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Russian Geology and Geophysics

2018 year, number 12

EXTREMELY MAGNESIAN OLIVINE IN IGNEOUS ROCKS

P.Yu. Plechov1,2, V.D. Shcherbakov2, N.A. Nekrylov3,4
1Fersman Mineralogical Museum RAS, pr. Leninskii 18, str. 2, Moscow, 119071, Russia
2Lomonosov Moscow State University, Leninskie Gory 1, 119991, Moscow, Russia
3Institute of Experimental Mineralogy RAS, pr. Leninskii 18, str. 2, Moscow, 119071, Russia
4Institute of Experimental Mineralogy RAS, ul. Osypyana 4, Chernogolovka, 142432, Russia
Keywords: Оливин, форстерит, малые компоненты, окисление, десерпентинизация, переуравновешивание, Olivine, forsterite, minor components, oxidation, deserpentinization, re-equilibraton

Abstract

Published data on extremely magnesian olivine (>96 mol.% forsterite) in igneous rocks were generalized and compared with data of new high-precision electron probe microanalyses of olivine from oxidized lavas of the Volcano Tolbachik (Kamchatka), chromitites from the Ray-Iz deposit (Russia), alkaline ultrabasic lavas from San Venanzo volcanoes (Italy), and skarns from the Kuh-i-Lal deposit (Tajikistan). All the found olivines resulted from low-temperature processes, such as subsurface oxidation, interaction with carbonates, and subsolidus re-equilibration. Low-temperature formation of olivine is reflected in its structure (hematite lamellae and abundance of inclusions of ore minerals) and abnormal contents of minor components (Mn, Ni, and Ca). The Mg content of olivine increases under the influence of postmagmatic processes and can be manifested in different rocks. This gives grounds to refine the genesis of olivine of exotic composition (93-96 mol.% forsterite) in some kimberlites, komatiites, and peridotites.