Dissolving Macrocrists of Olivine in a Kimberlite Melt at High <i>P-T </i>Parameters
Olivine is found in kimberlite in the form of two types of phenocrysts: olivine I in the form of relatively large grains of round or irregular shape and olivine II in the form of small (up to 0,5 rarely 1 mm) idiomorphic crystals, but having irregularly shaped cores. It is believed that type I olivine xenocrysts appear due to the disintegration of nodules of mantle rocks, and their specific morphology is results by the processes of dissolution and / or abrasion of grains in kimberlite (or even in protokimberlite) magma. The determination of the composi tion of the initial kimberlite magma and, ultimately, the concept of the genesis of the kimberlites themselves, depends on the solution of this question. It was previously experimentally established that among silicate minerals from mantle xenoliths, olivine is most stable when dissolved in kimberlite melt at high temperatures and pressures. However, the occurrence of a specific rounded form of olivine grains which were dissolved in a kimberlite melt was not experimentally confirmed yet. This report presents the first results of experiments on this topic. The experiments were carried out on a pressless multi-anvil high-pressure apparatus of the “split sphere” type (BARS). A series of four experiments was carried out at a pressure of 4 GPa, temperatures of 1300 °С and 1500 °С, with a duration of 1 and 2 hours. Kimberlite N-1 from Nyurbinskaya pipe (Yakutia, Russia) was used as a starting sample. Olivine grains were extracted from the nodules of spinel lherzolite from alkaline basaltoids of Mongolia. Following the hypothesis of the xenogenic nature of olivine phenocrysts, the composition of the proto-kimberlite magma should be enriched with iron in comparison with the composition of kimberlite. In addition, a substantially carbonatite composition of the protokimberlite magma is assumed. Therefore, Fe2O3 and CaCO3 are added to the kimberlite samples. The results of the experiments show that the olivine xenocrysts used are not in equilibrium with the N-1 molten kimberlite enriched with iron and calcium. On the contrary, the kimberlite melt is an aggressive medium capable of dissolving the xenocrysts of magnesian olivine. At the same time, their initially irregular shape is transformed into a rounded one, which is also characteristic for natural kimberlites. The large, rounded xenocrysts of kimberlite olivine are also often contains many cracks. In our experiments, this phenomenon probably occurred even at the stage of pressure increase due to the existing stress in the olivine crystals. After that, when the kimberlite powder was melted, the melt penetrated into these cracks and disintegrated the xenocrysts into separate fragments. A similar process may have occurred in natural objects under pressure gradient conditions during the rising of kimberlite magma. Thus, it was experimentally confirmed the possibility of the formation of rounded phenocrysts of olivine during their interaction with a carbonated melt.