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 ${\delta }^{44/40}$Ca 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 ${\delta }^{18}$O 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 ${\delta }^{18}$O 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 ${\delta }^{44/40}$Ca values for the Bellsbank eclogites range from +0.28 to +1.56‰. Although some of the xenoliths have ${\delta }^{44/40}$Ca 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 ${\delta }^{44/40}$Ca relative to mantle compositions are recorded by the eclogites, with a general negative correlation with ${\delta }^{18}$O suggestive of seawater-alteration of oceanic crust. The combined low ${\delta }^{44/40}$Ca (+0.28‰) and ${\delta }^{18}$O (+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 ${\delta }^{44/40}$Ca 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 ${\delta }^{44/40}$Ca–${\delta }^{18}$O–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 ${\delta }^{44/40}$Ca 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.

长期以来，深层钙地球化学循环的性质尚未解决，部分原因是缺乏关于太古宙循环利用的海洋地壳钙同位素组成的信息。这种古代洋壳的残余物以克拉通地幔榴辉岩的形式保存下来，在金伯利岩岩浆喷发中以异岩形式浮出水面。的${\delta }^{44/40}$本文介绍了来自南非Kaapvaal克拉通的Bellsbank金伯利岩的新鲜地幔衍生的榴辉辉长岩异岩（即石榴石和绿辉石矿物分离物）Ca及其微量元素组成，石榴石的Fe ³⁺含量和${\delta }^{\mathrm{18岁}}$O值。

所研究的Bellsbank榴辉岩异岩具有地球化学成分，表明海洋地壳原生岩，其Al ₂ O _3的含量为15至27 wt。％，Eu异常为0.8至2.6，并且石榴石明显${\delta }^{\mathrm{18岁}}$O值从+2.7到+ 6.2‰。石榴石Fe ³⁺ /ΣFe含量在110-180 km深度范围内产生的log f O ₂（ΔFMQ）值在-4.0和-1.2之间，记录了太古代Kaapvaal地幔岩石圈中榴辉岩组分的强氧化还原异质性。重建的榴辉岩MgO含量与f O ₂负相关，表明氧化还原成分与大洋地壳形成过程中的岩浆分异有关，不包括次生交代叠印。因此，这些数据可能强调，太古代的玄武质洋壳具有与现代MORB型地壳相似的可变氧化还原成分。

重建的散装 ${\delta }^{44/40}$Bellsbank榴辉岩的Ca值范围为+0.28至+ 1.56‰。尽管某些异种生物具有${\delta }^{44/40}$与平均地幔组成和现代MORB重叠的Ca值（+0.94±0.1和+0.83±0.05‰），我们数据集的一半显示出对更极端的Ca同位素组成的偏移。更高和更低${\delta }^{44/40}$钙云母记录的是相对于地幔成分的Ca，与 ${\delta }^{\mathrm{18岁}}$提示海水改变了地壳。组合低${\delta }^{44/40}$Ca（+ 0.28‰）和 ${\delta }^{\mathrm{18岁}}$对一种榴辉岩异岩体测得的O（+ 3.4‰）可能记录着富含碳酸盐的地幔熔体的微弱痕迹，已知该熔岩中含有同位素同位素轻钙，这些钙是由再循环沉积物贡献的。相比之下，高${\delta }^{44/40}$某些榴辉岩异岩中的Ca高达+ 1.56‰，加上强烈的LREE消耗，可以用部分熔融过程中的钙同位素分馏来解释。

由克拉通地幔榴辉岩探测到的可循环利用的海洋地壳的漫长历史记录是由其高度变化所记录的 ${\delta }^{44/40}$Ca–${\delta }^{\mathrm{18岁}}$O– f O ₂签名。尽管这种异质性中的某些可以与在太古代海底进行的过程（例如玄武岩壳的海水蚀变）相关联，但在俯冲再循环和地幔居住期间，熔体的损失和添加会引入其他成分变异性来源。观察到的${\delta }^{44/40}$在单个地幔衍生的榴辉岩异岩体规模上，古代可循环利用的海洋地壳组分的Ca复杂性意味着，源自地幔柱的板内岩浆应显示出明显重要的可循环利用的地壳组分所造成的类似强的钙同位素变化-如全球海洋岛玄武岩中所观察到的记录。