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POLYMORPHS OF CALSIUM CARBONATE IN MANTLE XENOLITHS FROM KIMBERLITES OF THE SIBERIAN CRATON

А. В. Головин

Abstract

Aragonite and calcite were identified among the daughter phases within secondary melt inclusions in rock-forming olivines and among interstitial minerals in sheared garnet peridotite xenoliths from kimberlites of the Udachnaya diamond deposit (Siberian Craton, Russia). This type of the xenoliths are the deepest mantle rocks sampled by kimberlite magma from 180?230 km depth. According to experimental data, aragonite is a high-pressure polymorph of CaCO3, which is stable at upper mantle pressures and temperatures. Thereby aragonite is used as indicator of high P-T parameters in studies of magmatic and ultrahigh-pressure metamorphic rocks. Here we report the first finding of aragonite in the mantle xenoliths from kimberlites worldwide. Calcium carbonate in mantle xenoliths were identified initially by composition using scanning electron microscopy. Then Raman spectroscopy was used to determine CaCO3 polymorphs. Both polymorphs are characterized by strong Raman band at 1086 cm?1 and medium band at 283 cm?1. However, aragonite has additional three medium Raman bands at 154, 208 and 704 cm?1 as well as three weak bands at 181, 191 and 1462 cm?1, whereas calcite has two medium bands at 156 and 713 cm?1 and one weak band at 1436 cm?1. High impurities of Na2O (0.4-1.2 wt%) and SrO were identified in both carbonates by electron probe microanalysis. However, aragonite has a higher SrO content (0.8-5.1 wt%) in comparison with calcite (0.6-0.1 wt%). Silicate metasomatism events of the upper mantle is considered as ordinary in the Earth?s history, but high-pressure experimental works also point to the important role of carbonate-rich melts in the mantle modifications. The presence of aragonite in the deepest mantle xenoliths is direct evidence of existence carbonatite/carbonate-rich melts in the deep mantle (?230 km) near lithosphere basement of ancient cratons. Such melts can percolate through the upper mantle rocks and should inevitably modify their mineralogy and element budgets of individual minerals. Alkali-rich primitive kimberlite melts are a good candidate on the role of mantle metasomatic agents for such type of mantle rocks.

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