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Carbonatite Melts and Electrical Conductivity in the
Asthenosphere
Fabrice Gaillard, Mohammed Malki, Giada Iacono-Marziano, Michel
Pichavant, Bruno Scaillet
To cite this version:
Fabrice Gaillard, Mohammed Malki, Giada Iacono-Marziano, Michel Pichavant, Bruno Scaillet. Car-
bonatite Melts and Electrical Conductivity in the Asthenosphere. Science, American Association for
the Advancement of Science (AAAS), 2008, 322 (5906), pp.1363 - 1365. �10.1126/science.1164446�.
�insu-00343685�
1
SMALL AMOUNT OF CARBONATITE MELTS EXPLAINS HIGH ELECTRICAL
CONDUCTIVITY IN THE ASTHENOSPHERE
Gaillard F
1
, Malki M
2
, Iacono-Marziano G
1
, Pichavant M
1
, Scaillet B
1
1
Corresponding author: gaillard@cnrs-orleans.fr
CNRS/INSU, Université d'Orléans, Université François Rabelais - Tours,
Institut des Sciences de la Terre d'Orléans - UMR 6113
Campus Géosciences, 1A, rue de la Férollerie, 41071 Orléans cedex 2, France.
2
CEMHTI-CNRS, UPR3079, 1D avenue de la Recherche Scientifique, 45071 Orléans cedex2,
France.
2
Polytech’Orléans – Université d’Orléans, 8 rue Léonard de Vinci, 45072 Orléans cedex 2, France.
One sentence summary:
We unambiguously show that molten carbonates have electrical conductivities exceeding by far the
one of any mantle phases. We therefore propose that mantle electrical anomalies observed by
geophysical surveys image the presence at depth of small quantities of molten carbonates providing
us the possibility to better constrain the distribution of carbon in the deep Earth.
2
Abstract: (140 words)
Electrically conductive regions in the Earth mantle are classically interpreted to reflect the
presence at depth of either molten silicates or water dissolved in olivine. However, geochemical
studies have largely recognized molten carbonates as important fluid agents in the mantle, but
their impact on the electrical properties of the mantle has never been evaluated. Laboratory
measurements reported here show that molten carbonates have electrical conductivities
exceeding by 3 orders of magnitude those of molten silicate and by 5 orders of magnitude those
of hydrated olivine. Many conductive regions of the mantle can thus reflect the presence of
small amounts of pervasive molten carbonates, which are thermodynamically stable in the
upper mantle. In particular, the deep conductive oceanic asthenosphere can be explained by 30-
460 ppm CO
2
in the form of molten carbonates, consistent with estimated CO
2
fluxes at mid
ocean ridges.
3
Laboratory measurements on anhydrous peridotite and olivine single crystals indicate that the
electrical conductivity of the upper mantle, if dry, should be in the range 10
-4
-10
-2
S.m
-1
with high
conductivity values essentially reflecting high mantle temperatures (1,2). Deep magnetotelluric
sounding however indicates that the electrical conductivity of some mantle regions exceeds those
values (3,4,5). Underneath the Pacific Ocean, for example, conductivities of 10
-1
S.m
-1
have been
clearly recognized deeper than 60 km (5). Such zones require the presence at depth of conductive
phases; silicate melts or hydrated olivine crystals are commonly considered (3,5,6,7). Silicate melts
have electrical conductivity in the range 10
-2
-10 S.m
-1
(6,8,9) but can only be present if the
temperature of the mantle is high enough to allow peridotite melting (10). Incorporation of trace
amounts of hydrogen in olivine is therefore the most accepted way to interpret high mantle
conductivity (4,5,7,11). Direct measurements in mantle xenoliths indeed provide compelling evidence
for hydrated mantle olivine (12), however, the magnitude of the effect of water on olivine
conductivity remains under debate (13,14). Furthermore, measurements performed on hydrated single
olivine crystals cannot explain high electrical anisotropy in the asthenospheric mantle (14). We
present here molten carbonates (i.e. carbonatites) as an additional phase potentially explaining high
conductivity in the mantle. Hereafter, we review the petrological supports for their presence and
stability in the mantle and report laboratory evidence of their very high electrical conductivity. We
conclude that mantle regions with high conductivity probably image the presence of small amounts of
carbonatite melts, in keeping with geochemical observations.
Carbonatites are very rare at the Earth surface and only one volcano, Ol Doinyo Lengai, Tanzania, is
currently emitting such magmas (15). The reason classically put forward to explain such a scarcity is
that these carbonate melts, initially present in the mantle, are diluted and masked by silicate melts that
constitute the overwhelming part of extrusive rocks at the Earth surface (16). The carbon dioxide
content of mantle derived magmas is in the range of a few hundredth ppm in mid ocean ridge basalts
and can reach a few thousand ppm in specific settings (17,18), which constrains the CO
2
content of
the mantle source to a few tens to hundreds ppm (17,18). Under most of the P-T-redox conditions
4
prevailing in the upper mantle, carbon is likely to be present in the form of molten carbonates (19-
21). Such carbonatite melts have exceedingly large wetting properties (22): they form interconnected
liquid networks at olivine grain boundaries even at very low volume fractions (22,23) and could
therefore contribute to the electrical conductivity of the mantle. The available data on the electrical
conductivity of molten carbonates cover Li-rich compositions of industrial interest (24). However,
mantle carbonatites are particularly Li-poor and Mg, Ca-rich with K and Na in variable proportions
(25). In this paper, we extend the database on electrical conductivity of molten carbonates to Li-free
and Ca-rich system. The measurements were performed at 1 atmosphere of CO
2
pressure using a 4-
electrode experimental method specifically adapted to greatly conductive molten materials (26; see
Note and Supporting online material, SOM). No magnesium carbonate was included in our material
because Mg-bearing molten carbonates are not stable at 1 atm pressure and the very high
conductivities of carbonate melts shown below makes the deployment conductivity measurements
with conventional setup very challenging at high pressure. Potential pressure effects are nevertheless
revealed below to be probably minor.
The electrical conductivity of molten carbonate compositions measured here increases from 50 to 200
S.m
-1
for temperature increasing from 400 to 1000°C (Table 1, SOM). Such conductivity values are
comparable to those reported on Li-rich molten carbonates (24, SOM). In Figure 1, we show the
temperature dependence of molten carbonate electrical conductivity together with the one of other
mantle phases such as dry and hydrous olivine (2,13,14) and silicate melts (6,8,9). Molten carbonates
at 1000°C are 1000 times more conductive than molten silicates at the same temperature and 100,000
times more conductive than hydrous olivine single crystals. Our measurements show that the
electrical conductivity of molten carbonates varies slightly with their chemical composition. Calcium-
free carbonates containing 30% Li
2
CO
3
is only about 2 to 3 times more conductive than a Li-free melt
containing 50% of CaCO
3
(Fig. 1, Table 1, see also SOM). Another striking feature of molten
carbonate conductivities is their small temperature dependence. The temperature dependence of
molten carbonate electrical conductivities (σ) can be adequately fitted using an Arrhenius law:
