Helium, neon and argon in alkaline basalt-related corundum megacrysts: Implications for their origin and forming process

Abstract

Corundum megacrysts from Cenozoic alkaline basaltic fields, notably in eastern Asia and Australia, are economically important but poorly understood in their genesis. Here we present new crushing noble gas (He, Ne and Ar) data of alkali basalt-related corundum and some other mineral megacrysts from several localities worldwide to better constrain their ultimate fluid (material) source and forming process. Markedly, the corundum megacrysts have widespread mid ocean ridge basalts (MORB)-like 3He/4He ratios (8 ± 1 RA, where RA is atmospheric 3He/4He ratio of 1.39 × 10−6) in each locality, although lower than MORB 3He/4He ratios also occur in some cases. The latter could be interpreted as the addition of ingrowth 4He but not reflect the initially trapped compositions. The initially MORB-like 3He/4He and high 40Ar/36Ar isotopic ratios, combined with major element and fluid inclusion data, support that the asthenosphere-derived melts/fluids are the primary source for their formation, which is genetically related to the generation of the Cenozoic continental alkaline basalts and mafic megacrysts like clinopyroxene and garnet. Elemental fractionation between helium and argon, enriched in helium compared to the current mantle production value, was observed in most blue-green-yellow (BGY) sapphires, notably for the samples from Changle, eastern China and Ban Huai Sai, Laos. However, the blue sapphires from Wenchang, China do not show such an enrichment. The elevated 4He/40Ar* (40Ar* is radiogenic Ar corrected for atmospheric Ar) ratios are interpreted as a solubility-controlled fractionation during magma degassing. Based on the degree of fractionation, the degassing scheme is most likely to be a single-stage process in closed systems (e.g., in a magma chamber), and the original volatile (assuming pure CO2) content of the parental melts before vesiculation is estimated to be ∼3.5 wt.% in minimum. The degassing process may have caused a sudden Al-saturation of the parental magma and facilitated the crystallization of sapphires by losing alkalis from the magma and decreasing the system's temperature. However, the “degassing” model, so far, has limitations to interpret the Wenchang data. After crystallization of the BGY sapphires at depths of deep to middle crust levels, a new pulse of alkaline basaltic melt carries them to the surface.