Unusual olivine zoning ? Evidence for complex physico-chemical changes during the evolution of olivine melilitite and kimberlite magmas

Abstract

Olivine melilitites from Namaqualand, South Africa are characterized by a broad range in olivine compositions on the scale of individual hand specimens. It is possible to distinguish four petrographically and chemically distinct olivine populations in both the northern and southern pipe clusters studied: (a) Scarce anhedral or subhedral olivines that display marked disequilibrium features with the surrounding matrix, and which are characterized by having high iron and extremely low nickel contents (referred to as HILN olivines) relative to the other olivines in the same rock, (b) A dominant population of euhedral and often skeletal (hopper) olivines that are richer in Mg and Ni than the HILN olivines in the same rock. There are in addition ‘unusual’ hopper olivines that are petrographically similar to the skeletal olivines, but show aberrant zonation patterns. Hopper and HILN type olivines contain fluid and carbonate inclusions which apparently record the loss of a vapour phase and an immiscible carbonate liquid during magma ascent, (c) A third population consists of large rounded olivines (megacrysts), up to 40 mm in greatest diameter. Individuals are chemically homogeneous, but megacrysts from the same pipe collectively define a trend of decreasing Mg and Ni (Fo92, 0.36% Ni to Fo75, 0.17% Ni). The most fayalitic megacrysts are depleted in Mg and Ni relative to the hopper olivines in the same rock, (d) Scarce magnesium-rich (Fo91) anhedral olivines which show strained extinction are believed to be xenocrysts.