High Temperature Geochemistry and Cosmochemistry

The cratonic lithosphere–asthenosphere boundary is commonly invoked as the site of sheared peridotite and megacryst formation, a well-recognized petrologic assemblage whose genetic relationships – if any – remain poorly understood. We have undertaken a comprehensive petrology and Sr-Nd-Hf-Ca isotope study of sheared peridotite xenoliths and clinopyroxene megacrysts from the ca. 1150 Ma Premier kimberlite pipe on the central Kaapvaal craton in South Africa. New textural and mineral trace element evidence suggests that strong tectonic and magmatic overprinting affected the lower cratonic mantle over a vertical distance of ≥ 50 km from the lithosphere–asthenosphere boundary located at ∼200-225 km depth. Although modification of the central Kaapvaal cratonic mantle is commonly linked to the ca. 2056 Ma Bushveld large igneous event, our thermobarometry, mantle redox, and Sr-Nd-Hf-Ca isotope data support a model in which volatile-rich low-volume melts and associated high-density fluids refertilized the lithosphere base shortly before or during asthenosphere-derived kimberlite and carbonatite magmatism at around 1150 Ma. This episode of lithospheric mantle enrichment was facilitated by exceptionally strong shear movements, as are recorded in the plastically deformed peridotites. We argue that stress-driven segregation of percolating carbonated melts contributed to megacryst formation along, or in close proximity to, shear zones within the cratonic mantle lithosphere.

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Mass‐Independent and Mass‐Dependent Ca Isotopic Compositions of Thirteen Geological Reference Materials Measured by Thermal Ionisation Mass Spectrometry

Wang

Zhu

et al. 2016

Geostandard Geoanalytic Res

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We report mass‐independent and mass‐dependent Ca isotopic compositions for thirteen geological reference materials, including carbonates (NIST SRM 915a and 915b), Atlantic seawater as well as ten rock reference materials ranging from peridotite to sandstone, using traditional ε and δ values relative to NIST SRM 915a, respectively. Isotope ratio determinations were conducted by independent unspiked and 43Ca‐48Ca double‐spiked measurements using a customised Triton Plus TIMS. The mean of twelve measurement results gave ε40/44Ca values within ± 1.1, except for GSP‐2 that had ε40/44Ca = 4.04 ± 0.15 (2SE). Significant radiogenic 40Ca enrichment was evident in some high K/Ca samples. At an uncertainty level of ± 0.6, all reference materials had the same ε43/44Ca and ε48/44Ca values. We suggest the use of δ44/42Ca to report mass‐dependent Ca isotopic compositions. The precision under intermediate measurement conditions for δ44/42Ca over eight months in our laboratory was ± 0.03‰ (with n ≥ 8 repeat measurements). Measured igneous reference materials gave δ44/42Ca values ranging from 0.27‰ to 0.54‰. Significant Ca isotope fractionation may occur during magmatic and metasomatism processes. Studied reference materials with higher (Dyn/Ybn) tend to have lower δ44/42Ca, implying a potential role of garnet in producing magmas with low δ44/42Ca. Sandstone GBW07106 had a δ44/42Ca value of 0.22‰, lower than all igneous rocks studied so far.

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High Temperature Geochemistry and Cosmochemistry

Cited by 3 publications

References 95 publications

Calcium Isotope Constraints on Recycled Carbonates in Subduction‐Related Magmas

Calcium Isotope Constraints on Recycled Carbonates in Subduction‐Related Magmas

Sheared Peridotite and Megacryst Formation Beneath the Kaapvaal Craton: a Snapshot of Tectonomagmatic Processes across the Lithosphere–Asthenosphere Transition

Mass‐Independent and Mass‐Dependent Ca Isotopic Compositions of Thirteen Geological Reference Materials Measured by Thermal Ionisation Mass Spectrometry

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