Magma rheology variation in sheet intrusions
Craig Magee1, Brian O’Driscoll2, Michael S. Petronis3, Carl T.E. Stevenson4
1
Department of Earth Science and Engineering, Imperial College, Prince Consort Road, London,
SW7 2BP, UK
2
School of Physical and Geographical Sciences, Keele University, Keele, ST5 5BG, England, UK
3
Environmental Geology, Natural Resource Department, New Mexico Highlands University, Las
Vegas, NM 87701, USA
4
School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston,
Birmingham, B15 2TT, UK
The rheology of magma fundamentally controls igneous intrusion style as well as the
explosivity and type of volcanic eruptions. Importantly, the dynamic interplay between the
viscosity of magma and other processes active during intrusion (e.g., crystallisation, magma
mixing, assimilation of crystal mushes and/or xenolith entrainment) will likely bear an
influence on the temporal variation of magma rheology. Constraining the timing of
rheological changes during magma transit therefore plays an important role in understanding
the nuances of volcanic systems.
However, the rheological evolution of actively emplacing igneous intrusions cannot be
directly studied. While significant advances have been made via experimental modelling and
analysis of lava flows, how these findings relate to intruding magma remains unclear. This
has led to an increasing number of studies that analyse various characteristics of fully
crystallised intrusions in an attempt to ‘back-out’ the rheological conditions governing
emplacement. For example, it has long been known that crystallinity affects the rheology and,
consequently, the velocity of intruding magma. This means that quantitative textural analysis
of crystal populations (e.g., crystal size distribution; CSD) used to elucidate crystallinity at
different stages of emplacement can provide insights into magma rheology. Similarly,
methods that measure flow-related fabrics (e.g., anisotropy of magnetic susceptibility; AMS)
can be used to discern velocity profiles, a potential proxy for the magma rheology.
To illustrate these ideas, we present an integrated AMS and petrological study of several
sheet intrusions located within the Ardnamurchan Central Complex, NW Scotland. We focus
on the entrainment and transport dynamics of gabbroic inclusions that were infiltrated by the
host magma upon entrainment. Importantly, groundmass magnetic fabrics within and external
to these inclusions are coaxial. This implies that a deviatoric stress was transmitted into the
inclusions during magma flow. We suggest that this represents a modification of the magma
dynamics from Newtonian-like to Bingham-like behaviour. Furthermore, the spatial
restriction of inclusions within the sheet intrusions suggest that subtle variations in magma
rheology may partition apparently continuous intrusions, perhaps affecting lateral mixing and
the longevity of discrete sheet segments. Detailed fabric analysis of other inclusion-free
intrusions in the Ardnamurchan Central Complex supports this interpretation. Our results
highlight that the crystalline cargo of a magma can result in temporal and spatial variations in
magma rheology. This can partition coalesced magma bodies into ‘zones’ characterised by
different magma properties, potentially affecting the location of magma flow pathways or
even eruption sites.