Speculations on the generation and movement of komatiites

McKenzie, D. P. (2020) Speculations on the generation and movement of komatiites. Journal of Petrology. ISSN Print ISSN 0022-3530 - Online ISSN 1460-2415 (In Press)

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Abstract

The discovery of komatiites, first in South Africa and then in many other Archean greenstone belts,with MgO concentrations of 20-30% and eruption temperatures of more than∼1600◦C, showed thatsome parts of the mantle were hotter in the Archean than they are now. Since their discovery therehave been many speculative proposals as to how such magmas can form. At present melt is producedby mantle upwelling, because the solidus temperature gradient of the mantle is steeper than that ofisentropic decompression gradient at depths of less than 300 km. In contrast, in the lower half of theupper mantle the solidus gradient is shallower than the isentropic gradient, and therefore isentropicupwelling cannot generate melt. At the base of the upper mantle limited melting can occur, eitherin the thermal boundary layer at the base of the upper mantle, or in the upper part of the lowermantle where the solidus gradient is steeper than the isentropic gradient. In both cases melting canoccur at depths of more than 600 km, where Ca perovskite, CaPv, is a stable phase on the solidus.A surprising feature of the partitioning between melt and solid CaPv is that most trace elements arecompatible in the solid. Partitioning into CaPv can therefore account for the low concentrations ofsuch elements in komatiites. The temperatures required to generate such magmas in plumes needbe no more than∼50◦C above those of Phanerozoic plumes. The presence of komatiites in theArchean therefore requires plume temperatures in the first half of the Earth’s history to have beensomewhat hotter than they are now, but does not constrain the average temperature of the Archeanupper mantle.

Item Type: Article
Uncontrolled Keywords: 2020AREP; IA76
Subjects: 02 - Geodynamics, Geophysics and Tectonics
Divisions: 02 - Geodynamics, Geophysics and Tectonics
08 - Green Open Access
Journal or Publication Title: Journal of Petrology
Depositing User: Sarah Humbert
Date Deposited: 30 Jun 2020 00:25
Last Modified: 30 Jun 2020 00:26
URI: http://eprints.esc.cam.ac.uk/id/eprint/4772

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