Sulfide inclusions in alkali basalt-associated garnet megacrysts shed light on the mysterious megacryst nature

We have carefully studied an unusual sulfide-bearing garnet megacryst from the ever-surprising Cenozoic Shavaryn-Tsaram basaltic cone (Tariat Platou, Mongolia). Similar sulfide inclusions in minerals constituting mantle xenoliths and clinopyroxene megacrysts related to alkali basalts were already known (Peterson and Francis, 1977, Chaussidon et al, 1989, Ionov et al, 1992) but they have never been found in garnet megacrysts. Since these garnets are believed to be mantle-derived material, their sulfide inclusions provide information on the deep sulfur cycle.

The sulfide-rich garnet megacryst from Shavaryn Tsaram pyroclastic strata is a chip of a large (up to 3 cm) cracked and partly quenched glassy crystal (fig. 1A, fig.1B) with melt pockets (Aseeva et al, 2021) inside (fig. 1C).

 

Sulfide inclusions are primary, isometric, elongated, and orientated towards crystal growth with a distinctive arrangement (3D X-ray images, Skyscane, fig. 2A). Swarms of inclusions contour the growth planes typical for the deltoidal icositetrahedron (fig. 2B).

Sulfide inclusions mainly consist of Ni-bearing pyrrhotite (1.66-2), scarce chalcopyrite (fig.3A and B), and rarely of pentlandite. Incompletely crystallized droplets of MSS (monosulfide solid solution) occur periodically as thin crystal pyrrhotite and pentlandite intergrowths (fig. 3C). These MSS inclusions are thought to be a product of the sulfide melt exsolution caused by undercooling (Chaudison et all, 1989).

The multi-isotope sulfur composition of these sulfide inclusions has been studied to define whether the sulfur source is crustal or mantle-derived. Thus, their δ³⁴S values account for 0.2-0.4‰, δ³³S for 0.1-0.2‰, and Δ³³S for 0.00-0.03‰, which is characteristic of mantle, meteoric, MORB, and volcanic settings. As for the host garnet, its oxygen isotope composition (Δ¹⁸О 5.4 to 5.8‰) also suggests the volcanic origin of these sulfides.

Submicron surface analysis (Bruker Dimension Icon and Solver NT-MDT) reveals the linear-globular structure of garnet (fig. 4A). Being probable nuclei, nearly 1 μm globules compose layers of garnet. We assume that garnet crystal formed via epitaxial growth from the gas phase. Garnet megacryst linear structures consisting of globules differ significantly from the metamorphic garnet crystal lattice (fig. 4B). Sulfur redundancy causes sulfide droplets, immiscible with silicate material (fig. 4C), to gather and form bulbs on top of a growing crystal due to surface tension (fig. 4C). 

The following conclusions may be drawn: 1. Sulfide inclusions in alkali basalt-associated garnet megacrysts are primary. 2. Sulfides hosted in garnet are mantle-derived according to isotopic data. 3. Garnet megacryst formation was caused by epitaxial growth.