Abstract
Most eclogitic mantle xenoliths brought to the surface exhibit a certain degree of enrichment with incompatible elements, usually attributed to the effect of mantle metasomatism by a putative metasomatic fluid. The metasomatic overprint is represented mainly by enrichments in Na, K, Ba, Ti and LREE and the original source of this fluid remains unknown. In this paper, we present a detailed petrological study of a typical eclogitic mantle xenolith from the Roberts Victor kimberlite mine in South Africa. We find that its textural and mineralogical features present strong evidence for incipient melting. The melting assemblage we observe did not necessarily require introduction of additional components, that is: in-situ melting alone could produce highly incompatible element enriched melt without involvement of a hypothetical and speculative “metasomatic event”. Due to the higher abundance in incompatible elements and lower solidus temperature than peridotites, mantle eclogites, some of which represent previously subducted oceanic crust, are much more plausible sources of mantle metasomatism, but on the other hand, they can be considered as highly metasomatised themselves. This brings us to the “chicken or egg” dilemma – was the secondary mineral assemblage in mantle lithologies a result or a source of mantle metasomatism?
Highlights
Studies of metasomatism date back to Goldschmidt (1922), who defined it as “a process of alteration which involves enrichment of the rock by new substances brought in from the outside”
Introduction of “metasomatic” components affects the chemistry of the original mantle rock and decreases its solidus temperature, meaning that mantle metasomatism or enrichment in highly incompatible elements should be considered one of the main driving forces of mantle melting
Replacement textures and development of secondary, usually hydrous phases) and “cryptic” (with enrichment in trace and rare earth elements (REE), but without petrographic evidence of metasomatism, i.e. no secondary minerals are formed) (Dawson, 1984). These metasomatic overprints are usually so complex that they are considered to have been caused by multiple metasomatic events (e.g. Heaman et al, 2006; Huang et al, 2014; Misra et al, 2004)
Summary
Studies of metasomatism date back to Goldschmidt (1922), who defined it as “a process of alteration which involves enrichment of the rock by new substances brought in from the outside”. The two main types of metasomatic alteration, observed in most lithospheric mantle xenoliths, are “modal” (petrographically visible due to replacement textures and development of secondary, usually hydrous phases) and “cryptic” (with enrichment in trace and rare earth elements (REE), but without petrographic evidence of metasomatism, i.e. no secondary minerals are formed) (Dawson, 1984). These metasomatic overprints are usually so complex that they are considered to have been caused by multiple metasomatic events These metasomatic overprints are usually so complex that they are considered to have been caused by multiple metasomatic events (e.g. Heaman et al, 2006; Huang et al, 2014; Misra et al, 2004)
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