Ocean-island basalt-like source of kimberlite magmas from West Greenland revealed by high 3He/4He ratios

摘要

The depth of origin of kimberlite magmas remains controversial. Hypothesized sources include the asthenosphere, the lower part of the upper mantle, and the lower mantle. Based on Sr-Nd isotope systematics, kimberlites are divided into two groups. Group 1 kimberlites have Sr-Nd isotope compositions similar to bulk earth. These compositions are consistent with a kimberlite source that is either a relatively unfractionated ocean-island basalt (OIB)–like source, or a mixture of depleted mid-oceanic-ridge basalt and enriched mantle components. In order to distinguish between these two possibilities, we performed noble gas analyses on olivine crystals separated from extremely fresh kimberlites from South and West Greenland. We detected OIB-like (Loihi-type) high 3He/4He ratios of up to 26.6 R_A in kimberlites. This clearly indicates that group 1 kimberlite magmas have a source similar to that of mantle plumes, implying that the source of kimberlite magma is in the lower mantle.