Komatiites, kimberlites, and boninites

Abstract

[1] When the mantle melts, it produces ultramafic magma if the site of melting is unusually deep, the degree of melting is unusually high, or the source is refractory. For such melting to happen, the source must be unusually hot or very rich in volatiles. Differing conditions produce a spectrum of ultramafic magma types. Komatiites form by high degrees of melting, at great depths, of an essentially anhydrous source. Barbertontype komatiites are moderately high degree melts from a particularly hot and deep source; Munro-type komatiites are very high degree melts of a slightly cooler source. Kimberlites result from low-degree melting, also at great depth, of sources rich in incompatible elements and CO 2 + H 2 O. They become further enriched through interaction with overlying asthenospheric or lithospheric mantle. Boninites form by hydrous melting of metasomatized mantle above a subduction zone. Just like basalts, the different types of ultramafic magma, and the conditions in which they form, are readily identified using major and trace element criteria.

The Early Permian Tarim Large Igneous Province: Main characteristics and a plume incubation model

“…The magnitudes of thermal anomaly range up to 350°C (in the plume axis area) to~100°C (in the margin of plume head) (Campbell, 2005(Campbell, , 2007. High-temperature mantle is believed to be responsible for the occurrence of high magnesian rocks such as komatiites and picrites (Arndt, 2003).…”

Extracting low frequency climate signal from GRACE data

“…Independent evidence for the process, which could be described as mantle "defertilization", was presented by Arndt (2003) who attributed the high MgO contents in a global suite of kimberlites to the addition of olivine alone, without the addition of orthopyroxene and other minerals that should have been present in xenoliths from the lithospheric mantle.…”

“…If the interaction took place entirely at sub-lithosphere depths, the kimberlite will acquire an asthenospheric isotopic signature; if it continued into the lithosphere, the isotopic signature will reflect this interaction. The CO 2 content of the parental magma might control the depth interval over which such interaction takes place and influence the isotopic compositions of kimberlites (see Becker and Le Roex, 2006) for a recent summary of the isotopic compositions of kimberlites). In Group I kimberlites, which have asthenospheric isotopic signature and high CO 2 contents, exsolution might start at depths near the base of the lithosphere driving the magma rapidly to the surface and precluding subsequent interaction with the wall rocks.…”

Terrestrial aftermath of the Moon-forming impact

“…The hottest Archaean komatiites were probably the product of mantle plumes, indicating that Archaean mantle temperatures were heterogeneous. The fraction of partial melting within Archaean plumes was up to approximately 50% mantle [50], enough to form slurry if globally present.…”

“…[48,49]). Evidence of heterogeneous mantle temperatures is then provided by Archaean igneous rocks, but there are no relevant Hadean data [48][49][50].…”