Jyotiranjan S. Ray

Age of the Vindhyan Supergroup: A review of recent findings

The Vindhyan Supergroup of India is one of the largest and thickest sedimentary successions of the world. Deposited in an intra-cratonic basin, it is composed mostly of shallow marine deposits. It is believed to have recorded a substantial portion of Proterozoic time and therefore, likely to contain valuable information on the evolution of the atmosphere, climate, and life on our planet. It also contains some of the most disputed fossils of earliest animal life. Despite their importance, the absolute age of these rocks had remained unknown until recently. In this work I evaluate all the recent chronological information and discuss their implications. From the present findings it appears that the issues surrounding the age of the Lower Vindhyans in the Son valley are now resolved, whereas problems with the age of the Upper Vindhyans and that with the stratigraphic correlations remain to be answered.

Carbon isotopes in Kerguelen plume-derived carbonatites: evidence for recycled inorganic carbon

Carbonatites form from deep mantle melts that are believed to incorporate recycled crustal carbon. Most of the evidence in favour of this hypothesis is, however, circumstantial and comes from the study of radiogenic (Nd‐Sr‐Pb) isotopes that show HIMU and EM-I mantle signatures. In this work, we present direct evidence for the incorporation of recycled crustal carbon in carbonatites of Eastern India through a study of their stable isotope systematics. The 40 Ar= 39 Ar age of one of these coeval complexes is 107:2 0:8 Ma, which suggests that these carbonatites represent late magmatic pulses of the Rajmahal‐Bengal‐Sylhet flood basalt province. Their age, spatial proximity to the Sylhet traps, HIMU-EM I isotopic signatures, and Sr-isotopic similarity to the 115‐105 Ma old Kerguelen Plateau basalts are consistent with the hypothesis of their Kerguelen plume origin. The carbon and oxygen isotope compositions of three of these carbonatite complexes are homogeneous, unlike most of the carbonatites world-wide, and is suggestive of batch crystallization of these rocks under plutonic conditions. The d 18 O values of all the complexes are consistent with their derivation in equilibrium with mantle silicates, whereas d 13 C shows higher values than a ‘normal’ mantle ( d 13 CD 5.0 to 8.0‰). The homogeneity of isotope compositions, absence of 18 O enrichments, co-precipitation of calcite and dolomite in isotopic equilibrium and absence of any crustal contamination effects, preclude the possibility of any change in d 13 C of the carbonatite magmas=rocks by magmatic or secondary fractionation process. Therefore, the d 13 C values of these carbonatites directly reflect the d 13 C values of their source regions. As all these complexes probably belonged to a single magmatic episode, the higher d 13 C of the parent magma (average for all the complexes D 3.2‰) than that of a ‘normal’ mantle is clear evidence for incorporation of recycled inorganic carbon. We suggest that this incorporation is a result of entrainment of a subcontinental lithospheric mantle, which was already enriched in 13 C derived from subducted ancient oceanic crusts through mantle metasomatism.

Carbonatite alkaline magmatism associated with continental flood basalts at stratigraphic boundaries: Cause for mass extinctions

The debate between the impact scenario and the hypothesis of the Deccan volcanism leading to Cretaceous/Tertiary (K/T) extinctions is far from settled. We add a new dimension to this debate by introducing an overlooked aspect of the Deccan volcanism, the emplacement and eruption of associated carbonatite-alkaline complexes, which are capable of rapidly injecting catastrophic amounts of CO2 and SO2 into the atmosphere. 40Ar-39Ar dating of one of the several coeval late-Deccan alkaline pulses has revealed that this alkaline volcanism took place at the K/T boundary. A conservative estimate shows that these activities can indeed emit a substantial amount of CO2 and SO2, comparable to that of an impact scenario, which could have enhanced the catastrophic effects of the ongoing basaltic eruptions. Other major Phanerozoic extinctions also show contemporaneity with the late carbonatite-alkaline pulses of associated flood basalt provinces, suggesting a causal relationship between such carbonatite-alkaline magmatisms and the mass extinctions.

Rayleigh fractionation of stable isotopes from a multicomponent source

A formulation of the Rayleigh equation for the stable isotopic evolution of a multicomponent source reservoir is presented. Its applicability to the carbon and oxygen isotopic evolution of a fluid-rich carbonate magma and the crystallizing calcite carbonatite is demonstrated using data from the Amba Dongar carbonatite complex, Deccan province, India. The initial δ13C of the parent magma for this complex has been estimated to be −5.3 ± 0.2‰ relative to V-PDB.

Isotope and rare earth element chemistry of carbonatite–alkaline complexes of Deccan volcanic province: implications to magmatic and alteration processes

Abstract Results of different isotopic and trace element studies on three carbonatite–alkaline complexes (Amba Dongar, Mundwara and Sarnu-Dandali) of the Deccan flood basalt province, India, are presented. The Amba Dongar (Ambadungar) complex has been dated precisely to 65.0±0.3 Ma by the 40 Ar– 39 Ar method. The minimum initial Sr isotopic ratio of alkaline rocks of Amba Dongar is found to be same as that of the coexisting carbonatites, suggesting their derivation from a common parent magma, probably through liquid immiscibility. The rare earth element abundance in these rocks also supports the liquid immiscibility hypothesis. Further investigation revealed that the parent magma of this complex has been contaminated (∼5%) by the lower crustal material, which is clearly reflected in the initial 87 Sr/ 86 Sr variation of alkaline rocks but not in the carbonatites. Sr study also suggests that the mantle source of Amba Dongar like the other two complexes is a Rb/Sr enriched source. The temporal and spatial relationships of all the three complexes with the Deccan flood basalts support the hypothesis of reunion plume origin for these. Fractional crystallization and subsequent hydrothermal/meteoric alteration are found to have controlled the stable carbon and oxygen isotopic variations in carbonatites. This study suggests that all the complexes have been derived from isotopically average mantle except for a particular batch of parent magma at Amba Dongar, which appears to have incorporated recycled crustal carbon. In a plume origin scenario such incorporation indicates the entrainment of 13 C-enriched subcontinental lithospheric mantle by the plume.

Trace element and isotope evolution during concurrent assimilation, fractional crystallization, and liquid immiscibility of a carbonated silicate magma

Abstract Liquid immiscibility is an important magmatic process that causes unmixing of magmas into liquids of contrasting compositions. Such magmas may get modified by simultaneous wall rock assimilation and fractional crystallization during the liquid immiscibility in a crustal magma chamber. The element and isotope effects of such a process are likely to be reflected in the final products. To treat these effects and to understand the evolution of the immiscible liquids, a model has been developed modifying the assimilation-fractional crystallization (AFC) model of DePaolo (1981). I demonstrate the applicability of this model by an example using Sr isotope systematics of silicate-carbonate melt immiscibility. The initial 87Sr/86Sr ratio and Sr concentration variation in the silicate rocks of some alkaline-carbonatite complexes of Deccan Province are found to be a result of lower crustal contamination (up to 5%) of the parent carbonated silicate magma, while the 87Sr/86Sr of the carbonate melt separated out of the parent remained unaffected. Though the data on silicate rocks could also be explained by the conventional AFC model, the processes treated by the model do not include liquid immiscibility, needed for explaining the evolution of the cogenetic carbonatites. It appears from this study that the slightly higher initial 87Sr/86Sr (than that of the coexisting carbonatites) shown by the alkaline silicate rocks could be due to crustal contamination of the carbonated silicate parent magma during concurrent fractional crystallization of silicates and exsolution of carbonate melt. Though the model has been applied to a very specific case—that of carbonate-silicate melt immiscibility—it can be applied to any case in which both assimilation and immiscibility occur together.

Provenance of the Late Quaternary sediments in the Andaman Sea: Implications for monsoon variability and ocean circulation

We present a geochemical and Sr-Nd isotopic study on a sediment core collected from the Andaman Sea in an attempt to reconstruct the Late Quaternary weathering and erosion patterns in the watersheds of the river systems of Myanmar and understand their controlling factors. Age control is based on nine radiocarbon dates and δ18O stratigraphy. The rate of sedimentation was strongly controlled by fluctuations of the monsoon. We identify three major sediment provenances: (1) the Irrawaddy catchment, (2) the western slopes of the Indo-Burman-Arakan (IBA) mountain ranges and the Andaman Islands, and (3) the catchments of Salween and Sittang and the Bengal shelf, with the first two contributing 30–60% of the material. Enhanced contributions from juvenile sources and corresponding positive shifts of δ18O are observed at seven time periods (11–14, 20–23, 36, 45, 53, 57, and 62 ka) of which five are synchronous with cooling of the northern hemisphere, suggesting a link between the changes in sediment provenances and the shifting of the locus of the summer monsoon, southward from the Himalayas, without substantial reduction in intensity. Our data, and that from other cores in the region suggest that an eastward moving surface current disperses sediments, derived from the Bengal shelf and western margin of Myanmar, from the eastern Bay of Bengal into the western Andaman Sea and that its strength has increased since the LGM. The existence of this current during the LGM implies that the Andaman Sea and the Bay of Bengal were well connected during the last glacial period.

Recycling of Flow-Top Breccia Crusts into Molten Interiors of Flood Basalt Lava Flows: Field and Geochemical Evidence from the Deccan Traps

Thick flood basalt lava flows cool conductively inward from their tops and bases, usually developing columnar jointing. Although relatively rapid cooling in such flows due to meteoric water circulation has been previously demonstrated, mixing of the surface crust with the interior – as observed in active lava lakes – has not been shown. Here we report large radial columnar jointing structures (rosettes) with cores of highly brecciated, weathered and amygdaloidal material within Deccan flood basalt lava flows. The morphology of such breccia-cored rosettes, petrographic observations, and geochemical data, particularly Nd–Sr isotopic ratios, all suggest that the features formed due to the sinking of the flow-top breccia crusts into these flows’ molten interiors and the resultant warping of isotherms around these “cold anchors”. Thus, cooling in some thick flood basalt lava flows may be accelerated by sinking of cooler upper crusts into hotter, molten interiors.

Carbon isotopic variations in fluid‐deposited graphite: evidence for multicomponent Rayleigh isotopic fractionation

Carbon isotopic composition of fluid‐deposited graphite in a metamorphic terrain can be used as a tracer for sources of fluids, their compositions and temperatures of graphitization. However, δ13C of graphite almost always shows a large spread, mostly because of isotope fractionation during precipitation, making it necessary to understand these processes. In this work, a novel quantitative approach, involving Rayleigh isotope fractionation during formation of graphite from a realistic multicomponent metamorphic fluid, is proposed to explain the observed δ13C variations in fluid‐deposited graphite. Results of this study reveal that most graphite crystallizes from a mixed CO2–CH4 fluid that can acquire its isotopic composition from a variety of sources like organic matter, carbonate, mantle degassing or variable mixtures of these.

Stable Carbon and Oxygen Isotopic Compositions of Indian Carbonatites

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, δ18O variations are larger compared to those of δ13C. However, the average and mode of δ13C distributions in Indian carbonatites (∼-4 ‰) are clearly higher than the global average. In general, δ13C and δ18O 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 δ18O values of the mantle sources of Indian carbonatites (5.3-7.5‰) show the expected normal mantle signatures, but δ13C values appear to be more variable (-6 to -3.1‰) than expected for a normal mantle. The younger carbonatites (<107 Ma) in particular appear to have been derived from 13C enriched sources. Combined study of δ13C and 87Sr/86Sr data suggests that Indian carbonatites were derived from enriched mantle sources, and the enrichment probably took place some time in the Archean (∼2.4 Ga). We suggest that the Indian sub-continental mantle, which was metasomatized by fluids from subducted oceanic crusts around that time, has remained a continuous source of carbonatites.