A geochemical and Ca, Sr, Nd isotopic study of the mantle: case studies of Kimberlites, Ophiolites, Abyssal Peridotites, and Layered Igneous Complexes

Abstract

The Earth’s mantle constitutes 67% of its mass and despite mantle convection, it is geochemically and isotopically heterogeneous as evident from the compositions of mantle derived basaltic melts. It is a dynamic system that interacts with crustal material via subduction as well as the deep Earth by core-mantle interaction. Radiogenic isotopic compositions of magmatic rocks are powerful tracers of mantle sources and demonstrate heterogeneity in mantle compositions at varying spatial scales. Stable isotopes provide insights into processes like partial melting, crystal fractionation, metamorphism, metasomatism, and hydrothermal alteration, which can modify the mineralogical, elemental, and isotopic compositions of Earth’s mantle. This study utilizes variations in the abundances of stable isotopes of calcium, a major element, in different magmatic rocks and minerals derived from varying depths in the mantle. In addition to Ca stable isotopes, geochemical and Sr-Nd isotopic compositions are reported for kimberlites, ophiolites, abyssal peridotites, and rocks from the Sittampundi layered igneous complex. Kimberlites are volatile and trace element-enriched rocks originating under high pressure by low degree partial melting of the mantle. Geochemical data for global kimberlites reveal that the Al concentration of kimberlites correlates with their (Gd/Yb)N ratio, which is consistent with their derivation from the garnet stability zone, while their Nd-Sr isotopic compositions suggest a depleted mantle origin. The 44/40Ca values (reported relative to the NIST SRM 915a standard) of 1100 Ma old kimberlites from the Wajrakrur Kimberlite Field, South India, ranges from 0.91-1.22‰. These values are broadly overlap with the inferred composition of the bulk silicate Earth (BSE, 0.94 ± 0.05‰) suggesting minimal Ca isotopic fractionation during low-degree partial melting; some samples display slightly higher 44/40Ca values, which likely reflects a mineralogical control from either garnet + clinopyroxene or garnet + perovskite assemblages at possible depths of 200-400 km and 600-700 km, respectively. Ophiolites are obducted remnants of ancient oceanic lithosphere. Based on geochemical, tectonic, and geochronological constraints, ~117 Ma old Nagaland-Manipur ophiolites (NMO) are inferred as a segment of Tethyan oceanic lithosphere influenced by Kerguelen plume during the breakup of Gondwana supercontinent. The 44/40Ca values of mafic and ultramafic rocks of NMO range from 0.78-1.32‰; these values vary with (La/Sm)N, Mg concentrations, Mg#, Ca/Mg, Ca/Al, and Ca/Na ratios suggesting (high degree, 10-20%) partial melting induced fractionation of Ca stable isotopes with possible effect of mineral fractionation from the magma. The 87Sr/86Sr of the ultramafic rocks show effects of hydrothermal alteration, but the 44/40Ca values remain relatively unaffected, which is likely due to the varying behavior of these elements based on host rock mineralogy during water-rock interaction. Abyssal peridotites are oceanic lithospheric mantle partial melting residues after extraction of mid-oceanic basalts (MORB). Calcium stable isotopic compositions are reported for clinopyroxene and orthopyroxene mineral separates from abyssal peridotites from the Southwest Indian Ocean Ridge (SWIR). The 44/40Ca values provide evidence for metasomatism of the oceanic lithosphere by melts derived from the garnet-stability zone while radiogenic isotopes reflect relative enrichments due to the influence of the Bouvet plume. This observation has implications for the 44/40Ca of the BSE, which is estimated from fertile peridotites and does not consider the garnet contribution. Additionally, our results indicate that hydrothermal alteration of oceanic lithosphere can result in low 44/40Ca values (0.33-74‰) along with depleted trace element compositions. Such low 44/40Ca values provide additional sources of mantle heterogeneity as the oceanic lithosphere subducts back into the mantle. Layered igneous complexes provide insights into magma chamber processes. The 44/40Ca values of the Archean-age Sittampundi layered igneous complex (SIC) reflects the role of mineralogy, specifically, clinopyroxene (cpx) vs. orthopyroxene (opx) and anorthite vs. albite contents, which in turn depends on pressure and hydrous content of the magma chamber. Radiogenic isotopes indicate a depleted mantle origin while associated geochemical data suggest a high Al and Ca bearing mantle source. The Sittampundi anorthosites show 44/40Ca values overlapping with lunar anorthosites consistent with their high Ca content like the lunar anorthosites, which has implications for early lunar evolution.