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S P Kelley - One of the best experts on this subject based on the ideXlab platform.
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a Laser Probe 40ar 39ar investigation of poikilitic shergottite nwa 4797 implications for the timing of shock metamorphism
Geological Society London Special Publications, 2014Co-Authors: S P Kelley, Erin L Walton, C D K Herd, A J IrvingAbstract:Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of NW Africa (NWA) 4797. Shock heating of NWA 4797 completely melted and vesiculated precursor igneous plagioclase, which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition (Ca/K ratios). Using a focused ultraviolet Laser beam, is has been possible to distinguish between argon isotopic signatures from groundmass minerals (igneous olivine + pyroxene), plagioclase and a shock vein. This study focuses on the potential for this meteorite to shed light on shock ages of shergottites. Apparent 40 Ar/ 39 Ar ages of groundmass minerals show that there are large amounts of excess argon in this phase, yielding a wide range of calculated ages from 690 ± 30 Ma to several apparent ages older than 4.5 Ga. A traverse of Laser-Probe extractions across the 1 mm-diameter shock vein in NWA 4797 yielded apparent 40 Ar/ 39 Ar ages younger than the groundmass. A signature of the Martian atmosphere, identified by 40 Ar/ 36 Ar ratios of 1600-1900, was not found in the NWA 4797 shock vein. This is distinct from other shergottites where the products of shock melting contain a nearly pure sample of Martian atmosphere. We attribute this to a distinct formation mechanism, and hence gas-trapping mechanism, of the NWA 4797 shock vein. We undertook 44 analyses of plagioclase areas identified by SEM analysis. Ages ranged from 45 ± 27 to 3771 ± 109 Ma and yield an average age of 375 ± 77 Ma, considerably younger than ages obtained in this study from either the groundmass or the shock vein. A plot of age v. 37 Ar/ 39 Ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured. Older ages are correlated with higher Ca/K ratios of plagioclase, indicating contamination from groundmass minerals rich in excess argon. The youngest ages correlate to plagioclase extractions with the lowest Ca/K ratios, interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a K-rich mesostasis. We see no evidence for multiple shock events in NWA 4797. Rather, we favour the interpretation that the cosmic-ray exposure (CRE) age of 3.0 ± 0.5 Ma, obtained on NWA 4797 in this study using cosmogenic 38 Ar, approximates the timing of shock melting in this meteorite.
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shock implantation of martian atmospheric argon in four basaltic shergottites a Laser Probe 40ar 39ar investigation
Geochimica et Cosmochimica Acta, 2007Co-Authors: Erin L Walton, S P Kelley, John G SprayAbstract:Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of two Martian basalts (Los Angeles and Zagami) and two Martian olivine-phyric basalts (Dar al Gani (DaG) 476 and North West Africa (NWA) 1068). With a similar to 50 mu m diameter focused infrared Laser beam, it has been possible to distinguish between argon isotopic signatures from host rock (matrix) minerals and localized shock melt products (pockets and veins). The concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured J values, Ar-40/Ar-39 ratios and K2O wt% in each phase. Melt pockets contain, on average, 10 times more gas (7-24 ppb Ar-40) than shock veins and matrix minerals (0.3-3 ppb Ar-40). The Ar-40/Ar-36 ratio of the Martian atmosphere, estimated from melt pocket argon extractions corrected for cosmogenie Ar-36, is: Los Angeles (similar to 1852), Zagami (similar to 1744) and NWA 1068 (similar to 1403). In addition, Los Angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs: (1) Martian atmosphere in melt pockets, and (2) a trapped component, possibly Martian interior (Ar-40/Ar-16: 480-490) in matrix minerals. Average apparent Ar-40/Ar-39 ages determined for matrix minerals in the four analyzed meteorites are 1290 Ma (Los Angeles), 692 Ma (Zagami), 515 Ma (NWA 1068) and 1427 Ma (DaG 476). These Ar-40/Ar-39 apparent ages are substantially older than the similar to 170-0474 Ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped 40Ar. Cosmic ray exposure (CRE) ages were measured using spallogenic Ar-16 and Ar-38 production. Los Angeles (3.1 +/- 0.2 Ma), Zagami (2.9 +/- 0.4 Ma) and NWA 1068 (2.0 +/- 0.5 Ma) yielded ages within the range of previous determinations. DaG 476, however, yielded a young CRE age (0.7 +/- 0.25 Ma), attributed to terrestrial alteration. The high spatial variation of argon indicates that the incorporation of Martian atmospheric argon into near-surface rocks is controlled by localized glass-bearing melts produced by shock processes. In particular, the larger (mm-size) melt pockets contain near end-member Martian atmospheric argon. Based on petrography, composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection. Three processes may have led to atmosphere incorporation: (1) argon implantation due to atmospheric shock front collision with the Martian surface, (2) transformation of an atmosphere-filled cavity into a localized melt zone, and (3) shock implantation of atmosphere trapped in cracks, pores and fissures.
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an 40ar 39ar Laser Probe study of pseudotachylites in charnockite gneisses from the cauvery shear zone system south india
Gondwana Research, 2006Co-Authors: Y Bhaskar J Rao, S P Kelley, Nigel Harris, B L Narayana, C SrikantappaAbstract:The age of pseudotachylite formation in the crustal-scale Cauvery Shear Zone system of the Precambrian Southern Granulite Terrain (South India) has been analyzed by Laser-Probe Ar-40-Ar-39 dating. Laser spot analyses from a pseudotachylite from the Salem-Attur shear zone have yielded ages ranging from 1214 to 904 Ma. Some evidence for the presence of excess Ar is indicated by the scatter of ages from this locality. The host gneiss preserves Palaeoproterozoic Rb-Sr whole rock-biotite ages (2350 +/- 11 to 2241 +/- 11 Ma). A mylonite in the Koorg shear, ca. 200 km to the north, yielded an age of 895 17 Ma the consistency of the age distribution from spot analyses precludes the presence of significant excess Ar. Despite published evidence for the growth of high-grade minerals within some components of the Cauvery Shear Zone during the Pan-African event (700-550 Ma), the pseudotachylites in this study provide no evidence for Pan-African formation. Instead they document the first evidence for Mesoproterozoic tectonism in the Cauvery Shear Zone system, thus prompting a review of the correlation between the Cauvery Shear Zone system and the large-scale shear zones located elsewhere in eastern Gondwana. (c) 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
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Laser Probe 40ar 39ar investigation of a pseudotachylyte and its host rock from the outer isles thrust scotland
Geology, 1994Co-Authors: S P Kelley, Steven M Reddy, R MaddockAbstract:The formation of a friction melt or pseudotachylyte in the footwall of the Outer Isles thrust, Scotland, during thrust movement in the early Paleozoic Caledonian orogeny caused argon loss from biotites in the host gneiss. Mean 40 Ar/ 39 Ar ages for biotite grains decrease from 1450 to 923 Ma in a zone 730°C in the host gneiss; these high temperatures may have caused catastrophic argon loss rather than loss by volume diffusion. An 40 Ar/ 39 Ar age traverse across the pseudotachylyte vein revealed old ages adjacent to the margin, reflecting the incorporation of partially outgassed host-rock clasts. Apparent ages for the pseudotachylyte decrease unevenly toward the center of the vein. A weighted mean age of 430 ±6 Ma (2σ) obtained in the center corresponds closely to movement ages derived for the associated Moine thrust zone.
Erin L Walton - One of the best experts on this subject based on the ideXlab platform.
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A Laser Probe 40Ar/39Ar investigation of poikilitic shergottite NWA 4797: implications for the timing of shock metamorphism
2016Co-Authors: Erin L Walton, C D K Herd, Simon Kelley, A J IrvingAbstract:Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of NW Africa (NWA) 4797. Shock heating of NWA 4797 completely melted and vesiculated precursor igneous plagioclase, which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition (Ca/K ratios). Using a focused ultraviolet Laser beam, is has been possible to distinguish between argon isotopic signatures from groundmass minerals (igneous olivine + pyroxene), plagioclase and a shock vein. This study focuses on the potential for this meteorite to shed light on shock ages of shergottites. Apparent 40Ar/39Ar ages of groundmass minerals show that there are large amounts of excess argon in this phase, yielding a wide range of calculated ages from 690 ± 30 Ma to several apparent ages older than 4.5 Ga. A traverse of Laser-Probe extractions across the 1 mm-diameter shock vein in NWA 4797 yielded apparent 40Ar/39Ar ages younger than the groundmass. A signature of the Martian atmosphere, identified by 40Ar/36Ar ratios of 1600–1900, was not found in the NWA 4797 shock vein. This is distinct from other shergottites where the products of shock melting contain a nearly pure sample of Martian atmosphere. We attribute this to a distinct formation mechanism, and hence gas-trapping mechanism, of the NWA 4797 shock vein. We undertook 44 analyses of plagioclase areas identified by SEM analysis. Ages ranged from 45 ± 27 to 3771 ± 109 Ma and yield an average age of 375 ± 77 Ma, considerably younger than ages obtained in this study from either the groundmass or the shock vein. A plot of age v. 37Ar/39Ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured. Older ages are correlated with higher Ca/K ratios of plagioclase, indicating contamination from groundmass minerals rich in excess argon. The youngest ages correlate to plagioclase extractions with the lowest Ca/K ratios, interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a K-rich mesostasis. We see no evidence for multiple shock events in NWA 4797. Rather, we favour the interpretation that the cosmic-ray exposure (CRE) age of 3.0±0.5 Ma, obtained on NWA 4797 in this study using cosmogenic 38Ar, approximates the timing of shock melting in this meteorite.
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a Laser Probe 40ar 39ar investigation of poikilitic shergottite nwa 4797 implications for the timing of shock metamorphism
Geological Society London Special Publications, 2014Co-Authors: S P Kelley, Erin L Walton, C D K Herd, A J IrvingAbstract:Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of NW Africa (NWA) 4797. Shock heating of NWA 4797 completely melted and vesiculated precursor igneous plagioclase, which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition (Ca/K ratios). Using a focused ultraviolet Laser beam, is has been possible to distinguish between argon isotopic signatures from groundmass minerals (igneous olivine + pyroxene), plagioclase and a shock vein. This study focuses on the potential for this meteorite to shed light on shock ages of shergottites. Apparent 40 Ar/ 39 Ar ages of groundmass minerals show that there are large amounts of excess argon in this phase, yielding a wide range of calculated ages from 690 ± 30 Ma to several apparent ages older than 4.5 Ga. A traverse of Laser-Probe extractions across the 1 mm-diameter shock vein in NWA 4797 yielded apparent 40 Ar/ 39 Ar ages younger than the groundmass. A signature of the Martian atmosphere, identified by 40 Ar/ 36 Ar ratios of 1600-1900, was not found in the NWA 4797 shock vein. This is distinct from other shergottites where the products of shock melting contain a nearly pure sample of Martian atmosphere. We attribute this to a distinct formation mechanism, and hence gas-trapping mechanism, of the NWA 4797 shock vein. We undertook 44 analyses of plagioclase areas identified by SEM analysis. Ages ranged from 45 ± 27 to 3771 ± 109 Ma and yield an average age of 375 ± 77 Ma, considerably younger than ages obtained in this study from either the groundmass or the shock vein. A plot of age v. 37 Ar/ 39 Ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured. Older ages are correlated with higher Ca/K ratios of plagioclase, indicating contamination from groundmass minerals rich in excess argon. The youngest ages correlate to plagioclase extractions with the lowest Ca/K ratios, interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a K-rich mesostasis. We see no evidence for multiple shock events in NWA 4797. Rather, we favour the interpretation that the cosmic-ray exposure (CRE) age of 3.0 ± 0.5 Ma, obtained on NWA 4797 in this study using cosmogenic 38 Ar, approximates the timing of shock melting in this meteorite.
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shock implantation of martian atmospheric argon in four basaltic shergottites a Laser Probe 40ar 39ar investigation
Geochimica et Cosmochimica Acta, 2007Co-Authors: Erin L Walton, S P Kelley, John G SprayAbstract:Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of two Martian basalts (Los Angeles and Zagami) and two Martian olivine-phyric basalts (Dar al Gani (DaG) 476 and North West Africa (NWA) 1068). With a similar to 50 mu m diameter focused infrared Laser beam, it has been possible to distinguish between argon isotopic signatures from host rock (matrix) minerals and localized shock melt products (pockets and veins). The concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured J values, Ar-40/Ar-39 ratios and K2O wt% in each phase. Melt pockets contain, on average, 10 times more gas (7-24 ppb Ar-40) than shock veins and matrix minerals (0.3-3 ppb Ar-40). The Ar-40/Ar-36 ratio of the Martian atmosphere, estimated from melt pocket argon extractions corrected for cosmogenie Ar-36, is: Los Angeles (similar to 1852), Zagami (similar to 1744) and NWA 1068 (similar to 1403). In addition, Los Angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs: (1) Martian atmosphere in melt pockets, and (2) a trapped component, possibly Martian interior (Ar-40/Ar-16: 480-490) in matrix minerals. Average apparent Ar-40/Ar-39 ages determined for matrix minerals in the four analyzed meteorites are 1290 Ma (Los Angeles), 692 Ma (Zagami), 515 Ma (NWA 1068) and 1427 Ma (DaG 476). These Ar-40/Ar-39 apparent ages are substantially older than the similar to 170-0474 Ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped 40Ar. Cosmic ray exposure (CRE) ages were measured using spallogenic Ar-16 and Ar-38 production. Los Angeles (3.1 +/- 0.2 Ma), Zagami (2.9 +/- 0.4 Ma) and NWA 1068 (2.0 +/- 0.5 Ma) yielded ages within the range of previous determinations. DaG 476, however, yielded a young CRE age (0.7 +/- 0.25 Ma), attributed to terrestrial alteration. The high spatial variation of argon indicates that the incorporation of Martian atmospheric argon into near-surface rocks is controlled by localized glass-bearing melts produced by shock processes. In particular, the larger (mm-size) melt pockets contain near end-member Martian atmospheric argon. Based on petrography, composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection. Three processes may have led to atmosphere incorporation: (1) argon implantation due to atmospheric shock front collision with the Martian surface, (2) transformation of an atmosphere-filled cavity into a localized melt zone, and (3) shock implantation of atmosphere trapped in cracks, pores and fissures.
C Srikantappa - One of the best experts on this subject based on the ideXlab platform.
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an 40ar 39ar Laser Probe study of pseudotachylites in charnockite gneisses from the cauvery shear zone system south india
Gondwana Research, 2006Co-Authors: Y Bhaskar J Rao, S P Kelley, Nigel Harris, B L Narayana, C SrikantappaAbstract:The age of pseudotachylite formation in the crustal-scale Cauvery Shear Zone system of the Precambrian Southern Granulite Terrain (South India) has been analyzed by Laser-Probe Ar-40-Ar-39 dating. Laser spot analyses from a pseudotachylite from the Salem-Attur shear zone have yielded ages ranging from 1214 to 904 Ma. Some evidence for the presence of excess Ar is indicated by the scatter of ages from this locality. The host gneiss preserves Palaeoproterozoic Rb-Sr whole rock-biotite ages (2350 +/- 11 to 2241 +/- 11 Ma). A mylonite in the Koorg shear, ca. 200 km to the north, yielded an age of 895 17 Ma the consistency of the age distribution from spot analyses precludes the presence of significant excess Ar. Despite published evidence for the growth of high-grade minerals within some components of the Cauvery Shear Zone during the Pan-African event (700-550 Ma), the pseudotachylites in this study provide no evidence for Pan-African formation. Instead they document the first evidence for Mesoproterozoic tectonism in the Cauvery Shear Zone system, thus prompting a review of the correlation between the Cauvery Shear Zone system and the large-scale shear zones located elsewhere in eastern Gondwana. (c) 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
A J Irving - One of the best experts on this subject based on the ideXlab platform.
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A Laser Probe 40Ar/39Ar investigation of poikilitic shergottite NWA 4797: implications for the timing of shock metamorphism
2016Co-Authors: Erin L Walton, C D K Herd, Simon Kelley, A J IrvingAbstract:Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of NW Africa (NWA) 4797. Shock heating of NWA 4797 completely melted and vesiculated precursor igneous plagioclase, which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition (Ca/K ratios). Using a focused ultraviolet Laser beam, is has been possible to distinguish between argon isotopic signatures from groundmass minerals (igneous olivine + pyroxene), plagioclase and a shock vein. This study focuses on the potential for this meteorite to shed light on shock ages of shergottites. Apparent 40Ar/39Ar ages of groundmass minerals show that there are large amounts of excess argon in this phase, yielding a wide range of calculated ages from 690 ± 30 Ma to several apparent ages older than 4.5 Ga. A traverse of Laser-Probe extractions across the 1 mm-diameter shock vein in NWA 4797 yielded apparent 40Ar/39Ar ages younger than the groundmass. A signature of the Martian atmosphere, identified by 40Ar/36Ar ratios of 1600–1900, was not found in the NWA 4797 shock vein. This is distinct from other shergottites where the products of shock melting contain a nearly pure sample of Martian atmosphere. We attribute this to a distinct formation mechanism, and hence gas-trapping mechanism, of the NWA 4797 shock vein. We undertook 44 analyses of plagioclase areas identified by SEM analysis. Ages ranged from 45 ± 27 to 3771 ± 109 Ma and yield an average age of 375 ± 77 Ma, considerably younger than ages obtained in this study from either the groundmass or the shock vein. A plot of age v. 37Ar/39Ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured. Older ages are correlated with higher Ca/K ratios of plagioclase, indicating contamination from groundmass minerals rich in excess argon. The youngest ages correlate to plagioclase extractions with the lowest Ca/K ratios, interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a K-rich mesostasis. We see no evidence for multiple shock events in NWA 4797. Rather, we favour the interpretation that the cosmic-ray exposure (CRE) age of 3.0±0.5 Ma, obtained on NWA 4797 in this study using cosmogenic 38Ar, approximates the timing of shock melting in this meteorite.
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a Laser Probe 40ar 39ar investigation of poikilitic shergottite nwa 4797 implications for the timing of shock metamorphism
Geological Society London Special Publications, 2014Co-Authors: S P Kelley, Erin L Walton, C D K Herd, A J IrvingAbstract:Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of NW Africa (NWA) 4797. Shock heating of NWA 4797 completely melted and vesiculated precursor igneous plagioclase, which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition (Ca/K ratios). Using a focused ultraviolet Laser beam, is has been possible to distinguish between argon isotopic signatures from groundmass minerals (igneous olivine + pyroxene), plagioclase and a shock vein. This study focuses on the potential for this meteorite to shed light on shock ages of shergottites. Apparent 40 Ar/ 39 Ar ages of groundmass minerals show that there are large amounts of excess argon in this phase, yielding a wide range of calculated ages from 690 ± 30 Ma to several apparent ages older than 4.5 Ga. A traverse of Laser-Probe extractions across the 1 mm-diameter shock vein in NWA 4797 yielded apparent 40 Ar/ 39 Ar ages younger than the groundmass. A signature of the Martian atmosphere, identified by 40 Ar/ 36 Ar ratios of 1600-1900, was not found in the NWA 4797 shock vein. This is distinct from other shergottites where the products of shock melting contain a nearly pure sample of Martian atmosphere. We attribute this to a distinct formation mechanism, and hence gas-trapping mechanism, of the NWA 4797 shock vein. We undertook 44 analyses of plagioclase areas identified by SEM analysis. Ages ranged from 45 ± 27 to 3771 ± 109 Ma and yield an average age of 375 ± 77 Ma, considerably younger than ages obtained in this study from either the groundmass or the shock vein. A plot of age v. 37 Ar/ 39 Ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured. Older ages are correlated with higher Ca/K ratios of plagioclase, indicating contamination from groundmass minerals rich in excess argon. The youngest ages correlate to plagioclase extractions with the lowest Ca/K ratios, interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a K-rich mesostasis. We see no evidence for multiple shock events in NWA 4797. Rather, we favour the interpretation that the cosmic-ray exposure (CRE) age of 3.0 ± 0.5 Ma, obtained on NWA 4797 in this study using cosmogenic 38 Ar, approximates the timing of shock melting in this meteorite.
Y Bhaskar J Rao - One of the best experts on this subject based on the ideXlab platform.
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an 40ar 39ar Laser Probe study of pseudotachylites in charnockite gneisses from the cauvery shear zone system south india
Gondwana Research, 2006Co-Authors: Y Bhaskar J Rao, S P Kelley, Nigel Harris, B L Narayana, C SrikantappaAbstract:The age of pseudotachylite formation in the crustal-scale Cauvery Shear Zone system of the Precambrian Southern Granulite Terrain (South India) has been analyzed by Laser-Probe Ar-40-Ar-39 dating. Laser spot analyses from a pseudotachylite from the Salem-Attur shear zone have yielded ages ranging from 1214 to 904 Ma. Some evidence for the presence of excess Ar is indicated by the scatter of ages from this locality. The host gneiss preserves Palaeoproterozoic Rb-Sr whole rock-biotite ages (2350 +/- 11 to 2241 +/- 11 Ma). A mylonite in the Koorg shear, ca. 200 km to the north, yielded an age of 895 17 Ma the consistency of the age distribution from spot analyses precludes the presence of significant excess Ar. Despite published evidence for the growth of high-grade minerals within some components of the Cauvery Shear Zone during the Pan-African event (700-550 Ma), the pseudotachylites in this study provide no evidence for Pan-African formation. Instead they document the first evidence for Mesoproterozoic tectonism in the Cauvery Shear Zone system, thus prompting a review of the correlation between the Cauvery Shear Zone system and the large-scale shear zones located elsewhere in eastern Gondwana. (c) 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
