Main supervisor:
Olivier Bachmann
(ETH Zurich)
Co-supervisors and
collaborators:
Prof. Chris Huber
(Brown University),
Maria Schönbächler
(ETH Zurich),
Amir Khan
(ETH Zurich),
Tony Irving
(University of Washington)
Silicic igneous rocks (enriched in SiO2
and quartzo-feldspathic components like
granitoids and rhyolites), due to their
low densities, play a fundamental role
in many processes on Earth. The fact
that they “float” on the mantle, forming
stable lids (cratons) standing tall
above ocean floors, is at the origin of
the complex tectonic processes that
govern the dynamics of our planet and
have allowed the richness of
environments to develop on Earth since
the Archean. Despite their abundance
(> 85% of the upper crust is
granodioritic in composition; e.g.,
Wedepohl, 1995; Bonin et al., 2002),
these silicic rocks did not form on the
Moon, are rare or absent on Mars, and
only appear as clasts in a few
meteorites (Bonin et al., 2002). It has
been suggested that the presence of
abundant water on Earth is key to the
generation of granites (e.g., Campbell
and Taylor, 1983), but these rocks form
in every tectonic setting (even in the
driest hot spot conditions), and could
be abundant on ‘dry’ Venus (Bonin et
al., 2002), Hence, understanding the
necessary conditions for the
construction of low-density crust in
telluric planets warrants further study.
Mars appears to play a unique role in
this discussion. Despite detailed
assessment of its petrological diversity
over the past decades (using
spectroscopic methods, direct
measurements with rovers, and the study
of meteoritic samples; Nimmo and Tanaka,
2004) and evidence that mafic magmas
hold some water (McSween et al., 2001),
large bodies of silicic magmas have not
yet been discovered on the red planet.
The most common rock types that are
directly analyzable from meteorites
belong to the SNC meteorite group
(Shergottites, Nakhlites, Chassignites;
McSween, 1994), and they range from
dunites (Chassignites) to peridotites,
pyroxenites, and mafic to intermediate
volcanic units (basalts, basaltic
andesites; Nakhlites and Shergottites).
Of course, those meteorites may not
cover most of the surface of Mars, nor
do they represent the full time span of
its magmatic processes. These meteorites
likely come from the northern lowlands
(Hesperian and Amazonian material),
which covers the older Noachian rocks
(formed ~ 4.5 Gyr B.P. and forming much
of the ~ 50 km thick Martian crust;
Nimmo and Tanaka, 2005), and
crystallization ages appear fairly young
(clustering between 1300-900 Ma - with
some as low as 150-200 Ma; McSween,
1994). The InSight Mars Lander will
provide more opportunities to explore
previously unknown corners of Mars, but
it is likely that the amount of silicic
lithologies present on the Red Planet
will remain much lower than on Earth.
A key aspect that controls the physical
and chemical evolution of a magma
reservoir is the depth at which magmas
are trapped. As depth (i.e., pressure)
increases, the background temperature
rises, and the mineral phases that are
crystallizing from any given initial
magma composition varies significantly
(hence controlling the composition of
the residual melt). On Earth, many
subvolcanic reservoirs seem to be
located between ~5 and 10 km depth (~1.5
to 2.5 kb; Huber et al., 2019). Even
considering earth-like primordial melts,
the difference in thermal gradient
between Earth and Mars and lower gravity
on Mars will impact the optimal pooling
depth of magmas and the longevity of
magma reservoirs on Mars (Figure 1). We
propose to test this hypothesis with an
update to a recently developed
thermo-mechanical model (Degruyter and
Huber, 2014). This model (shown
schematically in Figure 2) allows us to
directly couple the thermal and
mechanical evolution of the magma
reservoir, assuming a system that grows
with periodic magma injection, loses
mass through magma withdrawal, and
accounts for gas exsolution and
crystallization from the melt (3-phase
model). The magma reservoir is allowed
to lose heat to its surroundings
(crystalline mush transitioning to
crustal wall-rocks), which responds
visco-elastically to volume changes
within the reservoir. Modifications of
the model to convert it to Mars
conditions include the determination and
definition of relevant melting curves,
volatile solubility and geotherms (under
Mars-like conditions).
