Abstract
Magmatic stoping is discussed to be a main mechanism of magma emplacement. As a consequence of stoping, abundant country-rock fragments should occur within, and at the bottom of, magma reservoirs as “xenolith graveyards”, or become assimilated. However, the common absence of sufficient amounts of both xenoliths and crustal contamination have led to intense controversy about the efficiency of stoping. Here, we present new evidence that may explain the absence of abundant country-rock fragments in plutons. We report on vesiculated crustal xenoliths in volcanic rocks that experienced devolatilisation during heating and partial melting when entrained in magma. We hypothesise that the consequential inflation and density decrease of the xenoliths allowed them to rise and become erupted instead of being preserved in the plutonic record. Our thermomechanical simulations of this process demonstrate that early-stage xenolith sinking can be followed by the rise of a heated, partially-molten xenolith towards the top of the reservoir. There, remnants may disintegrate and mix with resident magma or erupt. Shallow-crustal plutons emplaced into hydrous country rocks may therefore not necessarily contain evidence of the true amount of magmatic stoping during their emplacement. Further studies are needed to quantify the importance of frothy xenolith in removing stoped material.
Highlights
The lack of abundant country-rock xenoliths in plutons is a key point in the ongoing controversy on magmatic stoping
In addition to the floating xeno-pumice from El Hierro, similar frothy xenoliths in eruptive products have been reported from several continents and cover sandstone, granite, schist, and gneiss lithologies that originated from various depths beneath these volcanoes (Figs 1 and 2)
Frothy xenoliths may represent some of the missing stoped material that was expelled from the host magma reservoir and conduits as a result of heating, devolatilisation, inflation, and associated density decrease during magma storage within the crust
Summary
The lack of abundant country-rock xenoliths in plutons is a key point in the ongoing controversy on magmatic stoping. Stoped material has been thought to often sink to the bottom of the host magma reservoir where it should accumulate in xenolith graveyards or become gradually assimilated (e.g. refs 17 and 26). Such graveyards that would manifest as magmatic breccia horizons are extremely scarce, both at the bottom of plutons and within them[13]. There “frothy” xenoliths (“xeno-pumice”30) may represent the remnants of stoped material that experienced volatile exsolution during partial melting while entrained in magma. Volatile exsolution and associated density decrease of xeno-pumice may allow to efficiently remove xenolithic material from the plutonic record. We will describe a selection of such frothy xenolith samples and analyse the effect of partial melting and density decrease on xenolith buoyancy while emersed in magma though thermomechanical numerical models
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