Mineralogy and Geochemistry of Ocelli in the Damtjernite Dykes and Sills, Chadobets Uplift, Siberian Craton: Evidence of the Fluid–Lamprophyric Magma Interaction

Anna A Nosova,Elena O Dubinina,Elena A Minervina,Ludmila V Sazonova,Alexey V Kargin

doi:10.3390/min11070724

Abstract

The study reports petrography, mineralogy and carbonate geochemistry and stable isotopy of various types of ocelli (silicate-carbonate globules) observed in the lamprophyres from the Chadobets Uplift, southwestern Siberian craton. The Chadobets lamprophyres are related to the REE-bearing Chuktukon carbonatites. On the basis of their morphology, mineralogy and relation with the surrounding groundmass, we distinguish three types of ocelli: carbonate-silicate, containing carbonate, scapolite, sodalite, potassium feldspar, albite, apatite and minor quartz ocelli (K-Na-CSO); carbonate–silicate ocelli, containing natrolite and sodalite (Na-CSO); and silicate-carbonate, containing potassium feldspar and phlogopite (K-SCO). The K-Na-CSO present in the most evolved damtjernite with irregular and polygonal patches was distributed within the groundmass; the patches consist of minerals identical to minerals in ocelli. Carbonate in the K-Na-CSO are calcite, Fe-dolomite and ankerite with high Sr concentration and igneous-type REE patterns. The Na-CSO present in Na-rich damtjernite with geochemical signature indicates the loss of the carbonate component. Carbonate phases are calcite and Fe-dolomite, and they depleted in LREE. The K-SCO was present in the K-rich least-evolved damtjernite. Calcite in the K-SCO has the highest Ba and the lowest Sr concentration and U-shaped REE pattern. The textural, mineralogical and geochemical features of the ocelli and their host rock can be interpreted as follows: (i) the K-Na-CSO are droplets of an alkali–carbonate melt that separated from residual alkali and carbonate-rich melt in highly evolved damtjernite; (ii) the Na-CSO are droplets of late magmatic fluid that once exsolved from a melt and then began to dissolve; (iii) the K-SCO are bubbles of K-P-CO2 fluid liberated from an almost-crystallised magma during the magmatic–hydrothermal stage. The geochemical signature of the K-SCO carbonate shows that the late fluid could leach REE from the host lamprophyre and provide for REE mobility.

Highlights

Alkali-rich silica-undersaturated melts commonly contain significant amounts of volatiles that play a key role in the magmatic and postmagmatic evolution of lamprophyres, including kimberlite and ultramafic massifs with carbonatite and their surrounding areas (e.g., [1,2,3])
Ocelli are interpreted as amygdales—as vesicles filled by late-stage minerals—or as products of silicate–carbonate or silicate–silicate liquid immiscibility ([5,6] and reference therein)
Ocelli are interpreted as products of silicate melt and carbonate liquid or carbonate-rich fluid immiscibility that occurs in lamprophyre magma (e.g., [9,10,11])

Summary

Introduction

Alkali-rich silica-undersaturated melts commonly contain significant amounts of volatiles that play a key role in the magmatic and postmagmatic evolution of lamprophyres, including kimberlite and ultramafic massifs with carbonatite and their surrounding areas (e.g., [1,2,3]). Most igneous rocks lack direct evidence of magmatic fluid presence (except microinclusions in minerals) because the fluid that had once been exsolved and physically separated from the parental magma rapidly left it. If exsolved fluids or liquids have not physically separated from the parental magma, they can redissolve, and immiscible textures are not preserved (e.g., [4]). Ocelli have been interpreted as preserved pockets of solidified exsolved immiscible late (residual) magmatic fluid (liquid or gas) [7,8]. Ocelli are interpreted as products of silicate melt and carbonate liquid or carbonate-rich fluid immiscibility that occurs in lamprophyre magma (e.g., [9,10,11])

Methods

Discussion

Conclusion