Variety of Iron Oxide Inclusions in Sapphire from Southern Vietnam: Indication of Environmental Change during Crystallization

Doan Thi Anh Vu,Alongkot Fanka,Chakkaphan Sutthirat,Abhisit Salam

doi:10.3390/min11030241

Abstract

Sapphires from alluvial deposits associated with Cenozoic basalts in Southern Vietnam were collected for investigation of mineral inclusions. In this report, primary iron oxide inclusions were focused on, with detailed mineral chemistry using a Raman spectroscope and electron probe micro-analyzer. Consequently, a variety of iron oxide inclusions were recognized as wüstite, hercynite, and ilmenite. Ilmenite falling within an ilmenite–hematite series ranged in composition between Il24-30He36-38Mt35-40 and Il49-54He34-40Mt7-10, classified as titanomagnetite and titanohematite, respectively. Wüstite with non-stoichiometry, (Fe2+0.3-0.9)(Ti3+<0.179Al3+≤0.6Cr3+<0.1Fe3+≤0.46)☐≤0.23O, was associated with hercynite inclusions, clearly indicating cogenetic sapphire formation. Wüstite and sapphire appear to have been formed from the breakdown reaction of hercynite (hercynite = sapphire+wüstite) within a reduction magma chamber. Titanohematite and titanomagnetite series might have crystallized during iron–titanium reequilibration via subsolidus exsolution under a slightly oxidized cooling process.

Highlights

Iron oxide minerals have been considered as a significant geothermometer for their host rocks [1,2,3,4,5,6]
Izokh et al [7] reported an iron oxide inclusion, namely, Al–Ti–hematite, that was chemically analyzed by an electron probe micro-analyzer (EPMA); subsequently, they proposed that crystallization of host sapphire should relate to iron-rich syenitic melt and metasomatism between crustal rocks and contaminated basaltic melt in the Dak Nong deposit
The crystallization of sapphire and related zircon from Southern Vietnam may occur in the lithospheric mantle which is related to carbonatite-dominant melts as a result of partial melting of a metasomatized lithospheric mantle source, at over 900 ◦C [10]

Summary

Introduction

Iron oxide minerals have been considered as a significant geothermometer for their host rocks [1,2,3,4,5,6]. Previous investigations of gem sapphire from Southern Vietnam, such as Dak Nong, Di Linh, and Binh Thuan deposits, have reported several iron oxide inclusions [7,8,9]. Most of these iron oxides were identified as ilmenite, magnetite–hercynite, and chromite–hercynite, using a scanning electron microscope–energy dispersive spectrometer (SEM-EDS) and an X-ray diffractometer (XRD) [8]. Izokh et al [7] reported an iron oxide inclusion, namely, Al–Ti–hematite, that was chemically analyzed by an electron probe micro-analyzer (EPMA); subsequently, they proposed that crystallization of host sapphire should relate to iron-rich syenitic melt and metasomatism between crustal rocks and contaminated basaltic melt in the Dak Nong deposit. The crystallization of sapphire and related zircon from Southern Vietnam may occur in the lithospheric mantle which is related to carbonatite-dominant melts as a result of partial melting of a metasomatized lithospheric mantle source, at over 900 ◦C [10]

Results

Discussion

Conclusion