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Beach sands harbor appreciable amount of radioactive elements such as zircon which contains some important and radioactive elements (e.g. uranium, thorium). Very limited quantitative information is available of such elements in the sand of the beaches of Cox's Bazar in Bangladesh. The present study presents application of Neutron Activation Analysis (NAA) to quantify uranium and thorium elemental concentration in zircon assemblages from beach placers of Cox's Bazar. Zircon was separated from beach sands in a mineral processing pilot plant. The separated zircon assemblages were examined by a polarizing petrographic microscope and X-ray diffraction techniques and found that it contained more than 75% pure zircon. Thereafter, uranium and thorium concentrations in zircon were measured by NAA and revealed that zircon contained 94-141 ppm uranium and 127-506 ppm thorium. Therefore, this noticeable amount of uranium and thorium in zircon should be taken in consideration during any mining attempt of the placer minerals present in Cox's Bazar beaches.

Carbonatite melts derived from the mantle are enriched in CO2- and H2O-bearing fluids. This melt can metasomatize the peridotitic lithosphere and liberate a considerable amount of CO2. Experimental studies have also shown that a CO2–H2O-rich fluid can form Fe- and Mg-rich carbonate by reacting with olivine. The Sung Valley carbonatite of NE India is related to the Kerguelen plume and is characterized by rare occurrences of olivine. Our study shows that this olivine is resorbed forsterite of xenocrystic nature. This olivine bears inclusions of Fe-rich magnesite. Accessory apatite in the host carbonatite contains CO2–H2O fluid inclusions. Carbon and oxygen isotopic analyses indicate that the carbonatites are primary igneous carbonatites and are devoid of any alteration or fractionation. We envisage that the forsterite is a part of the lithospheric mantle that was reprecipitated in a carbonatite reservoir through dissolution–precipitation. Carbonation of this forsterite, during interaction between the lithospheric mantle and carbonatite melt, formed Fe-rich magnesite. CO2–H2O-rich fluid derived from the carbonatite magma and detected within accessory apatite caused this carbonation. Our study suggests that a significant amount of CO2 degassed from the mantle by carbonatitic magma can become entrapped in the lithosphere by forming Fe- and Mg-rich carbonates.

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The Sung Valley ultramafic–alkaline–carbonatite complex (UACC) of Meghalaya, NE, India, is a result of magmatic activity related to the Kerguelen mantle plume spanning from 101 to 115 Ma. In the present study, an integrated crystal size distribution (CSD), mineral chemistry, and melt inclusion analysis are carried out on the ijolites present within this UACC. The CSD analysis shows that these ijolites were formed in multiple stages through changes in the crystallization environment, such as cooling and nucleation rates. Raman spectroscopy of mineral inclusions of rutile, aphthitalite, apatite, carbonate–silicate melt inclusions, and disordered graphite within clinopyroxene and titanite, respectively, indicates a heterogeneous composition of the parental magma. These mineral and melt inclusion phases further suggest localized changes in oxygen fugacity (fO2) due to redox reactions in the lower crust. SEM–EDX analysis of the exposed melt inclusions reveals the presence of alkali-bearing diopside, phlogopite, and andradite, along with an unidentified carbonated silicate daughter phase. The studied melt inclusions are dominated by carbonate, whereas silicates are subordinate. The presence of this fully crystallized carbonate–silicate melt as calcite, diopside, phlogopite, magnetite, apatite, and andradite suggests the presence of “nano-calciocarbonatites” in these ijolites. Our study provides insights into different mechanisms of the loss of alkalies from initially entrapped alkaline carbonate melt in clinopyroxenes. The predominant occurrence of calcite as the only carbonate phase in the studied melt inclusions is a result of silicate–carbonate melt immiscibility, calcite-normative system in these inclusions, dealkalization of the alkaline carbonates in the presence of external fluid, and/or redistribution of the alkalies to the daughter alkali-bearing silicates.