Isotopically heavy Ti in kimberlites explained by assimilation of metasomatised lithospheric mantle
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2025
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Abstract
Kimberlites represent some of the Earth’s deepest sourced mantle melts, but the origin of their variable geochemical signatures is debated, including contributions by source variations and lithospheric interaction during ascent. To evaluate this further, the Ti isotope compositions of selected kimberlites (n = 24) from major cratons worldwide with emplacement ages from 1.15 Ga to the Holocene are reported. The kimberlites display variable Ti isotope compositions ranging from +0.007 ± 0.027 ‰ to +0.173 ± 0.027 ‰ for δ⁴⁹Ti (deviation of ⁴⁹Ti/⁴⁷Ti from the OL-Ti standard). The observed δ⁴⁹Ti variation of ∼0.17 ‰ is notably larger compared to typical mantle-derived primitive magmas (i.e. basalts and komatiites). Significant inter-province δ⁴⁹Ti variations are in contrast with negligible variations within each locality. Notably, primitive and highly differentiated kimberlites within the same kimberlite field display uniform Ti isotope compositions. These results contrast with the Ti isotope differentiation trends observed for silicate magmas, corroborating late-stage crystallisation of major Ti-carriers (i.e. spinel, perovskite, mica) in kimberlites, and may further indicate limited crystal-melt Ti isotope fractionation in carbonated silicate magmas. Heavy Ti isotope compositions observed in some kimberlites cannot be explained by the limited isotopic fractionation predicted between carbonated melts and peridotitic sources during partial melting, or by mixing of ambient mantle with subducted material, including terrigenous sediments with elevated δ⁴⁹Ti. Instead, isotopically heavy Ti appears to occur in kimberlites featuring elevated concentrations of mica and low Mg/Fe in olivine, both considered a hallmark of extensive interaction of primary kimberlite melts with strongly metasomatised, Fe-Ti-rich lithospheric mantle. These observations underscore a dominant role of the lithospheric mantle in shaping the anomalously heavy Ti isotope compositions of kimberlites.
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Elsevier
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03946 - Schönbächler, Maria / Schönbächler, Maria
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