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 that some parts of the mantle were hotter in the Archean than they are now. Since their discovery there have been many speculative proposals as to how such magmas can form. At present melt is produced by mantle upwelling, because the solidus temperature gradient of the mantle is steeper than that of isentropic decompression gradient at depths of less than 300 km. In contrast, in the lower half of the upper mantle the solidus gradient is shallower than the isentropic gradient, and, therefore, isentropic upwelling cannot generate melt. At the base of the upper mantle limited melting can occur, either in the thermal boundary layer at the base of the upper mantle, or in the upper part of the lower mantle where the solidus gradient is steeper than the isentropic gradient. In both cases melting can occur 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 are compatible in the solid. Partitioning into CaPv can, therefore, account for the low concentrations of such elements in komatiites. The temperatures required to generate such magmas in plumes need be no more than ∼50 ^°C above those of Phanerozoic plumes. The presence of komatiites in the Archean, therefore, requires plume temperatures in the first half of the Earth’s history to have been somewhat hotter than they are now, but does not constrain the average temperature of the Archean upper mantle.

中文翻译：

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关于科马提人的产生和运动的推测

摘要

首先在南非，然后在许多其他太古代绿岩带中发现科马铁矿，MgO浓度为20–30％，喷发温度超过1600  ^°CC表示，在太古代，地幔的某些部分比现在更热。自从他们被发现以来，已经有许多关于这种岩浆如何形成的推测。目前，由于地幔上升流所产生的熔体是因为在小于300 km的深度处，地幔的固相线温度梯度比等熵减压梯度的固相线温度梯度陡。相反，在上地幔的下半部分，固相线梯度比等熵梯度浅，因此，等熵上升流不会产生熔体。在上地幔的底部，有限的熔化可能发生在上地幔的底部的热边界层中，或者在下地幔的上部，那里的固相线梯度比等熵梯度陡。在这两种情况下，熔化都可能发生在600公里以上的深度，其中钙钛矿CaPv是固相线上的稳定相。熔体和固体CaPv之间分配的一个令人惊讶的特征是，大多数痕量元素与固体相容。因此，分配到CaPv中可解释科马铁矿中此类元素的低浓度。产生羽状岩浆所需的温度不超过约50 ^° C高于生代烟羽。因此，在太古代中存在科马铁矿，这就要求地球历史上半年的羽流温度比现在要高一些，但并不限制太古宙上地幔的平均温度。