Abstract:

Few“Immiscible small balls”(called “balls”)were discovered in kimberlite heavy mineral concentrates. These balls are dominantly <1mm in diameter and have isotropic structure except few balls with microlite, which implies that they are quenching products from kimberlitic melts. Twenty-nine major element compositions and three trace element analyses are provided in this paper. According to the major element compositions, three types of balls can be divided:(1)high Fe-Ti balls;(2)S-Fe-Ni balls;(3)light-colored sialic balls. The high Fe-Ti balls have the highest FeO(up to 99.39%)and TiO₂(up to 45.90%)contents. They also have relatively high MnO contents(up to 23.75%). Fe, Ti, and Mn elements have high electro negativity, which have penitential to form very strong bone with oxygen in melts. Thus they are easier to separate from the host melts through the immiscibility. The S-Fe-Ni balls have SO₃, FeO and NiO contents ranging from 38.27%-51.95%, 0.31%-23.10%，and 25.24%-61.05%, respectively. SiO₂, Al₂O₃ and CaO oxides are dominant components in light-colored sialic balls with the SiO₂ contents in a range of 24.01%-52.64%, corresponding to the compositions of basic to ultra-basic magmas. Characteristics of major and trace elements and the discovery of olivine xenocryst(Fo=0.95)in a S-Fe-Ni ball demonstrate that these balls were formed from kimberlitic magma. The occurrence of twophase(high-Fe and sialic)intergrown ball illustrates that high-Fe and sialic balls were produced from magma immiscibility. The formation of three type balls can be clearly explained with the SiO₂-FeS-FeO phase diagram. We consider that immiscible small balls were formed at late stage of magma crystallization with decreasing temperature and pressure. The magma enriched in CO₂ 、SO₃、FeO、TiO_2、MnO uplift and chilled very rapidly to form many local order areas in melts that produced these balls.

Key words: immiscible small balls, kimberlite, high Fe-Ti ball, S-Fe-Ni ball, light-colored sialic ball, magma immiscibility