Ilmenite and magnetite microfabrics in shocked gneisses from the Vredefort impact structure, South Africa
Abstract We investigated microfabrics of shocked Archean gneisses from two, 10 m-deep drill cores located near the center of the Vredefort impact structure in an area that is characterized by a prominent, long-wavelength negative magnetic anomaly (< − 3000 nT) together with short-w...
| Main Authors: | , , , , , |
|---|---|
| Other Authors: | |
| Format: | Article |
| Language: | English |
| Published: |
Springer Berlin Heidelberg
2022
|
| Online Access: | https://hdl.handle.net/1721.1/145354 |
| _version_ | 1826191338584932352 |
|---|---|
| author | Dellefant, Fabian Trepmann, Claudia A. Gilder, Stuart A. Sleptsova, Iuliia V. Kaliwoda, Melanie Weiss, Benjamin P. |
| author2 | Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences |
| author_facet | Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences Dellefant, Fabian Trepmann, Claudia A. Gilder, Stuart A. Sleptsova, Iuliia V. Kaliwoda, Melanie Weiss, Benjamin P. |
| author_sort | Dellefant, Fabian |
| collection | MIT |
| description | Abstract
We investigated microfabrics of shocked Archean gneisses from two, 10 m-deep drill cores located near the center of the Vredefort impact structure in an area that is characterized by a prominent, long-wavelength negative magnetic anomaly (< − 3000 nT) together with short-wavelength, high-amplitude anomalies attributed to lightning strikes. Planar fractures and feather features in quartz, which can be partially recrystallized, indicate shock conditions less than 20 GPa. Micrometer-sized magnetite and ilmenite along shock-related shear fractures in quartz and feldspar emanate from adjacent deformed coarse (> 100 µm) ilmenite and magnetite host grains. These fine-scaled veins suggest mobilization of magnetite and ilmenite during shear deformation of host Fe-phases and adjacent silicates, probably associated with frictional heating. Coarse ilmenite has fine-lamellar mechanical twins parallel to {10
$$\overline{1}$$
1
¯
1} and single (0001) twins, indicative of dislocation-glide-controlled deformation under non-isostatic stresses related to shock. A few µm-wide magnetite lamellae parallel to {10
$$\overline{1}$$
1
¯
1} and spheroidal magnetite (diameter ≈10 µm) within coarse ilmenite document exsolution after shock. Dauphiné twins associated with planar features in quartz imply cooling from 650 to 725 °C after shock, which accords with estimates of pre-impact basement temperatures from petrographic studies. The Curie temperature of magnetite is 580 °C; therefore, the central negative magnetic anomaly was produced as a thermoremanent magnetization acquired during cooling of the initially hot crust. The long-wavelength anomaly was likely amplified by the newly created magnetite that also acquired a thermal remanence. Although the magnetic properties of surface samples are often influenced by lightning strikes, we found no microstructural evidence for lightning-related processes. |
| first_indexed | 2024-09-23T08:54:22Z |
| format | Article |
| id | mit-1721.1/145354 |
| institution | Massachusetts Institute of Technology |
| language | English |
| last_indexed | 2024-09-23T08:54:22Z |
| publishDate | 2022 |
| publisher | Springer Berlin Heidelberg |
| record_format | dspace |
| spelling | mit-1721.1/1453542023-04-14T17:56:09Z Ilmenite and magnetite microfabrics in shocked gneisses from the Vredefort impact structure, South Africa Dellefant, Fabian Trepmann, Claudia A. Gilder, Stuart A. Sleptsova, Iuliia V. Kaliwoda, Melanie Weiss, Benjamin P. Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences Abstract We investigated microfabrics of shocked Archean gneisses from two, 10 m-deep drill cores located near the center of the Vredefort impact structure in an area that is characterized by a prominent, long-wavelength negative magnetic anomaly (< − 3000 nT) together with short-wavelength, high-amplitude anomalies attributed to lightning strikes. Planar fractures and feather features in quartz, which can be partially recrystallized, indicate shock conditions less than 20 GPa. Micrometer-sized magnetite and ilmenite along shock-related shear fractures in quartz and feldspar emanate from adjacent deformed coarse (> 100 µm) ilmenite and magnetite host grains. These fine-scaled veins suggest mobilization of magnetite and ilmenite during shear deformation of host Fe-phases and adjacent silicates, probably associated with frictional heating. Coarse ilmenite has fine-lamellar mechanical twins parallel to {10 $$\overline{1}$$ 1 ¯ 1} and single (0001) twins, indicative of dislocation-glide-controlled deformation under non-isostatic stresses related to shock. A few µm-wide magnetite lamellae parallel to {10 $$\overline{1}$$ 1 ¯ 1} and spheroidal magnetite (diameter ≈10 µm) within coarse ilmenite document exsolution after shock. Dauphiné twins associated with planar features in quartz imply cooling from 650 to 725 °C after shock, which accords with estimates of pre-impact basement temperatures from petrographic studies. The Curie temperature of magnetite is 580 °C; therefore, the central negative magnetic anomaly was produced as a thermoremanent magnetization acquired during cooling of the initially hot crust. The long-wavelength anomaly was likely amplified by the newly created magnetite that also acquired a thermal remanence. Although the magnetic properties of surface samples are often influenced by lightning strikes, we found no microstructural evidence for lightning-related processes. 2022-09-12T13:23:51Z 2022-09-12T13:23:51Z 2022-09-05 2022-09-11T03:11:54Z Article http://purl.org/eprint/type/JournalArticle https://hdl.handle.net/1721.1/145354 Contributions to Mineralogy and Petrology. 2022 Sep 05;177(9):88 PUBLISHER_CC en https://doi.org/10.1007/s00410-022-01950-5 Creative Commons Attribution https://creativecommons.org/licenses/by/4.0/ The Author(s) application/pdf Springer Berlin Heidelberg Springer Berlin Heidelberg |
| spellingShingle | Dellefant, Fabian Trepmann, Claudia A. Gilder, Stuart A. Sleptsova, Iuliia V. Kaliwoda, Melanie Weiss, Benjamin P. Ilmenite and magnetite microfabrics in shocked gneisses from the Vredefort impact structure, South Africa |
| title | Ilmenite and magnetite microfabrics in shocked gneisses from the Vredefort impact structure, South Africa |
| title_full | Ilmenite and magnetite microfabrics in shocked gneisses from the Vredefort impact structure, South Africa |
| title_fullStr | Ilmenite and magnetite microfabrics in shocked gneisses from the Vredefort impact structure, South Africa |
| title_full_unstemmed | Ilmenite and magnetite microfabrics in shocked gneisses from the Vredefort impact structure, South Africa |
| title_short | Ilmenite and magnetite microfabrics in shocked gneisses from the Vredefort impact structure, South Africa |
| title_sort | ilmenite and magnetite microfabrics in shocked gneisses from the vredefort impact structure south africa |
| url | https://hdl.handle.net/1721.1/145354 |
| work_keys_str_mv | AT dellefantfabian ilmeniteandmagnetitemicrofabricsinshockedgneissesfromthevredefortimpactstructuresouthafrica AT trepmannclaudiaa ilmeniteandmagnetitemicrofabricsinshockedgneissesfromthevredefortimpactstructuresouthafrica AT gilderstuarta ilmeniteandmagnetitemicrofabricsinshockedgneissesfromthevredefortimpactstructuresouthafrica AT sleptsovaiuliiav ilmeniteandmagnetitemicrofabricsinshockedgneissesfromthevredefortimpactstructuresouthafrica AT kaliwodamelanie ilmeniteandmagnetitemicrofabricsinshockedgneissesfromthevredefortimpactstructuresouthafrica AT weissbenjaminp ilmeniteandmagnetitemicrofabricsinshockedgneissesfromthevredefortimpactstructuresouthafrica |