{"title":"Northwest africa 16273: 4Vesta's mantle sampled during the giant impact","authors":"Ratiba Kared , Bertrand N. Moine , Jean-Luc Devidal , Abdelmadjid Seddiki","doi":"10.1016/j.jafrearsci.2025.105604","DOIUrl":null,"url":null,"abstract":"<div><div>We report here the results of a comprehensive petrological and geochemical study of Northwest Africa (NWA) 16273. NWA 16273 is classified as a mesosiderite-B2/3 and contains a significant amount of metal and silicates phases. It is composed of pyroxene, olivine, plagioclase, iron-metal + sulfide phases, and minor amounts of phosphate, chromite, phosphide, rutile, ilmenite and quartz. Olivines appear as xenocrysts surrounded by reaction rims composed of pyroxene and chromite. Some pyroxenes occur as xenocrysts and as small grains in the matrix. Pyroxene shows significant variation in composition: Low-Ca pyroxene (Fs<sub>18-36</sub> Wo<sub>3-4</sub>; n = 22) and pigeonite Fs<sub>22-26</sub> Wo<sub>12-20</sub> (n = 2). The Fe/Mn ratios of all pyroxenes show a large variation and have molar Fe/Mn rations of 27.6 ± 3.1 (n = 24). Whereas, olivines have molar Fe/Mn ratios of 45.5 ± 4.2 (n = 20). These ratios are compatible with 4Vesta compositions. Plagioclase are bytownites and anorthite (An<sub>87-92</sub> Or<sub>0.2-0.6</sub>). Additionally, Olivine displays a bimodal distribution with two populations: olivine-Mg-rich (Fo<sub>72-81</sub> Fa<sub>19-28</sub>) and olivine less magnesian (Fo<sub>60-67</sub> Fa<sub>33-40</sub>). The combination of an harzburgitic composition and a high metal content in the NWA 16273 suggests a complex origin involving several parent bodies, which may have undergone a collision, melting and ejection to form the mesosiderite. Textural, mineral and geochemical characteristics indicate that a metallic impactor melted an eucritic crust of 4Vesta. The eucritic magma and molten metal were injected into the harzburite mantle prior to ejection. Nickel phosphides oxidized on contact with silicate magma to form secondary phosphates. Olivine underwent a more or less intense peritectic reaction to produce secondary orthoproxenes and chromites. The orthopyroxene xenocrysts are zoned and partially chemically re-equilibrated. These features show that a fraction of 4Vesta mantle was sampled during the giant impact.</div></div>","PeriodicalId":14874,"journal":{"name":"Journal of African Earth Sciences","volume":"227 ","pages":"Article 105604"},"PeriodicalIF":2.2000,"publicationDate":"2025-03-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of African Earth Sciences","FirstCategoryId":"89","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S1464343X25000718","RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"GEOSCIENCES, MULTIDISCIPLINARY","Score":null,"Total":0}
引用次数: 0
Abstract
We report here the results of a comprehensive petrological and geochemical study of Northwest Africa (NWA) 16273. NWA 16273 is classified as a mesosiderite-B2/3 and contains a significant amount of metal and silicates phases. It is composed of pyroxene, olivine, plagioclase, iron-metal + sulfide phases, and minor amounts of phosphate, chromite, phosphide, rutile, ilmenite and quartz. Olivines appear as xenocrysts surrounded by reaction rims composed of pyroxene and chromite. Some pyroxenes occur as xenocrysts and as small grains in the matrix. Pyroxene shows significant variation in composition: Low-Ca pyroxene (Fs18-36 Wo3-4; n = 22) and pigeonite Fs22-26 Wo12-20 (n = 2). The Fe/Mn ratios of all pyroxenes show a large variation and have molar Fe/Mn rations of 27.6 ± 3.1 (n = 24). Whereas, olivines have molar Fe/Mn ratios of 45.5 ± 4.2 (n = 20). These ratios are compatible with 4Vesta compositions. Plagioclase are bytownites and anorthite (An87-92 Or0.2-0.6). Additionally, Olivine displays a bimodal distribution with two populations: olivine-Mg-rich (Fo72-81 Fa19-28) and olivine less magnesian (Fo60-67 Fa33-40). The combination of an harzburgitic composition and a high metal content in the NWA 16273 suggests a complex origin involving several parent bodies, which may have undergone a collision, melting and ejection to form the mesosiderite. Textural, mineral and geochemical characteristics indicate that a metallic impactor melted an eucritic crust of 4Vesta. The eucritic magma and molten metal were injected into the harzburite mantle prior to ejection. Nickel phosphides oxidized on contact with silicate magma to form secondary phosphates. Olivine underwent a more or less intense peritectic reaction to produce secondary orthoproxenes and chromites. The orthopyroxene xenocrysts are zoned and partially chemically re-equilibrated. These features show that a fraction of 4Vesta mantle was sampled during the giant impact.
期刊介绍:
The Journal of African Earth Sciences sees itself as the prime geological journal for all aspects of the Earth Sciences about the African plate. Papers dealing with peripheral areas are welcome if they demonstrate a tight link with Africa.
The Journal publishes high quality, peer-reviewed scientific papers. It is devoted primarily to research papers but short communications relating to new developments of broad interest, reviews and book reviews will also be considered. Papers must have international appeal and should present work of more regional than local significance and dealing with well identified and justified scientific questions. Specialised technical papers, analytical or exploration reports must be avoided. Papers on applied geology should preferably be linked to such core disciplines and must be addressed to a more general geoscientific audience.