detectors were corrected using an appropriate calibration. Ring artifacts caused by nonlinear response of detector elements to a signal were corrected via removing vertical lines in the reconstructed image converted to polar coordinates according to the technique described in the article [4]. 3. Results and discussion The 3D visualization technique consists of volume rendering, in which each grayscale value in the data set is assigned a color and an opacity value. This allows some mineral phases to be rendered transparent and others partially or entirely opaque, providing unique opportunities for petrographic analysis in three dimensions. The simplest segmentation method for data is to define a grayscale threshold or range which is unique to the phase of interest. However, this approach is complicated by the finite resolution of the imagery. Because each data voxel (volume elements) can encompass a range of material, if more than one material is present in that voxel, the resulting grayscale will be some average of the phases present; this is known as the partial volume effect. Furthermore, some blurring is inevitable, causing the grayscale value within a voxel to be influenced by surrounding material. Thus, the material boundaries often extend across two voxel featuring a gradual grayscale transition between the corresponding values characterizing each phase. The threshold level for proper location of a boundary between two different materials is the mean of their exact values. For the diamondiferous xenoliths scan data, the threshold value for sulfide was 150, while garnet had a value about 100, clinopyroxene had a value of 80, and diamond had a value of 50. Because garnet is most frequently situated in contact with clinopyroxene, its appropriate threshold value is 90. Zones of secondary mineralization appear as dark thin lines on X-ray images presumably due to their porous nature, resulting in lower X-ray attenuation. In this manner, it is easy to distinguish diamonds, sulfides, silicates (garnet and clinopyroxene), and their alteration products in xenolith (Fig. 5). Several such 2D
ARTICLE IN PRESS K.E. Kuper et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 255–258
257
Fig. 2. The HRXCT installation mounted on the base of the LDRD ‘‘Siberia’’ (left) and an example of the data obtained on a logarithmic scale (right).
Fig. 3. The system for sample scanning with SR (left) and an example of projection of a diamondiferous rock on logarithmic scale (right).
Photons/sec./keV/cm2
3.5x108 3.0x108 2.5x108 2.0x108 1.5x108 1.0x108 5.0x107 0.0 50
60
70
80
90
100
X-ray energy, keV Fig. 4. Theoretical photon energy spectrum, after the copper filter 5 mm thick.
images are stacked together using volume–visualization software to produce a 3D model of the xenolith. This essentially represents a density map of the sample, from which one can extract the sizes, shapes, textures, and locations of individual crystals that have dimensions exceeding the spatial resolution of the scans.
3D models of the samples reveal clearly the spatial relationships between diamonds and their surroundings. This gives clues to the processes that control diamond crystallization. These relationships are determined by rotating and viewing the model in different perspectives to look for any mineral associations and alignments. It is possible to make some phases transparent and display only selected mineral phases at a time. For example, a 3D HRCXT model of diamondiferous eclogite 107 is shown in Fig. 6. For better clarity, the garnet and clinopyroxene grains are made invisible so that locations and sizes of diamonds could be seen easily. A series of macro-diamonds is clearly visible in the interior of this sample, which could not have been seen by any other technique. It seems that many of the diamonds in the eclogite are located linearly along a diagonal passing through the eclogite. The spatial resolution of this technique is close to 100 mm, which is essentially helpful in visualization of diamonds less than 1 mm in size. It is interesting to note that sometimes diamonds were observed to be in direct contact with or to be enclosed within a fresh garnet or clinopyroxene. Furthermore, sulfide minerals are not preferentially associated with diamonds. It may be concluded from these observations that some diamonds are syngenetic with the garnet and
ARTICLE IN PRESS K.E. Kuper et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 255–258
258
Fig. 5. Grayscale (left) and mapped version of the one slice (right) of a diamondiferous sample (Cpx is for clinopyroxene; Grt is for garnet).
diamonds. The LDRD-based system allows getting images of rather big samples with a special resolution of approximately 200 mm. The SR application gives a possibility to reach a 1% contrast resolution for minerals in a diamondiferous sample with a spatial resolution higher than 100 mm. The samples of eclogite xenoliths from Udachnaja in the present study are exceptionally rich in diamonds. It has been found that more than 70–90% of all diamonds in these xenoliths are located inside the xenolith, which was found out without destroying the sample. Acknowledgments This work has been supported by Siberian Branch of Russian Academy of Sciences (interdisciplinary integration Grant no. 7, young scientist Grant no. 137), Russian Foundation for Basic Research (Grant no. 05-05-64246), and The Foundation for Assistance to Small Innovative Enterprises, Project no. 4814. Fig. 6. 3D model of rock sample with diamonds distinguished in the volume.
References clinopyroxene in host eclogites. The association of diamonds with primary minerals together with some other in our studies of diamond inclusions and minerals in host eclogites suggests that most of the diamonds could have formed in the eclogite in conjunction with metasomatic input(s) of C–O–H–N–S fluids [5,6]. 4. Conclusion HRCXT is an effective tool for mapping xenoliths and for definition of relative positions of minerals, including
[1] L.A. Taylor, R.A. Keller, G.A. Snyder, W. Wang, W.D. Carlson, E.H. Hauri, T. McCandless, K.R. Kim, N.V. Sobolev, S.M. Bezborodov, Int. Geol. Rev. 42 (2000) 952. [2] R.A. Ketcham, J. Struct. Geol. 27 (2005) 1217. [3] E.A. Babichev, S.E. Baru, V.R. Groshev, A.G. Khabakhpashev, G.S. Krainov, V.V. Leonov, V.A. Neustroev, V.V. Porosev, G.A. Savinov, L.I. Shekhtman, Nucl. Instr. and Meth. A 461 (2001). [4] J. Sijbers, A. Postnov, Phys. Med. Biol. 49 (14) (2004). [5] V.S. Shatsky, D.A. Zedgenizov, A.L. Ragozin, S.I. Mityukhin, N.V. Sobolev, Dokl. Earth Sci. 402 (4) (2005) 587. [6] A.L. Ragozin, V.S. Shatsky, D.A. Zedgenizov, S.I. Mityukhin, Dokl. Earth Sci. 407A (3) (2006) 465.
Nuclear Instruments and Methods in Physics Research A 575 (2007) 255–258 www.elsevier.com/locate/nima
Three-dimensional distribution of minerals in diamondiferous eclogites, obtained by the method of high-resolution X-ray computed tomography K.E. Kupera,, D.A. Zedgenizovb, A.L. Ragozinb, V.S. Shatskyb, V.V. Poroseva, K.V. Zolotareva, E.A. Babicheva, S.A. Ivanova a
Budker Institute of Nuclear Physics of SB RAS, Lavrentiev prospect 11, Novosibirsk 630090, Russia b Institute of Geology and Mineralogy of SB RAS, Novosibirsk 630090, Russia Available online 16 January 2007
Abstract High-resolution X-ray computed tomography (HRXCT) is a technology ideally applicable to a wide range of geological investigations. It is an express non-destructive method to produce images corresponding to series of slice projections through a sample. In the present study, HRXCT was applied to rock samples with the use of synchrotron radiation from the VEPP-3 storage ring, at the ‘‘X-ray microscopy and tomography’’ station. The method was calibrated and preliminary measurements were carried out on the Low-Dose Digital Radiographic Device ‘‘Siberia’’. The data obtained have determined the internal structure of rock samples with a spatial resolution of 100 mm. HRXCT has been applied to the xenoliths of diamondiferous eclogites from the Udachnaya kimberlitic pipe located in Yakutia, Russia. It has allowed determination of the distribution of rock-forming (garnet and clinopyroxene) and accessory (diamond, rutile, and sulfide) minerals of different X-ray absorption. This is important to find out the genetic relationship of diamonds with associated minerals and the sequence of crystallization. r 2007 Elsevier B.V. All rights reserved. PACS: 41.50.+h; 07.85.Tt; 91.65. n Keywords: X-ray tomography; Petrography; Diamond; Eclogite
1. Introduction High-resolution X-ray computed tomography (HRXCT) gives a detailed three-dimensional (3D) imagery of the interiors of rocks, realized in a non-destructive manner. The essence of the method is scanning a sample with X-ray beams that are differently absorbed in its various areas. The attenuation of X-rays along the beam is an integrated characteristic of the density of the object to investigate. The degree of distinguishing different components of rock volume in HRXCT data depends on the difference in their linear attenuation coefficients (Fig. 1). The resulting data are reconstructed from the projections obtained during rotation of a sample as series two-dimennsional (2D) slices. Collecting a sequence of contiguous slices allows reconCorresponding author. Tel.: +7 3832 394 259.
E-mail address: [email protected] (K.E. Kuper). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.01.053
struction of a full 3D volume. The HRXCT method can be applied ideally to a wide range of geological investigations. Geological applications include interior examination of textural analysis of igneous and metamorphic rocks and other tasks demanding 3D data that previously required serial sectioning. Some HRXCT studies of the internal structure of geological samples were previously reported (e.g., [1,2]). These data revealed general relationships among rock components in the 3D space. It is important that this non-destructive technique maps and locates minerals within rocks, thereby enabling one to investigate the nature of their formation in their original setting. The rare specimens of diamondiferous eclogite xenoliths make them invaluable samples of the Earth’s mantle. As part of comprehensive study of diamondiferous xenoliths, we have conducted HRCXT studies of several xenoliths of diamondiferous eclogites from the Udachnaya kimberlitic pipe (Yakutia).
ARTICLE IN PRESS K.E. Kuper et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 255–258
256
Linear attenuation coefficient, cm-1
6 5 Garnet Mica Clinopyroxene Sulfide Diamond
4 3 2 1 0 50
60
70 80 X-ray energy, keV
90
100
Fig. 1. Linear X-ray attenuation coefficients for different minerals in a diamondiferous sample.
2. Experiment and setup Two subsystems were used to image geological specimens across a range of size classes. The high-resolution subsystem uses an X-ray source and linear array detectors to image samples of 50–80 mm in diameter with a slice thickness of 0.2 mm. The second subsystem uses synchrotron radiation (SR) from the VEPP-3 storage ring and 2D detector to image samples of 10–40 mm in diameter at slice thickness down to 100 mm. Both systems used polychromatic sources. The first system was mounted on the base of the LowDose Digital Radiographic Device (LDRD) ‘‘Siberia’’. We used an X-ray tube with a tungsten target with a focal spot size of 0.6 mm and a typical voltage of 110 kVp. An additional filter of 0.6 mm of Cu was installed on the output window of the X-ray tube. A slit collimator formed a thin fanlike beam. A detector measured the distribution of radiation intensity in the horizontal direction. It is possible to consider the detector as 1024 independent channels with a strip of 0.4 mm [3]. The detector was filled with Kr gas under a pressure of 40 bar. The vertical beam size was defined by the collimator, and the X-ray focal spot size equaled 0.2 mm in the object area. During scanning, the object was rotated with a constant angular speed and data were registered line by line every 5 ms. For the next slice projection, the sample was vertically scanned with a step of 200 mm. A layout of the scanning technique is presented in Fig. 2. An example of the data obtained (sinogram) is also shown in the figure. The second part of experiment was carried out at the station ‘‘X-ray microscopy and tomography’’. SR is a very strong source of radiation in the X-ray range that is four times brighter than a photon flux from an X-ray tube. It enables us to use a detector with higher spatial resolution than in the previous experiment. In these measurements, the size of samples did not exceed 40 mm, and the
resolution was about 100 mm. The 2D RadIcon image sensor with gadolinium oxysulfide scintillator is a 1024 1000 CMOS matrix with a 50 mm pixel. Fig. 3 demonstrates the installation for sample scanning and an example of projection of a diamondiferous rock on a logarithmic scale. Each scan consists of 360 projections with an angular step of 0.51 (from 01 to 1801). Small-angle SR deviation of about 0.2 mrad makes it possible to use an algorithm for a parallel beam geometry, which simplifies the process of 3D reconstruction of the object and significantly improves the quality of the image. Five millimeter copper filter was used to cut off soft components in the photon spectrum, (Fig. 4). In both cases, the effects caused by heterogeneities in the incident X-ray beams and non-uniformity in the response of the detectors were corrected using an appropriate calibration. Ring artifacts caused by nonlinear response of detector elements to a signal were corrected via removing vertical lines in the reconstructed image converted to polar coordinates according to the technique described in the article [4]. 3. Results and discussion The 3D visualization technique consists of volume rendering, in which each grayscale value in the data set is assigned a color and an opacity value. This allows some mineral phases to be rendered transparent and others partially or entirely opaque, providing unique opportunities for petrographic analysis in three dimensions. The simplest segmentation method for data is to define a grayscale threshold or range which is unique to the phase of interest. However, this approach is complicated by the finite resolution of the imagery. Because each data voxel (volume elements) can encompass a range of material, if more than one material is present in that voxel, the resulting grayscale will be some average of the phases present; this is known as the partial volume effect. Furthermore, some blurring is inevitable, causing the grayscale value within a voxel to be influenced by surrounding material. Thus, the material boundaries often extend across two voxel featuring a gradual grayscale transition between the corresponding values characterizing each phase. The threshold level for proper location of a boundary between two different materials is the mean of their exact values. For the diamondiferous xenoliths scan data, the threshold value for sulfide was 150, while garnet had a value about 100, clinopyroxene had a value of 80, and diamond had a value of 50. Because garnet is most frequently situated in contact with clinopyroxene, its appropriate threshold value is 90. Zones of secondary mineralization appear as dark thin lines on X-ray images presumably due to their porous nature, resulting in lower X-ray attenuation. In this manner, it is easy to distinguish diamonds, sulfides, silicates (garnet and clinopyroxene), and their alteration products in xenolith (Fig. 5). Several such 2D
ARTICLE IN PRESS K.E. Kuper et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 255–258
257
Fig. 2. The HRXCT installation mounted on the base of the LDRD ‘‘Siberia’’ (left) and an example of the data obtained on a logarithmic scale (right).
Fig. 3. The system for sample scanning with SR (left) and an example of projection of a diamondiferous rock on logarithmic scale (right).
Photons/sec./keV/cm2
3.5x108 3.0x108 2.5x108 2.0x108 1.5x108 1.0x108 5.0x107 0.0 50
60
70
80
90
100
X-ray energy, keV Fig. 4. Theoretical photon energy spectrum, after the copper filter 5 mm thick.
images are stacked together using volume–visualization software to produce a 3D model of the xenolith. This essentially represents a density map of the sample, from which one can extract the sizes, shapes, textures, and locations of individual crystals that have dimensions exceeding the spatial resolution of the scans.
3D models of the samples reveal clearly the spatial relationships between diamonds and their surroundings. This gives clues to the processes that control diamond crystallization. These relationships are determined by rotating and viewing the model in different perspectives to look for any mineral associations and alignments. It is possible to make some phases transparent and display only selected mineral phases at a time. For example, a 3D HRCXT model of diamondiferous eclogite 107 is shown in Fig. 6. For better clarity, the garnet and clinopyroxene grains are made invisible so that locations and sizes of diamonds could be seen easily. A series of macro-diamonds is clearly visible in the interior of this sample, which could not have been seen by any other technique. It seems that many of the diamonds in the eclogite are located linearly along a diagonal passing through the eclogite. The spatial resolution of this technique is close to 100 mm, which is essentially helpful in visualization of diamonds less than 1 mm in size. It is interesting to note that sometimes diamonds were observed to be in direct contact with or to be enclosed within a fresh garnet or clinopyroxene. Furthermore, sulfide minerals are not preferentially associated with diamonds. It may be concluded from these observations that some diamonds are syngenetic with the garnet and
ARTICLE IN PRESS K.E. Kuper et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 255–258
258
Fig. 5. Grayscale (left) and mapped version of the one slice (right) of a diamondiferous sample (Cpx is for clinopyroxene; Grt is for garnet).
diamonds. The LDRD-based system allows getting images of rather big samples with a special resolution of approximately 200 mm. The SR application gives a possibility to reach a 1% contrast resolution for minerals in a diamondiferous sample with a spatial resolution higher than 100 mm. The samples of eclogite xenoliths from Udachnaja in the present study are exceptionally rich in diamonds. It has been found that more than 70–90% of all diamonds in these xenoliths are located inside the xenolith, which was found out without destroying the sample. Acknowledgments This work has been supported by Siberian Branch of Russian Academy of Sciences (interdisciplinary integration Grant no. 7, young scientist Grant no. 137), Russian Foundation for Basic Research (Grant no. 05-05-64246), and The Foundation for Assistance to Small Innovative Enterprises, Project no. 4814. Fig. 6. 3D model of rock sample with diamonds distinguished in the volume.
References clinopyroxene in host eclogites. The association of diamonds with primary minerals together with some other in our studies of diamond inclusions and minerals in host eclogites suggests that most of the diamonds could have formed in the eclogite in conjunction with metasomatic input(s) of C–O–H–N–S fluids [5,6]. 4. Conclusion HRCXT is an effective tool for mapping xenoliths and for definition of relative positions of minerals, including
[1] L.A. Taylor, R.A. Keller, G.A. Snyder, W. Wang, W.D. Carlson, E.H. Hauri, T. McCandless, K.R. Kim, N.V. Sobolev, S.M. Bezborodov, Int. Geol. Rev. 42 (2000) 952. [2] R.A. Ketcham, J. Struct. Geol. 27 (2005) 1217. [3] E.A. Babichev, S.E. Baru, V.R. Groshev, A.G. Khabakhpashev, G.S. Krainov, V.V. Leonov, V.A. Neustroev, V.V. Porosev, G.A. Savinov, L.I. Shekhtman, Nucl. Instr. and Meth. A 461 (2001). [4] J. Sijbers, A. Postnov, Phys. Med. Biol. 49 (14) (2004). [5] V.S. Shatsky, D.A. Zedgenizov, A.L. Ragozin, S.I. Mityukhin, N.V. Sobolev, Dokl. Earth Sci. 402 (4) (2005) 587. [6] A.L. Ragozin, V.S. Shatsky, D.A. Zedgenizov, S.I. Mityukhin, Dokl. Earth Sci. 407A (3) (2006) 465.