Some igneous rocks such as carbonatites and ultramafic lamprophyres have primary magmatic carbonates as their essential mineral constituent, similarly many other rocks including kimberlites, lamproites, lamprophyres,etc. also possess primary magmatic carbonates as accessory phases. However, many a times recrystallized secondary carbonates occur as veins and segregations in igneous rocks and also as cavity filling. Therefore, it is immensely important to make sure whether the carbonate is primary or secondary while studying the rocks mentioned above.
To identify igneous carbonate in carbonatite usually requires several methods to be certain. Optically, calcite crystals may show a variety of euhedral forms, but igneous zoning is usually rare, although sometimes visible in cathodoluminescence. When analysed by electron microprobe, trace elemnts are often characteristic and usually include, measurable strontium up to the percent level (this is not usually the case in sedimentary rocks). Sometimes the carbonates are associated with elevated rare earth elements, although these are more likely to be contained in accompanying apatite or other REE minerals like pyrochlore or perovskite. Isotopic features include oxygen isotopes close to the field defined for primary carbonatites (d18O 6 to 9 per mil), and carbon isotopes close to mantle values (d13C -5 to -7.5 per mil).
An old but useful reference illustrating igneous calcite and apatite morphologies including cathodoluminescence is here :
http://www.minersoc.org/pages/Archive-MM/Volume_55/55-381-591.pdf
A recent review of carbonatites is available for download for free at http://www.minsocam.org/msa/rim/RiM75/RIM075_C10_novideo.pdf
Thank you Sir. This surely is a great help. I have some good cases where establishing primary and secondary status of carbonates is crucial.
An increased phosphorus content is diagnostic for carbonatite melts. We have described a situation, when Sr-rich aragonite phenocrysts are associated with Mg-rich calciocarbonatite melt. See our paper published in American Mineralogist 2013, vol. 98, p. 1074. No phosphorus is present in secondary carbonatites. Note that our carbonatites had stable isotope ratios different from those in primary igneous carbonatites, partly due to explosive devolatilization of the calciocarbonatite melt, partly due to its origin in subducted slab of oceanic crust.
Geochemical tests are almost always diagnostic for carbonates crystallised from melts like carbonatite, but sometimes petrographic textures can also be clear, for example as interlocking poikilitic calcite grains around fresh olivine in kimberlites (as demonstrated isotopically with the ion probe reference 1) . More rarely you may identify distinctive feathery-textured calcite which has quenched from a carbonate liquid, as seen in some impact deposits or commonly in high pressure laboratory experiments (reference 2) , or as immiscible segregations as in some lamprophyres , or smaller microscopic scale as rare inclusions in mantle peridotite xenoliths (reference 3).
REFERENCES 1; (Exley RA and Jones AP Contrib Mineral Petrol (1983) volume 83, pp 288-292;
2: http://link.springer.com/article/10.1007%2FBFb0027767/lookinside/001.png from Jones AP et al Impacts and the Early Earth Lecture Notes in Earth Sciences Volume 91, 2000, pp 343-361
3: Ionov A et al Contributions to Mineralogy and Petrology (1996) Volume 125, pp 375-392
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