Oxidation States of Fe in Constituent Minerals of a Spinel Lherzolite Xenolith from the Tariat Depression, Mongolia: The Significance of Fe3+ in Olivine

Abstract

The oxidation states of Fe within olivine, orthopyroxene, clinopyroxene, and spinel in a spinel lherzolite xenolith from the Tariat Depression, Mongolia were investigated using 57 Fe Mössbauer spectroscopy to evaluate the redox condition of the upper mantle from which the Tariat spinel lherzolite xenolith was derived. The purity of separated minerals for the Mössbauer spectroscopic analysis was examined using X-ray powder diffraction, Raman spectroscopy, and transmission electron microscopy. Average Fo and Fe contents of olivine at the core part of the xenolith are 89.9(4) mol % and 0.195(3) atoms per formula unit, respectively. The Fe 3+ /ΣFe values of the olivine, orthopyroxene, clinopyroxene, and spinel, determined by Mössbauer spectroscopic analysis, are 0.027(2), 0.15(1), 0.26(3), and 0.34(5), respectively. The Mössbauer spectrum of olivine consists of two doublets assigned to Fe 2+ at the octahedral sites and one doublet, with I.S. of 0.40(2) mm/s and Q.S. of 0.69(3) mm/s assigned to Fe 3+ at the octahedral site. Since the Tariat spinel lherzolite xenolith in this study shows no evidence of metasomatism or thermal alteration, the existence of a small amount of Fe 3+ in olivine and the fairly high Fe 3+ contents of clinopyroxene, orthopyroxene, and spinel imply that the upper mantle under the Tariat area was in a rather oxidized condition.

Effect of redox on Fe–Mg–Mn exchange between olivine and melt and an oxybarometer for basalts

“…(1) is the need to know the Fe 3+ content of the silicate melt, which is not readily measurable by conventional electron microprobe techniques (see review by Hughes et al 2018). Olivine contains negligible Fe 3+ (less then a few thousand ppm and not more than a few percent of the total Fe, Fe T ; Ejima et al 2018) so that where Fe 3+ in the melt is low, i.e., in relatively reduced systems, Kd Fe 2+ −Mg can be used with confidence assuming that Fe 2+ = Fe T . In such cases (1) Fe 2 SiO 4 (melt) + Mg 2 SiO 4 (olivine) = Fe 2 SiO 4 (olivine) + Mg 2 SiO 4 (melt)…”

A synthesis and meta-analysis of the Fe chemistry of serpentinites and serpentine minerals

“…Studies on the oxidation state of Fe in the primary minerals show Fe(III)/Fe T in clinopyroxene > orthopyroxene > olivine though orthopyroxene often contributes more Fe(III) overall because it is more Fe rich than clinopyroxene (e.g. [66][67][68]). The data presented here show that serpentinites with harzburgitic protoliths have significantly less FeO than those with dunitic and lherzolitic protoliths (figure 1c).…”

Editorial for Special Issue “Petrology, Geochemistry and Mineralogy of the Mantle as Tools to Read Messages from the Earth’s Interior”

“…This Special Issue contains nine articles [1][2][3][4][5][6][7][8][9]. Seven of them [1][2][3][4][5][6]8] deal with mantle-derived rocks, which have played an essential and important role in providing us with direct information about the chemistry and processes that take place in the deep part of Earth [10,11]. Their geochemistry and mineralogy give us excellent tools to unravel various mantle processes, i.e., partial melting, melt extraction, melt-rock interaction, metallogeny, metasomatism and metamorphism.…”

“…Their geochemistry and mineralogy give us excellent tools to unravel various mantle processes, i.e., partial melting, melt extraction, melt-rock interaction, metallogeny, metasomatism and metamorphism. Five papers [1,3,4,6,8] describe and discuss mantle-derived peridotites from Asia. Interesting new data are presented regarding the origin of mantle wedges and the nature of ophiolites [1,3,4,6].…”

“…Ejima et al [8] examined the redox state of the upper mantle using the ferric iron content of olivine in mantle-derived peridotite xenoliths from a rift zone in Mongolia. This method is different from the conventional oxygen barometer based on olivine-orthopyroxene-spinel equilibrium [29].…”