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.