Thermal model of successive dike injections and implications for the development of intraplate volcanoes

Résumé 0

Temperatures in the root zones of volcanoes play a critical role in the development and persistence of shallow-level magmatic reservoirs in the crust. Here, we present a 1D thermal model allowing evaluation of the thermal impact of magma travelling in conduits to the surface on the root zone of a volcano. This thermal model has been developed to better understand the formation of a vertical intrusion located in the root zone of a dismembered Miocene volcano on Fuerteventura, Canary Archipelago. This intrusion, named PX1, constitutes an almost pure amalgamation of dikes of either clinopyroxenitic or gabbroic composition. Both types of dikes display cumulate textures and are interpreted as resulting from the protracted crystallization of a mafic magma. The formation of clinopyroxenitic, in contrast to gabbroic dikes, requires that the residual melt was extracted at high temperature (N1050°) to avoid plagioclase crystallization. Simulations of multiple dike injections show that the temperature in the root zone increases significantly with the addition of dikes, but the maximum temperature reached in the system depends on the duration of magma flow in the conduits and the time interval between dike injections (i.e., repose period). Active flow is the critical parameter that distinguishes instantaneous dike injection from a magmatic conduit. Without significant magma flow (N1 month), high-temperature conditions (N1000 °C) cannot be maintained in the pluton unless dikes are very thick and the repose period is extremely small. On the other hand,magma flow times of one to several months, combined with short time intervals between dike injections (b25 years), which are conditions comparable to those recorded for historical eruptions of oceanic island volcanoes, allowthe production and preservation of temperatures above the plagioclase liquidus for significant durations, as required to generate clinopyroxenitic dikes such as those observed in the PX1 pluton. Persistent high temperature in the vicinity of magma conduits limits the differentiation of melts in transit to the surface, providing a potential explanation forwhy lavas ofmafic to intermediate composition predominate in intraplate volcanoes such as Fuerteventura or Fogo Island (Cape Verde Archipelago). In extreme cases, when temperatures over 1000–1050 °C in the central part of the feeding zone are maintained for years, the remaining magma in the conduit does not solidify but is preserved in a mushy state. New pulses of magma would not be able to cross this zone butwould rather amalgamate in the incipientmagma reservoir. The present model differs from previous models of sill intrusion in that magmas do not need to pond at depth to create a reservoir but merely supply heat while travelling to the surface. Depending on the time interval between dike injections and the duration ofmagma flow through the crust, magma rising in vertical conduits could directly feed the volcanic edifice or could lead to the formation of magma reservoirs. This process may explain why some volcanoes erupt mafic or differentiated magmas during distinct periods of activity.