Fluid Transport and Metasomatic Storage in the Mantle
Alkaline intra-plate magmatism ranges from felsic to ultramafic, with the latter melts showing extremely low SiO2 activity, carbonatite being associated throughout, and all having mantle signatures. In all its forms this magmatism shows evidence of enhanced volatile activity, ranging from the chemistry of the melts through to the style of eruption, which frequently takes the form of high velocity eruptions that entrain mantle xenoliths. Volatile activity is thus not a function of degree of differentiation (“evolution”) and must signify high activity in the source mantle. The paradox of primitive magmas rich in alkalis, volatiles, and incompatible elements has led to proposals for source enrichment. Much is now known about the chemistry of the magmas and mantle nodules, and in addition to the more obvious alkalis, hydrogen, carbon and sulphur, the magmatism requires provision of significant P, Ti and Fe, and especially Ca. Minerals containing these elements, such as phlogopite, amphibole, clinopyroxene, phosphates and titanates, are present in metasomatised mantle xenoliths. These could not be samples of lithosphere that had experienced previous temperatures significantly higher than the vapour-present peridotite solidus, unless there had been subsequent cooling followed by a new influx of lithophile elements. Percolation of fluids, and of flux-induced melts, along geotherms ranging from “shield” to “oceanic” could lead to intensive lithophile enrichment near the solidus, and metasomatism in the sub-solidus. Combined petrographic and chemical data provide some limits on the essential composition of the fluids, which must be able to introduce at least Ca, K, Al, Si, H, and C to deep mantle peridotite. Experimental results provide the framework for exploring the consequences of fluid activity under different PT conditions. Most of the observed variations in magmatism and mantle xenoliths can be related to the interplay between geothermal gradients and the vapour-present mantle solidus.
Repetition of alkaline igneous activity through old lesions in the lithosphere, from the Precambrian onwards, requires a magma generating system in which the source of the energy and the special chemistry is below the lithpsphere but the control of its siting is in the lithosphere itself. As the lithosphere plates are continually moving, the ultimate cause of the igneous activity cannot be a unique anomaly in the deep mantle. Hence, the most likely means of introduction of the alkaline characteristics into the lithosphere is by migration of tenuous fluids. Alkaline ultramafic melts erupted at high velocity must achieve their distinctive eruption chemistry by interaction with enriched mantle below the point of lift-off; and pick up their nodule suite from this level and above.
KeywordsGeothermal Gradient Mantle Xenolith Mantle Peridotite Chemical Transport Alkaline Magmatism
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