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Temporal evolution of δ44/40Ca and 87Sr/86Sr of carbonatites: Implications for crustal recycling through time

Banerjee, A and Chakrabarti, R and Simonetti, A (2021) Temporal evolution of δ44/40Ca and 87Sr/86Sr of carbonatites: Implications for crustal recycling through time. In: Geochimica et Cosmochimica Acta, 307 . pp. 168-191.

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Official URL: https://doi.org/10.1016/j.gca.2021.05.046


Carbonatites are igneous rocks composed of over 50 modal carbonate minerals and their exact origin and possible mode(s) of formation are still debated. We report stable Ca isotopic compositions (δ44/40CaSRM915a) of globally distributed carbonatites (n = 46) from 22 different locations ranging in age from 2.61 Ga to recent along with their major, trace element concentrations and 87Sr/86Sr ratios. The δ44/40CaSRM915a values for carbonatites older than 300 Ma, except for four samples from the Neoproterozoic south Indian carbonatites and the Phalaborwa complex of South Africa, display limited variation (~0.18�, n = 12) with δ44/40Ca values overlapping (given their associated analytical uncertainties) with the estimated δ44/40CaSRM915a value of the bulk silicate Earth (BSE, 0.94 ± 0.10�). The 87Sr/86Sr(t) of these samples (0.70149�0.70394) are consistent with derivation from a depleted mantle source with a time-integrated Rb/Sr ratio of ~0.017. In contrast, carbonatites with emplacement ages younger than 300 Ma display much wider variations in δ44/40Ca values (0.63�, n = 29) with sixteen samples displaying values lower than that for the BSE and radiogenic 87Sr/86Sr compositions (0.70313�0.71044). The low δ44/40Ca values for the younger carbonatites are consistent with the presence of recycled crustal carbonates, with compositions similar to those of 100�200 Ma and 400�500 Ma old carbonates, in their mantle source, which is also consistent with their radiogenic 87Sr/86Sr ratios. Model calculations, based on δ44/40Ca and 87Sr/86Sr(t) ratios for mantle peridotite and eclogite sources and marine carbonates of different ages, suggest that the δ44/40Ca and 87Sr/86Sr for most young carbonatites (<300 Ma) examined here can be explained by the presence of <10 wt recycled crustal carbonates in their mantle source. The contrasting Ca and Sr isotope compositions between younger (<300 Ma) and older (>300 Ma) carbonatites may reflect changing geodynamic conditions. The prevalent Ca and Sr isotope signatures attributed to crustal recycling that is observed in carbonatites younger than 300 Ma most likely reflect two global-scale processes: (i) an increased amount of subduction flux due to high convergence rates subsequent to the break-up of the Pangea supercontinent, and (ii) greater amount of plume activity during the last 300 million years of Earth history. Additionally, the low δ44/40Ca values of younger carbonatites could have been accentuated by subduction of marine carbonates (aragonitic) with relatively low δ44/40Ca. In contrast to the younger carbonatites, Ca and Sr isotope compositions for the carbonatites older than 300 Ma investigated here suggest that crustal recycling was much more sporadic in the Precambrian. © 2021 Elsevier Ltd

Item Type: Journal Article
Publication: Geochimica et Cosmochimica Acta
Publisher: Elsevier Ltd
Additional Information: The copyright for this article belongs to Elsevier Ltd
Department/Centre: Division of Mechanical Sciences > Centre for Earth Sciences
Date Deposited: 01 Sep 2021 09:08
Last Modified: 01 Sep 2021 09:08
URI: http://eprints.iisc.ac.in/id/eprint/69463

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