Rheology of Martian lava flows

In recent years, high-resolution topographic images from Mars’
surface as well as mineralogical and chemical data, have rapidly
become more accessible. Martian volcanic landforms are
characterized by giant low slope shield volcanoes, abundant lava
flood plains and long lava flows. In-situ rock analysis and remote
sensing spectroscopy reveal mainly basaltic compositions with
particularly high iron concentrations, distinct from terrestrial
basalts. As yet, very little is known about the rheological
properties of such iron-rich Martian magmas that are essential to
understand magmatic processes. Understanding the chemical and
physical contributions to lava rheology is fundamental to provide
constraints on magma ascent and lava flow emplacement that shaped
the volcanic landforms on Mars. This study provides an experimental
investigation of the rheological properties of Martian lavas and
discusses the diversity of compositions in terms of lava viscosity
/ flow morphology relationship. The effect of iron, and its redox
state on silicate melt viscosity is experimentally investigated and
the viscosities of five synthetic silicate liquids having
compositions representative of the diversity of Martian volcanic
rocks were measured under controlled ambient oxygen fugacity. The
results highlight the low viscosity of the iron-rich Martian melts
that is consistent with viscosity values derived from morphological
observations. A solidified lava flow on Earth was studied by
combining analyses of remote sensing images (as commonly done on
Mars), as well as experimental investigations of the rheological
properties of the sampled rocks, in order to describe the viscous
behavior of lava as emplacement, cooling, and crystallization
occur. We show that a cooling-limited basaltic flow seemingly stop
flowing when it reaches a critical viscosity value that is function
of crystals content and shapes. As a result, the lava apparent
viscosity appears to be largely influenced by the details of the
crystallization sequence and is not uniquely and simply related to
the bulk chemical composition of the erupted material. Variation of
the chemical evolution of Martian primary mantle melts through the
volcanic history is not large to produce an significant shift of
the viscosity range that could be observed them from their
morphologies. Low apparent viscosities inferred from lava flow
morphology on Mars may in turn be attributed to lavas with primary
mantle melt composition crystallizing high proportion of olivine
and possibly forming spinifex textures. Higher viscosity values
derived from the morphology are compatible with mildly alkaline or
trachybasalts and do not necessarily imply the occurrence of
silica-rich lavas.