DOI: 10.1016/j.epsl.2018.03.014
Scopus记录号: 2-s2.0-85044577190
论文题名: Inclusions in diamonds constrain thermo-chemical conditions during Mesozoic metasomatism of the Kaapvaal cratonic mantle
作者: Weiss Y. ; Navon O. ; Goldstein S.L. ; Harris J.W.
刊名: Earth and Planetary Science Letters
ISSN: 0012821X
出版年: 2018
卷: 491 起始页码: 134
结束页码: 147
语种: 英语
英文关键词: diamond
; high density fluids (HDFs)
; Kaapvaal craton
; oxygen fugacity (fO2)
; sub-continental lithospheric mantle (SCLM)
; thermobarometry
Scopus关键词: Basalt
; Diamonds
; Garnets
; Mineralogy
; Minerals
; Nitrogen
; Oxygen
; Structural geology
; Tectonics
; Thermometers
; Wall rock
; high density fluids (HDFs)
; Kaapvaal
; Oxygen fugacity
; Sub-continental lithospheric mantles
; Thermobarometry
; Diamond deposits
; diamond
; fugacity
; kimberlite
; lithospheric structure
; magmatism
; mantle structure
; Mesozoic
; metasomatism
; thermochemistry
; thermometry
; xenolith
; Kaapvaal Craton
英文摘要: Fluid/melt inclusions in diamonds, which were encapsulated during a metasomatic event and over a short period of time, are isolated from their surrounding mantle, offering the opportunity to constrain changes in the sub-continental lithospheric mantle (SCLM) that occurred during individual thermo-chemical events, as well as the composition of the fluids involved and their sources. We have analyzed a suite of 8 microinclusion-bearing diamonds from the Group I De Beers Pool kimberlites, South Africa, using FTIR, EPMA and LA-ICP-MS. Seven of the diamonds trapped incompatible-element-enriched saline high density fluids (HDFs), carry peridotitic mineral microinclusions, and substitutional nitrogen almost exclusively in A-centers. This low-aggregation state of nitrogen indicates a short mantle residence times and/or low mantle ambient temperature for these diamonds. A short residence time is favored because, elevated thermal conditions prevailed in the South African lithosphere during and following the Karoo flood basalt volcanism at ∼180 Ma, thus the saline metasomatism must have occurred close to the time of kimberlite eruptions at ∼85 Ma. Another diamond encapsulated incompatible-element-enriched silicic HDFs and has 25% of its nitrogen content residing in B-centers, implying formation during an earlier and different metasomatic event that likely relates to the Karoo magmatism at ca. 180 Ma. Thermometry of mineral microinclusions in the diamonds carrying saline HDFs, based on Mg–Fe exchange between garnet–orthopyroxene (Opx)/clinopyroxene (Cpx)/olivine and the Opx–Cpx thermometer, yield temperatures between 875–1080 °C at 5 GPa. These temperatures overlap with conditions recorded by touching inclusion pairs in diamonds from the De Beers Pool kimberlites, which represent the mantle ambient conditions just before eruption, and are altogether lower by 150–250 °C compared to P–T gradients recorded by peridotite xenoliths from the same locality. Oxygen fugacity (fO2) differs as well. The fO2 calculated for the saline HDF compositions (ΔlogfO2(FMQ)=−2.47 to −1.34) are higher by about a log unit compared with that recorded by xenoliths at 4–7 GPa. We conclude that enriched saline HDFs mediated the metasomatism that preceded Group I kimberlite eruptions in the southwestern Kaapvaal craton, and that their ‘cold and oxidized’ nature reflects their derivation from a deep subducting slab. This event had little impact on the temperature and redox state of the Kaapvaal lithosphere as a reservoir, however, it likely affected its properties along limited metasomatized veins and their wall rock. To reconcile the temperature and oxygen fugacity discrepancy between inclusions in diamonds and xenoliths, we argue that xenoliths did not equilibrate during the last saline metasomatic event or kimberlite eruption. Thus the P–T–fO2 gradients they record express pre-existing lithospheric conditions that were likely established during the last major thermal event in the Kaapvaal craton (i.e. the Karoo magmatism at ca. 180 Ma). © 2018 Elsevier B.V. Citation statistics:
资源类型: 期刊论文
标识符: http://119.78.100.158/handle/2HF3EXSE/109931
Appears in Collections: 影响、适应和脆弱性 气候变化事实与影响
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作者单位: Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, United States; The Freddy and Nadine Herrmann Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel; Department of Earth and Environmental Sciences, Columbia University, Palisades, NY 10964, United States; School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, United Kingdom
Recommended Citation:
Weiss Y.,Navon O.,Goldstein S.L.,et al. Inclusions in diamonds constrain thermo-chemical conditions during Mesozoic metasomatism of the Kaapvaal cratonic mantle[J]. Earth and Planetary Science Letters,2018-01-01,491