Time-resolved, defect-hosted, trace element mobility in deformed Witwatersrand pyrite
The Pb isotopic composition of rocks is widely used to constrain the sources and mobility of melts and hydrothermal fluids in the Earth's crust. In many cases, the Pb isotopic composition appears to represent mixing of multiple Pb reservoirs. However, the nature, scale and mechanisms responsibl...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
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Elsevier
2019-01-01
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| Series: | Geoscience Frontiers |
| Online Access: | http://www.sciencedirect.com/science/article/pii/S1674987118300896 |
| _version_ | 1797713913131827200 |
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| author | Denis Fougerouse Steven M. Reddy Christopher L. Kirkland David W. Saxey William D. Rickard Robert M. Hough |
| author_facet | Denis Fougerouse Steven M. Reddy Christopher L. Kirkland David W. Saxey William D. Rickard Robert M. Hough |
| author_sort | Denis Fougerouse |
| collection | DOAJ |
| description | The Pb isotopic composition of rocks is widely used to constrain the sources and mobility of melts and hydrothermal fluids in the Earth's crust. In many cases, the Pb isotopic composition appears to represent mixing of multiple Pb reservoirs. However, the nature, scale and mechanisms responsible for isotopic mixing are not well known. Additionally, the trace element composition of sulphide minerals are routinely used in ore deposit research, mineral exploration and environmental studies, though little is known about element mobility in sulphides during metamorphism and deformation. To investigate the mechanisms of trace element mobility in a deformed Witwatersrand pyrite (FeS2), we have combined electron backscatter diffraction (EBSD) and atom probe microscopy (APM). The results indicate that the pyrite microstructural features record widely different Pb isotopic compositions, covering the entire range of previously published sulphide Pb compositions from the Witwatersrand basin. We show that entangled dislocations record enhanced Pb, Sb, Ni, Tl and Cu composition likely due to entrapment and short-circuit diffusion in dislocation cores. These dislocations preserve the Pb isotopic composition of the pyrite at the time of growth (∼3 Ga) and show that dislocation intersections, likely to be common in deforming minerals, limit trace element mobility. In contrast, Pb, As, Ni, Co, Sb and Bi decorate a high-angle grain boundary which formed soon after crystallisation by sub-grain rotation recrystallization. Pb isotopic composition within this boundary indicates the addition of externally-derived Pb and trace elements during greenschist metamorphism at ∼2 Ga. Our results show that discrete Pb reservoirs are nanometric in scale, and illustrate that grain boundaries may remain open systems for trace element mobility over 1 billion years after their formation. Keywords: Atom probe microscopy, Nanoscale, Nanogeochronology, Microstructure, Isotope geochemistry, Common Pb |
| first_indexed | 2024-03-12T07:43:54Z |
| format | Article |
| id | doaj.art-bc9cf9cdd8a64d94a5250eff8a774e67 |
| institution | Directory Open Access Journal |
| issn | 1674-9871 |
| language | English |
| last_indexed | 2024-03-12T07:43:54Z |
| publishDate | 2019-01-01 |
| publisher | Elsevier |
| record_format | Article |
| series | Geoscience Frontiers |
| spelling | doaj.art-bc9cf9cdd8a64d94a5250eff8a774e672023-09-02T21:07:23ZengElsevierGeoscience Frontiers1674-98712019-01-011015563Time-resolved, defect-hosted, trace element mobility in deformed Witwatersrand pyriteDenis Fougerouse0Steven M. Reddy1Christopher L. Kirkland2David W. Saxey3William D. Rickard4Robert M. Hough5School of Earth and Planetary Sciences, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA, 6845, Australia; Geoscience Atom Probe, Advanced Resource Characterisation Facility, John de Laeter Centre, Curtin University, GPO Box U1987, Perth, WA, 6845, Australia; Corresponding author. School of Earth and Planetary Sciences, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA, 6845, Australia.School of Earth and Planetary Sciences, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA, 6845, Australia; Geoscience Atom Probe, Advanced Resource Characterisation Facility, John de Laeter Centre, Curtin University, GPO Box U1987, Perth, WA, 6845, AustraliaSchool of Earth and Planetary Sciences, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA, 6845, AustraliaGeoscience Atom Probe, Advanced Resource Characterisation Facility, John de Laeter Centre, Curtin University, GPO Box U1987, Perth, WA, 6845, AustraliaGeoscience Atom Probe, Advanced Resource Characterisation Facility, John de Laeter Centre, Curtin University, GPO Box U1987, Perth, WA, 6845, AustraliaCSIRO Minerals Resources, Kensington, WA, AustraliaThe Pb isotopic composition of rocks is widely used to constrain the sources and mobility of melts and hydrothermal fluids in the Earth's crust. In many cases, the Pb isotopic composition appears to represent mixing of multiple Pb reservoirs. However, the nature, scale and mechanisms responsible for isotopic mixing are not well known. Additionally, the trace element composition of sulphide minerals are routinely used in ore deposit research, mineral exploration and environmental studies, though little is known about element mobility in sulphides during metamorphism and deformation. To investigate the mechanisms of trace element mobility in a deformed Witwatersrand pyrite (FeS2), we have combined electron backscatter diffraction (EBSD) and atom probe microscopy (APM). The results indicate that the pyrite microstructural features record widely different Pb isotopic compositions, covering the entire range of previously published sulphide Pb compositions from the Witwatersrand basin. We show that entangled dislocations record enhanced Pb, Sb, Ni, Tl and Cu composition likely due to entrapment and short-circuit diffusion in dislocation cores. These dislocations preserve the Pb isotopic composition of the pyrite at the time of growth (∼3 Ga) and show that dislocation intersections, likely to be common in deforming minerals, limit trace element mobility. In contrast, Pb, As, Ni, Co, Sb and Bi decorate a high-angle grain boundary which formed soon after crystallisation by sub-grain rotation recrystallization. Pb isotopic composition within this boundary indicates the addition of externally-derived Pb and trace elements during greenschist metamorphism at ∼2 Ga. Our results show that discrete Pb reservoirs are nanometric in scale, and illustrate that grain boundaries may remain open systems for trace element mobility over 1 billion years after their formation. Keywords: Atom probe microscopy, Nanoscale, Nanogeochronology, Microstructure, Isotope geochemistry, Common Pbhttp://www.sciencedirect.com/science/article/pii/S1674987118300896 |
| spellingShingle | Denis Fougerouse Steven M. Reddy Christopher L. Kirkland David W. Saxey William D. Rickard Robert M. Hough Time-resolved, defect-hosted, trace element mobility in deformed Witwatersrand pyrite Geoscience Frontiers |
| title | Time-resolved, defect-hosted, trace element mobility in deformed Witwatersrand pyrite |
| title_full | Time-resolved, defect-hosted, trace element mobility in deformed Witwatersrand pyrite |
| title_fullStr | Time-resolved, defect-hosted, trace element mobility in deformed Witwatersrand pyrite |
| title_full_unstemmed | Time-resolved, defect-hosted, trace element mobility in deformed Witwatersrand pyrite |
| title_short | Time-resolved, defect-hosted, trace element mobility in deformed Witwatersrand pyrite |
| title_sort | time resolved defect hosted trace element mobility in deformed witwatersrand pyrite |
| url | http://www.sciencedirect.com/science/article/pii/S1674987118300896 |
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