DOI: 10.1016/j.epsl.2020.116720
论文题名: Constraints on Archean crust recycling and the origin of mantle redox variability from the δ44/40Ca – δ18O – fO2 signatures of cratonic eclogites
作者: Smart K.A. ; Tappe S. ; Woodland A.B. ; Greyling D.R. ; Harris C. ; Gussone N.
刊名: Earth and Planetary Science Letters
ISSN: 0012821X
出版年: 2021
卷: 556 语种: 英语
中文关键词: Archean oceanic crust
; calcium and oxygen isotopes
; craton formation
; mantle redox
; metasomatism
; subduction recycling
英文关键词: Alumina
; Aluminum oxide
; Calcium
; Garnets
; Geochemistry
; Isotopes
; Magnesia
; Metamorphic rocks
; Oxide minerals
; Seawater
; Trace elements
; Volcanic rocks
; Compositional variability
; Geochemical composition
; Isotopic composition
; Mantle composition
; Negative correlation
; Oceanic island basalts
; Recycled oceanic crusts
; Trace element composition
; Recycling
; Archean
; calcium
; chemical composition
; craton
; crustal recycling
; eclogite
; formation mechanism
; mantle source
; metasomatism
; oceanic crust
; oxygen isotope
; paleoceanography
; paleogeography
; redox conditions
; trace element
; Kaapvaal Craton
英文摘要: The nature of the deep calcium geochemical cycle through time is unresolved, in part due to the dearth of information about the calcium isotope composition of Archean recycled oceanic crust. Remnants of such ancient oceanic crust are preserved in the form of cratonic mantle eclogites, brought to surface as xenoliths in kimberlite magma eruptions. The δ44/40Ca of fresh mantle-derived eclogite xenoliths (i.e., garnet and omphacite mineral separates) from the Bellsbank kimberlite on the Kaapvaal craton in South Africa are presented here in combination with their trace element compositions, garnet Fe3+ contents and δ18O values. The studied Bellsbank eclogite xenoliths have geochemical compositions that indicate oceanic crustal protoliths, with bulk Al2O3 from 15 to 27 wt.%, Eu anomalies from 0.8 to 2.6 and, significantly, garnet δ18O values from +2.7 to +6.2‰. Garnet Fe3+/ΣFe contents yield logfO2(ΔFMQ) values between −4.0 and −1.2 for a depth range of 110–180 km, recording strong redox heterogeneity of the eclogite component within the Archean Kaapvaal mantle lithosphere. Reconstructed bulk eclogite MgO contents correlate negatively with fO2, suggesting that the redox compositions are related to magmatic differentiation during oceanic crust formation, excluding secondary metasomatic overprints. These data may thus emphasize that Archean basaltic oceanic crust had a similarly variable redox composition to modern MORB-type crust. Reconstructed bulk δ44/40Ca values for the Bellsbank eclogites range from +0.28 to +1.56‰. Although some of the xenoliths have δ44/40Ca values that overlap with the average mantle composition and modern MORB (+0.94 ± 0.1 and +0.83 ± 0.05‰), half of our dataset shows excursions to more extreme Ca isotopic compositions. Both higher and lower δ44/40Ca relative to mantle compositions are recorded by the eclogites, with a general negative correlation with δ18O suggestive of seawater-alteration of oceanic crust. The combined low δ44/40Ca (+0.28‰) and δ18O (+3.4‰) measured for one eclogite xenolith may record a subtle imprint by carbonate-rich mantle melts, which are known to contain isotopically light calcium contributed by recycled sediments. In contrast, the high δ44/40Ca of up to +1.56‰ for some eclogite xenoliths, coupled with strong LREE depletion, can be explained by calcium isotope fractionation during partial melting. The protracted history of recycled oceanic crust as probed by cratonic mantle eclogites is recorded by their highly variable δ44/40Ca–δ18O–fO2 signatures. Whereas some of this heterogeneity can be linked to processes that operated on the Archean ocean floor such as seawater-alteration of basaltic crust, other sources of compositional variability are introduced by loss and addition of melts during subduction recycling and mantle residence. The observed δ44/40Ca complexity of ancient recycled oceanic crust components at the scale of a single mantle-derived eclogite xenolith suite implies that mantle plume sourced intraplate magmas should reveal similarly strong calcium isotope variations contributed by apparently essential recycled crust components – as observed in the global oceanic island basalt record. © 2020 Elsevier B.V. Citation statistics:
资源类型: 期刊论文
标识符: http://119.78.100.158/handle/2HF3EXSE/165677
Appears in Collections: 气候变化与战略
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作者单位: School of Geosciences, University of the Witwatersrand, Johannesburg, 2050, South Africa; Deep & Early Earth Processes (DEEP) Research Group, Department of Geology, University of Johannesburg, Auckland Park, 2006, South Africa; Institut für Geowissenschaften, Goethe-Universität, Frankfurt am Main, 60438, Germany; Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa; Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 24, Münster, 48149, Germany
Recommended Citation:
Smart K.A.,Tappe S.,Woodland A.B.,et al. Constraints on Archean crust recycling and the origin of mantle redox variability from the δ44/40Ca – δ18O – fO2 signatures of cratonic eclogites[J]. Earth and Planetary Science Letters,2021-01-01,556