Experimental Constraints on the Chemical - USRA

46th Lunar and Planetary Science Conference (2015)
MANTLE. A. Boujibar1, K. Righter1, K. Pando2, L. Danielson3, 1NASA Johnson Space Center, 2101 Nasa Parkway, Houston, TX 77058, 2UTAS – Jacobs JETS Contract, NASA Johnson Space Center, Houston, TX 77058,
Jacobs JETS, NASA Johnson Space Center, 2101 NASA Pkwy, Houston, TX 77058 ([email protected]).
Introduction: Mercury is known as being the most
reduced terrestrial planet with the highest core/mantle
ratio. Results from MESSENGER spacecraft have
shown that its surface is FeO-poor (2-4 wt%) and Srich (up to 6-7 wt%) [1-2], which confirms the reducing nature of its silicate mantle [3]. In addition several
features suggest important melting stages of the Mercurian mantle: widespread volcanic deposits on its surface [4], a high crustal thickness (~10% of the planet’s
volume) [5] and chemical compositions of its surface
suggesting several stages of differentiation and remelting processes [6]. Therefore it is likely that igneous
processes like magma ocean crystallization and continuous melting have induced chemical and mineralogical
heterogeneities in the Mercurian mantle.
The extent and nature of compositional variations
produced by partial melting remains poorly constrained
for the particular compositions of Mercury (very reducing conditions, low FeO-contents and high sulfurcontents). Melting experiments with bulk Mercuryanalogue compositions are scarce and with poorly controlled starting compositions. Therefore additional experimental data are needed to better understand the
differentiation processes that lead to the observed
chemical compositions of Mercury’s surface.
Methods: Partial melting experiments have been
conducted with a piston cylinder apparatus at NASA
JSC using enstatite chondrites (EH4) with variable
oxygen fugacity and sulfur content at 1 GPa and temperatures between 1400°C and 1650°C. The fO2 of the
experiments was controlled by varying the Si/SiO2 ratio of the starting composition. Indeed it has been previously shown that the addition of Si metal allows the
reduction of the samples. However, the reaction of the
sample with the assembly leads usually to partial oxidation of Si, that can yield a SiO2-enrichment of the
silicate [7]. Therefore, instead we chose to vary the
Si/SiO2 ratio, in order to control the fO2 without affecting the elemental ratios of the bulk starting compositions.
We used two starting compositions: the first one
with 2 bulk wt% S, 5wt% Si in the starting metal and a
62/38 silicate/metal mass ratio and the second one
more reduced with 6 bulk wt% S, 12wt% Si in the
starting metal and a 50/50 silicate/metal mass ratio.
Experimental run products were analyzed with Cameca
and the JEOL EPMAs at NASA JSC. The new data are
combined with previous work, and then compared to
the chemical composition of Mercury’s surface in order
to better understand mantle mineralogy, chemical differentiation processes and evaluate the bulk composition of the planet.
Oxygen fugacity. The oxygen fugacity of the samples was calculated relative to the IW buffer as a function of the activity coefficients of FeO and Fe in the
silicate melt and in the liquid metal respectively. We
found fO2 comprised between IW-4.5 and IW-3. FeO
content in the silicate melt is found very low (comprised between 0.24 to 0.78 wt%). However its concentration does not present notable differences between
the runs performed with the moderately and highly
reduced compositions, despite significantly different
Si-content of the metals (2-3 wt% Simetal against 12-13
wt% Simetal in the most reduced samples). This can be
attributed to the solubility of FeS in the silicates melts
at very reducing conditions, as shown in recent studies
of [8-9].
Phase proportions. Run products from the SiO2poor and the SiO2-rich compositions are composed of
orthopyroxene, silicate melt and liquid metal at high
temperature but at low temperature (<1450°C), quartz
is also present in the runs synthesized with the SiO2rich starting composition. No sulfide phases were observed in all samples, which can be due to sulfur volatilization during the heating of the samples. Indeed mass
balance calculations show that at least half the initial
content of sulfur was volatilized during sample heating.
In addition the samples synthesized with the most Srich composition are also very reduced, so that S is
highly partitioned to the silicate melt (with Sconcentrations up to 9 wt% and S partitioning coefficients between metal and silicate of 0.07 to 0.1).
Moreover liquidus temperature is lower with the S-rich
SiO2-poor composition (1500°C) in comparison to the
S-poor SiO2-rich composition (1650°C).
Evolution of the melts compositions. The evolution
of Al2O3 and alkali elements (Na2O and K2O) is similar
with the two types of compositions. The CaO and MgO
are more enriched in the S-rich silicate melts in contrast to SiO2 which is more depleted in the melts relative to the S-poor samples (Fig. 1). This could be due
to MgS and CaS solubility in the silicate melts and to
the high starting SiO2 concentration of the S-poor samples.
46th Lunar and Planetary Science Conference (2015)
These results show that the ultramafic surface of
Mercury can simply be explained by partial melting of
EH chondrites at pressures comprised between 1 bar
and 1 GPa and very reducing conditions. The melts
observed on the surface of Mercury could have been
formed by melting of a chondritic mantle at depths of
up to 55 km. Therefore, it is not required to invoke
several stages of differentiation and remelting processes to explain the Mercurian surficial compositions derived from MESSENGER results [2] as previously
proposed [6]. Further studies to better determine the
effect of pressure, S and SiO2 contents on phase relations and melting processes are underway and will provide better constraints on the physical and chemical
properties of the Mercurian mantle.
Fig. 1 Chemical composition of the silicate melts as a
function the degree of melting of the S-poor SiO2-rich
EH-like composition (blue circles) and S-rich SiO2poor EH-like composition (yellow squares).
Comparison with previous data and Mercury’s surface:
Ca/Si, Mg/Si and Al/Si element ratios of our silicate melts and that of previous studies [7-10] are compared to that of Mercury’s surface in Fig. 2. We found
that the melts produced with the S-poor composition
and that previously synthesized at 1 GPa with EH4
Indarch composition [7] have lower Al/Si and Ca/Si
ratios than Mercury’s surface. This can be originated
by the high SiO2 of both compositions. Indeed, in [7],
as stated above, the oxygen fugacity of the samples was
controlled by adding Si in the starting metals. However
during the equilibration at high pressure and temperature, Si can be oxidized and can yield high SiO2 concentrations.
In contrast, the compositions of Mercury’s surface
are in a good match with our silicate melts synthesized
with the S-rich starting material and the samples run at
1 bar by [10] with EH4 chondrites. In these run products, the fO2 was controlled by varying the Si/SiO2
ratio. In the present study, this was done directly in the
starting powder and in [10] with an external buffer
within the gas-mixing furnace.
Fig. 2 Comparison between Ca/Si, Al/Si and Mg/Si
ratios of Mercury’s surface compositions [2] (grey area
and diamonds) with the silicate melts obtained in this
study (blue circles and yellow squares) and that of [10]
(green triangles) and [7] (blue crosses).
References: [1] McClintock W. E. et al. (2008)
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JGR, 118, 138-146. [4] Solomon S. C. et al. (2011)
Planet Space Sci., 59, 1827-1828. [5] Padovan S. et al.
(2014) AGU, P21C-3938. [6] Charlier B. et al. (2013)
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