Floating stones off El Hierro, Canary Islands: xenoliths of pre-island sedimentary origin in the early products of the October 2011 eruption

摘要

A submarine eruption started off the south coast of El Hierro, CanaryIslands, on 10 October 2011 and continues at the time of this writing (February2012). In the first days of the event, peculiar eruption products were foundfloating on the sea surface, drifting for long distances from the eruptionsite. These specimens, which have in the meantime been termed"restingolites" (after the close-by village of La Restinga), appeared asblack volcanic "bombs" that exhibit cores of white and porous pumice-likematerial. Since their brief appearance, the nature and origin of these"floating stones" has been vigorously debated among researchers, withimportant implications for the interpretation of the hazard potential of theongoing eruption. The "restingolites" have been proposed to be either (i) juvenilehigh-silica magma (e.g. rhyolite), (ii) remelted magmatic material(trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites orzeolite from the submarine slopes of El Hierro.Here, we provide evidence that supports yet a different conclusion. We haveanalysed the textures and compositions of representative "restingolites"and compared the results to previous work on similar rocks found in theCanary Islands. Based on their high-silica content, the lack of igneoustrace element signatures, the presence of remnant quartz crystals, jasperfragments and carbonate as well as wollastonite (derived from thermaloverprint of carbonate) and their relatively high oxygen isotope values, weconclude that "restingolites" are in fact xenoliths from pre-islandsedimentary layers that were picked up and heated by the ascending magma,causing them to partially melt and vesiculate. As they are closelyresembling pumice in appearance, but are xenolithic in origin, we refer tothese rocks as "xeno-pumice". The El Hierro xeno-pumices hence representmessengers from depth that help us to understand the interaction betweenascending magma and crustal lithologies beneath the Canary Islands as wellas in similar Atlantic islands that rest on sediment-covered ocean crust(e.g. Cape Verdes, Azores). The occurrence of "restingolites" indicates that crustal recycling is a relevant process in ocean islands,too, but does not herald the arrival of potentially explosive high-silicamagma in the active plumbing system beneath El Hierro.