Comptes Rendus Géoscience

Jean-François Oehler

Marie-Françoise Lequentrec-Lalancette

Handled by Philippe Cardin

Volume 351 (2019) – No. 1 – January
Cover illustration: Vietnamese spinels from Luc Yen and An Phu mining districts. Spinels (Spl) in a calcitic (Cal) 
marble, Cong Troi. Left: octahedral spinel associated with calcite (Cal). Right: gem crystals of pink spinel in a calcitic 
(Cal) marble. Photographs: Vincent Pardieu. From Anthony E. Fallick et al., this issue, pp. 27–36.
Internal Geophysics 
(Applied Geophysics) 
• The contribution of marine magnetics in the Gulf of Saint-Malo (Brittany, France) to the understanding 
of the geology of the North Armorican Cadomian belt
Jean-François Oehler and Marie-Françoise Lequentrec-Lalancette  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
(Sismology)
• Simulation of seismic-prospecting random noise in the desert by a Brownian-motion-based parametric 
modeling algorithm
Tie Zhong, Shuo Zhang, Yue Li and Baojun Yang  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Petrology, Geochemistry
(Isotope Geochemistry)
• Paleoclimatic implications of hydrogen and oxygen isotopic compositions of Cretaceous–Tertiary kaolins 
in the Douala Sub-Basin, Cameroon
Nenita Bukalo, Georges-Ivo Ekosse, John Odiyo and Jason Ogola  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
• Remarkably uniform oxygen isotope systematics for co-existing pairs of gem-spinel and calcite in marble, 
with special reference to Vietnamese deposits
Anthony E. Fallick, Gaston Giuliani, Thomas Rigaudier, Adrian J. Boyce, Van Long Pham 
and Vincent Pardieu  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Hydrology, Environment
(Surface Geochemistry)
• Phosphate removal from water by naturally occurring shale, sandstone, and laterite: The role of iron oxides 
and of soluble species
Dan Eude Kpannieu, Martine Mallet, Lacina Coulibaly, Mustapha Abdelmoula and Christian Ruby  . . . . . . 37
Tectonics, Tectonophysics
• Automatic correction and simplifi cation of geological maps and cross-sections for numerical simulations
Pierre Anquez, Jeanne Pellerin, Modeste Irakarama, Paul Cupillard, Bruno Lévy and Guillaume Caumon  . . . . 48


The contribution of marine magnetics in the Gulf of Saint-Malo (Brittany, France) to the understanding of the geology of the North Armorican Cadomian belt

Fei Qin

Xiang Wu

Shan Qin

Dongzhou Zhang

Vatali B. Prakapenka

Steven D. Jacobsen

Handled by Robert C. Liebermann

Pressure-induced dehydration of dioptase: A single-crystal X-ray diffraction and Raman spectroscopy study

Fabian Wagle

Gerd Steinle-Neumann

Nico de Koker

Resistivity saturation in liquid iron–light-element alloys at conditions of planetary cores from first principles computations

Denis Andrault

Handled by Yanbin Wang

Thermodynamical constraints on the crystallization of a deep magma-ocean on Earth

Tony Yu

Yanbin Wang

Mark L. Rivers

Stephen R. Sutton

Handled by Guillaume Fiquet

An upgraded and integrated large-volume high-pressure facility at the GeoSoilEnviroCARS bending magnet beamline of the Advanced Photon Source

Ting Chen

Gabriel D. Gwanmesia

Lars Ehm

Charles Le Losq

Daniel R. Neuville

Brian L. Phillips

Baosheng Li

Robert C. Liebermann

Synthesis and characterization of polycrystalline KAlSi<sub>3</sub>O<sub>8</sub> hollandite [liebermannite]: Sound velocities vs. pressure to 13 GPa at room temperature

Takashi Yoshino

Handled by James Badro

Penetration of molten iron alloy into the lower mantle phase

Mohammad El Hajj

Nicolas Baghdadi

Nicolas Labrière

Jean-Stéphane Bailly

Ludovic Villard

Mapping of aboveground biomass in Gabon

Morgane Rondet

Caroline Martel

Jean-Louis Bourdier

Comptes rendus - Geoscience 351 (2019) 523e524
lable at ScienceDirectContents lists avaiComptes rendus - Geoscience
www.journals.elsevier.com/comptes-rendus-geoscienceForewordNew aspects of magma storage and transferMagmatic processes have played, and still play, a key
role in the evolution of Earth. Their surface manifestations
via explosive eruptions that propel gases and ashes high
into the atmosphere, or high-temperature geothermal
fields that fuel power plants, continuously remind us that
our planet is still active, striving daily to dissipate its in-
ternal energy via upward transport of hot material. Despite
decades of intensive research, the largely hidden nature of
source processes driving magmas has hampered our
quantitative understanding of this phenomenon.
Answering fundamental questions such as the lifetime of a
magmatic system, the occurrence and style of the next
eruptive episode of volcanoes still requires a wealth of as-
sumptions or simplifications that almost always limit cur-
rent model predictions to qualitative assessments at best.
This is why there exists a very active community
worldwide whose purpose is to make progress in the field.
Among the main issues at stake are the conditions
attending the construction of magmatic systems (e.g.,
Scaillet et al., 2016). We know these conditions in sufficient
detail to fill-up textbooks, but our level of knowledge falls
short when sound modelling of magmatic processes is
demanded. We know that magmas involve hot, partly or
fully molten silicates, at temperatures generally ranging
from 650C to 1200C, and we also know that magmas may
hold considerable amounts of dissolved volatiles, particu-
larly water and CO2, whose abundances and exsolution
largely dictate eruptive behaviour. Yet, the fact of accurately
determining temperature and volatile contents, which
exert a prime control on transport properties of magmas
(viscosity, density), still remains a challenge in most in-
stances. We know for sure that the molten lava comes from
deep in the Earth, but howdeep themagma sourced is, how
long it stays at rest, or how much time it takes to travel
towards the surface are open questions for many magmatic
systems, even for those benefiting from detailed laboratory
and field data, including geophysical surveys.
Factors controlling magmas, such as those mentioned
above, vary within or across centres, and are strongly
interrelated (i.e. the pressure control on volatile solubil-
ities), making the elaboration of a unique model or answer
difficult, if not illusory. The diversity of magma types andhttps://doi.org/10.1016/j.crte.2019.11.001
1631-0713/© 2019 Published by Elsevier Masson SAS on behalf of Académie de
(http://creativecommons.org/licenses/by-nc-nd/4.0/).tectonic contexts, together with the likely influence of the
latter on magma dynamics, call for an inevitably time-
consuming approach on a case-by-case basis. One good
illustration of our lack of detailed understanding of system
functioning is the current debate on what is a magmatic
system (e.g., Lundstrom and Glazner, 2016): field excur-
sions across many granitic terranes reveal that magmas in
the continental crust tend to amalgamate at rather
restricted locations forming rather homogeneous plutons,
which, for some, represent fossil reservoirs. However, the
very existence of large magma reservoirs in the shallow
crust has been called into question, in part because
geophysical methods deployed in active volcanic fields
have so far failed to reveal the existence of such large res-
ervoirs of molten material. The ordinary view of a single
level of magma accumulation is now challenged by the
concept of trans-crustal magma system, in which magma
accumulation develops almost everywhere across the
crust, the chemical diversity of magmas being mostly ac-
quired in the deep hot crust (Cashman et al., 2017). Robust
petrological evidence for such an architecture is scant,
however, and if any, it is merely available in broad outline
and only for a restricted number of areas, which prevents
the worldwide generalisation of that concept. The nature of
the relationships between frozen plutons and active sys-
tems is difficult to establish because exposed intrusives
represent the end-result of time-integrated processes,
while volcanoes yield only discontinuous snapshots along
this chain (assuming that plutons and volcanoes belong to
the same chain). Altogether, the questions of where and
why magma ponding in the crust occurs (shallow, deep, or
everywhere), and howmuch of it is liquid at any given time
(e.g., Bachmann and Huber, 2016), stand out as priority is-
sues to be solved if progress is sought in the area.
The papers collected in this thematic issue of Comptes
rendus Geoscience under the heading of Labex VOLTAIRE
(see http://www.univ-orleans.fr/fr/osuc/news/granites-
summer-school) illustrate some of the various ways the
community is currently handling to answer the above
questions. Rondet et al. (2019) have performed a series of
phase equilibrium experiments on a representative rock
coming from the Chaîne des Puys, which is host to the mosts sciences. This is an open access article under the CC BY-NC-ND license
Foreword / Comptes rendus - Geoscience 351 (2019) 523e524524recent volcanic manifestation in the French Massif Central.
Their work exemplifies the use of experiments to set robust
limits on critical parameters such as P, T, or the water
content prior to eruption. They also shed light on frac-
tionation mechanisms controlling the chemical evolution
of magmas once at rest in the crust, revealing in this case
the critical role of so-called minor phases (biotite) on liquid
evolution. Pichavant et al. (2019) report detailed experi-
ments on the role of calcium on the phase relationships of
the haplogranite system. This system is known to be a good
analogue of the chemical composition of residual liquids
produced during the fractionation of metaluminous
magmas. Recent work has suggested that the pressure ef-
fect on the haplogranite system, as documented by the
pioneering work of Tuttle and Bowen (1958), can be used to
infer the pressure evolution of Qz-saturated felsic melts,
but debate has arisen over the role of components such as
Ca on the phase relationships of the QzeAbeAf system.
Pichavant et al. (2019) report new phase equilibria and
related compositions that explore such an effect and sug-
gest that caution is required when applying the available
models, in particular those empirical, to infer the crystal-
lisation pressure of Qz-saturated felsic compositions.
Pressure appears to be one of the most difficult param-
eters to constrain in magmatic systems, in particular when
magmasevolve in the low-pressure range, say thefirst 10kb,
where the vast majority of (transient?) reservoirs feeding
felsic eruptions are believed to lie. Unfortunately, most
petrological geobarometers are associatedwith large errors,
on the order of ±2 kb, which makes their application to
shallow-crust processes rather impractical. Much effort has
been thus dedicated to improving the reliability of petro-
logical tools in recent years (e.g., Putirka, 2008). Erdmann
et al. (2019) have made experiments on felsic magmas,
focusing on the stability of titanite, a common accessory
mineral of granites. They suggest that titanite occurrence
and composition (its aluminium content) both depend on
pressure, offering a newapproach for constraining pressure
conditions of frozen felsic intrusives: this is a valuable
addition to the existing panel of barometric tools.
The conditions of magma production and evolution can
be also tackled via numerical models, assisted by field ob-
servations and thermophysical data, which altogether
permit a large-scale picture approach, such as the multi-
million year thermal evolution of a partially melted crus-
tal section. Vanderhaeghe et al. (2019) present the results
of 1D numerical simulations aimed at modelling the ther-
mal evolution of continental crust during the Archean eon.
Their results suggest that the lower crust may have
remained partially molten for several hundred million
years in the early Earth, an admittedly thought provoking
view that is likely to fuel further debate on the evolution of
the continental crust.
Magma evolution is in large part governed by its ability
to move, which depends on its density and viscosity. Yet,
the viscous behaviour of polyphase suspensions such as
magmas remains unclear, in particular how the percentageof crystals of a magmatic slurry affects viscosity. Arbaret
et al. (2019) provide new experimental data on this topic,
focusing their efforts on crystal-poor, or near-liquidus,
magmas. Their results show that suspensions with about
15% crystal (volume) behave in a Newtonian fashion when
sheared, owing to the absence of particle interactions.
They also document that the fabric, or preferential orien-
tation of anisotropic crystals, does record information
about the conditions of magma flow, in particular the
sense of shear. Both results are important, since they
inform numerical models and help interpreting field
observations.
This series of papers illustrates perfectly that nowadays
the petrological community is firmly engaged in a quanti-
tative evaluation of magmatic processes, in an approach
that uses state-of-the-art experimental, analytical, or nu-
merical tools, building upon but reaching well beyond the
traditional practice of rock description and categorisation.
The majority of first-order processes controlling the origin
and evolution of magmas has already been identified.What
is on the agenda now is to put figures on them so as to
develop truly predictive models.References
Arbaret, L., Bystricky, M., Launeau, P., et al., 2019. Crystal clustering in
magmas: insights from HPeHT experiments. C. R. Geoscience 351 (8),
547e585.
Bachmann, O., Huber, C., 2016. Silicic magma reservoirs in the Earth's
crust. Am. Mineral. 101 (11), 2377e2404.
Cashman, K.V., Sparks, R.S.J., Blundy, J.D., 2017. Vertically extensive and
unstable magmatic systems: a unified view of igneous processes.
Science 355 (6331) eaag3055.
Erdmann, S., Wang, R., Huang, F., Scaillet, B., Zhao, K., Liu, H., Chen, C.,
Faure, M., 2019. Titanite: a potential solidus barometer for granitic
magma systems. C. R. Geoscience 351 (8), 551e561.
Lundstrom, C.C., Glazner, A.F., 2016. Silicic magmatism and the
volcaniceplutonic connection. Elements 12 (2), 91e96.
Pichavant, M., Weber, C., Villaros, A., 2019. Effect of anorthite on granite
phase relations: experimental data and models. C. R. Geoscience 351
(8), 540e550.
Putirka, K.D., 2008. Thermometers and barometers for volcanic systems.
Rev. Mineral. Geochem. 69 (1), 61e120.
Rondet, M., Martel, C., Bourdier, J.-L., 2019. The intermediate step in
fractionation trends of mildly alkaline volcanic suites: an experi-
mental insight from the Pavin trachyandesite (Massif Central, France).
C. R. Geoscience 351 (8), 525e539.
Scaillet, B., Holtz, F., Pichavant, M., 2016. Experimental constraints on the
formation of silicic magmas. Elements 12 (2), 109e114.
Tuttle, O.F., Bowen, N.L., 1958. Origin of granite in the light of experi-
mental studies in the system NaAlSi3O8eKAlSi3O8eSiO2eH2O. GSA
Mem. Geol. Soc. Am. 74.
Vanderhaeghe, O., Guergouz, C., Fabre, C., Duchêne, S., Baratoux, D., 2019.
Secular cooling and crystallization of partially molten Archaean
continental crust over 1 Ga. C. R. Geoscience 351 (8), 562e573.
Bruno Scaillet*, Laurent Arbaret, Fabrice Gaillard
Institut des sciences de la Terre d’Orleans, Orleans, France
 Corresponding author.
E-mail address: bscaille@cnrs-orleans.fr (B. Scaillet)


The intermediate step in fractionation trends of mildly alkaline volcanic suites: An experimental insight from the Pavin trachyandesite (Massif Central, France)

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