Extremely magnesian olivine in igneous rocks

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 Tolbachik Volcano (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.

Jianite: Massive Dunite Solely Made of Virtually Pure Forsterite from Ji’an County, Jilin Province, Northeast China

“…Further, the magmas derived from this partial melting process are incapable of crystallizing at shallow depths more refractory olivine (with Mg# higher than that of the corresponding residual olivine in the source regions) due to the Fe-Mg partitioning equilibrium [12,13]. Consequently, natural olivine with Mg# > 96 is not typical for any igneous rocks in general [14]. Natural olivine crystals with nearly pure forsteritic composition (with Mg# > 96) were sometimes found as phenocrysts in some basalts and kimberlites [15,16], as inclusions in chromites from some podiform chromitite [17], and as one of the metamorphic minerals in the contact zone between an igneous rock and a carbonate rock [18][19][20].…”

Fingerprints of Kamafugite-Like Magmas in Mesozoic Lamproites of the Aldan Shield: Evidence from Olivine and Olivine-Hosted Inclusions

“…Elevated Mg# is believed to result from crystallization or equilibration in highly oxidized conditions. It has been shown that under high oxygen fugacity, magmatic olivine becomes almost pure forsterite due to trivalent Fe preventing substitution of Mg 2+ within the olivine crystal lattice [56,69]. Highly oxidized conditions of olivine-3 crystallization are consistent with low V/Sc ratios ( Figure 10B).…”

“…Elements, which distribution was mapped are specified on each inset. [56] and [57]. Specified on insets (A-L) are parameters, which are plotted on a certain chart, and total number of the analyses.…”

“…Partly homogenized melt inclusions both olivine-1 and olivine-3 appear as light brown glasses with a gas bubble ( Figure 9C,D) and sometimes with Cr-spinel micro-grains (<1 µm). The size of analyzed inclusions ranges from 5-20 µm (generally [56] and [57]. Specified on insets (A-L) are parameters, which are plotted on a certain chart, and total number of the analyses.…”

“…Regardless, due to the geological setting of the Ryabinoviy pipe, such an origin is unlikely in this case. Mantle xenocrysts, derived from extremely depleted lithospheric mantle, can possess Mg# up to 95.5 mole % [56], but do not contain such anomalously high concentrations of CaO (Figures 7 and 8). Therefore, features of olivine-3 are best attributed to a magmatic process.…”