Depleted and enriched mantle sources for Paleo- and Neoproterozoic carbonatites of southern India: Sr, Nd, C–O isotopic and geochemical constraints

NEG LABISE Department of Geology, Federal University of Pernambuco, C.P. 7852, Recife-PE 50.732-970, Brazil
Chemical Geology (Impact Factor: 3.52). 09/2002; 189(1-2):69-89. DOI: 10.1016/S0009-2541(02)00136-5


Paleoproterozoic (Hogenakal) and Neoproterozoic (Samalpatti, Sevattur, Mulakkadu–Pakkanadu) carbonatites of Tamil Nadu, southern India, have been investigated for whole-rock geochemistry and Nd, Sr and C–O isotopes. These temporally distinct carbonatite complexes are located close to a tectonically active zone that marks the transition between cratonic non-charnockitic (low- to medium-grade) terrain to the north and the charnockitic mobile belt (granulite facies) to the south. The carbonatites are variably enriched in LREE; the Hogenakal carbonatites being extremely enriched, with the highest ∑REE among the data while the younger carbonatites show variable enrichment levels and broadly comparable REE patterns. The Hogenakal carbonatites have coherent and typically mantle C- and O-isotopic ratios (δ13CV-PDB∼−6‰ and δ18OV-SMOW∼8‰). The Neoproterozoic carbonatites are relatively enriched in 13C (δ13CV-PDB=−5.3‰ to −3.3‰) and also show a range of O-isotopic composition (δ18OV-SMOW=7.3‰ to 15.4‰; two samples ∼25‰). Higher δ18O values indicate variable degree of post-magmatic low-temperature alteration. The older carbonatites have marginally positive εNd (+0.54 and +1) and significantly low initial 87Sr/86Sr ratios (0.70161 and 0.70174) while younger carbonatites have rather low εNd (−16.5 to −6.23) and higher 87Sr/86Sr ratios (0.70486–0.70658). The Sr–Nd isotopic data are compatible with a depleted mantle source for the Hogenakal carbonatites and an EM-I-type enriched mantle component for the younger group. This is the first report of the existence of depleted mantle beneath the southern Indian continental crust. The stable isotopic ratios are interpreted as suggesting a depletion event (crustal extraction) in the south Indian subcontinental mantle ∼2.6 Ga ago. The depleted mantle was subsequently enriched by metasomatic fluids under the influence of the subducting Dharwar plate (sediments and modified oceanic crust).

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    • "It is for this reason that this is nowadays the fundamental criterion for attributing a mantle origin to any studied carbonate in igneous rocks, and not just " something established for comparison " as seems to be defined by Demény et al., in their Comment. Pandit et al. (2002) also explained that the range of C and O compositions of primary igneous carbonatites defined by Taylor et al. (1967) corresponds to carbonatites unmodified by secondary (crustal) processes. Taking all the above into account, we think that our conclusion that carbonatites from Fuerteventura have mantle-like C and O compositions, in agreement with our radiogenic isotope data, and that therefore crustal contamination can be discarded, is well supported. "

    Chemical Geology 07/2007; 242(s 1–2):292–297. DOI:10.1016/j.chemgeo.2007.04.001 · 3.52 Impact Factor
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    • "On the contrary, the Pb isotopic ratios define an array, suggesting a possible mixing between a depleted mantle and a U-enriched component (crust/enriched source) involved in the generation of these carbonatites (Schleicher et al., 1998). Pandit et al. (2002) offered a different explanation for the origin of these complexes by suggesting a depleted mantle for the ~2.4 Ga old Hogenakal carbonatite and subsequent enrichment of its mantle source prior to the generation of other Neoproterozoic (~800 Ma old) carbonatites from the same source. "
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    ABSTRACT: Stable carbon and oxygen isotopic compositions of carbonatites are results of fractionation caused by various magmatic and post-magmatic processes during their generation and evolution. In the present work, we review available stable isotopic data from Indian carbonatites that span in age from Precambrian to Cretaceous. We explain the observed variations using various theoretical models and attempt to decipher the nature and temporal evolution of the mantle source(s) of these carbonatites. As observed elsewhere, δO variations are larger compared to those of δC. However, the average and mode of δC distributions in Indian carbonatites (∼-4 ‰) are clearly higher than the global average. In general, δC and δO variations of Indian carbonatites can be grouped into (1) primary, unaltered carbonatites and (2) secondary, altered carbonatites. Primary variations are results of either batch crystallization under plutonic conditions, as observed in Hogenakal and northeastern Indian carbonatites, or fractional crystallization from CO2+H2O fluid-rich parent magmas, as observed in the rest. Secondary isotopic variations in all the carbonatites are apparently results of low temperature alteration by either meteoric water or CO2-bearing aqueous fluids. Estimated δO values of the mantle sources of Indian carbonatites (5.3-7.5‰) show the expected normal mantle signatures, but δC values appear to be more variable (-6 to -3.1‰) than expected for a normal mantle. The younger carbonatites (
    International Geology Review 01/2006; 48(1):17-45. DOI:10.2747/0020-6814.48.1.17 · 1.71 Impact Factor

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