Abstract We studied melt inclusions of two types (MI-1 and MI-2) coexisting in cumulus olivine Fo91–92 of ultramafic rocks from the Uitkomst layered intrusion, which is part of the Bushveld Magmatic Province. The inclusions differ in the association of daughter phases, their composition and behavior during heating, and temperature of partial homogenization. The main daughter phases of fully crystallized equigranular melt inclusions of type MI-1 are olivine and orthopyroxene. Some inclusions were not completely homogenized even at the maximum experimental temperature of 1450°C, and their melts contained up to 22.5 wt % MgO, 52 wt % SiO2, and minor amounts of CaO, alkalis, volatile components (including no more than 168 ppm H2O), and fluid-mobile elements. It was calculated that the primary magma in equilibrium with olivine Fo91 was generated at 1590°C and 2.5 GPa and corresponded to dry high-silica komatiite. The MI-1 inclusions coexist with type MI-2 melt inclusions of felsic composition. Among the daughter phases of MI-2, glass, apatite, amphibole, microgranophyric feldspar–quartz intergrowths, perovskite, titanite, ilmenite, and rutile were identified. The temperature of MI-2 entrapment was approximately 1063–1100°C, and their melt quenched under natural conditions contained up to 75 wt % SiO2, 10 wt % Na2O, and 0.6 wt % MgO on average. Raman spectroscopy indicated that the fluid in the inclusions is a methane–hydrogen mixture with admixtures of nitrogen, <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({\text{CO}}_{3}^{{2 - }}\)</EquationSource>  </InlineEquation>, OH– group, and liquid H2O and coexists with dispersed carbon (C). We considered possible mechanism of the formation of felsic melt in a flow-through magma chamber with chromite–olivine cumulate, which is replenished by magma portions with progressively increasing MgO content. The chemical compositions of the felsic melt enriched in alkalis and volatiles and associating fluid indicate their formation through the melting of a sedimentary (metapelitic) protolith. Dissolution and crystallization events in the zone of variable thermal gradient during multiple injections of fluid-saturated melt in the central part of the flow-through magma conduit did not affect olivine hosting two types of coexisting inclusions. The presence of inclusions of felsic anatectic xenomelt in highly magnesian olivine from the Uitkomst Complex exemplifies a universal mechanism of local contamination at the development of an intrusion.

Felsic Melt Inclusions in Olivine of Ultramafic Rocks of the Uitkomst Complex (Bushveld Province) as Evidence for Crustal Anatexis

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2026年7月22日

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I. P. Solovova,
M. A. Yudovskaya,
A. A. Averin,
J. Kinnaird

摘要

Abstract

We studied melt inclusions of two types (MI-1 and MI-2) coexisting in cumulus olivine Fo_91–92 of ultramafic rocks from the Uitkomst layered intrusion, which is part of the Bushveld Magmatic Province. The inclusions differ in the association of daughter phases, their composition and behavior during heating, and temperature of partial homogenization. The main daughter phases of fully crystallized equigranular melt inclusions of type MI-1 are olivine and orthopyroxene. Some inclusions were not completely homogenized even at the maximum experimental temperature of 1450°C, and their melts contained up to 22.5 wt % MgO, 52 wt % SiO₂, and minor amounts of CaO, alkalis, volatile components (including no more than 168 ppm H₂O), and fluid-mobile elements. It was calculated that the primary magma in equilibrium with olivine Fo₉₁ was generated at 1590°C and 2.5 GPa and corresponded to dry high-silica komatiite. The MI-1 inclusions coexist with type MI-2 melt inclusions of felsic composition. Among the daughter phases of MI-2, glass, apatite, amphibole, microgranophyric feldspar–quartz intergrowths, perovskite, titanite, ilmenite, and rutile were identified. The temperature of MI-2 entrapment was approximately 1063–1100°C, and their melt quenched under natural conditions contained up to 75 wt % SiO₂, 10 wt % Na₂O, and 0.6 wt % MgO on average. Raman spectroscopy indicated that the fluid in the inclusions is a methane–hydrogen mixture with admixtures of nitrogen, \({\text{CO}}_{3}^{{2 - }}\) , OH^– group, and liquid H₂O and coexists with dispersed carbon (C). We considered possible mechanism of the formation of felsic melt in a flow-through magma chamber with chromite–olivine cumulate, which is replenished by magma portions with progressively increasing MgO content. The chemical compositions of the felsic melt enriched in alkalis and volatiles and associating fluid indicate their formation through the melting of a sedimentary (metapelitic) protolith. Dissolution and crystallization events in the zone of variable thermal gradient during multiple injections of fluid-saturated melt in the central part of the flow-through magma conduit did not affect olivine hosting two types of coexisting inclusions. The presence of inclusions of felsic anatectic xenomelt in highly magnesian olivine from the Uitkomst Complex exemplifies a universal mechanism of local contamination at the development of an intrusion.