44th Lunar and Planetary Science Conference (2013)
1258.pdf
THE VISCOSITY OF LUNAR MAGMAS AT HIGH PRESSURE AND TEMPERATURE. Nachiketa Rai1,
Jean Phillipe Perrillat2, Mohamed Mezour3, Sylvain Petitgirard4, Aurelia Colin1 and Wim van Westrenen1. 1Faculty
of Earth and Life Sciences, VU University Amsterdam, The Netherlands, 2 Université Claude Bernard Lyon 1, Lab.
of Earth Sciences, Lyon, France, 3 European Synchrotron Radiation Facility, Grenoble, France, 4 Bayerisches
Geoinstitut, University of Bayreuth, Bayreuth, Germany. ([email protected])
Introduction and Aim: A combination of new analyses of Apollo-era samples [1,2] and lunar seismic
data [3], orbital measurements from recent lunar missions by India, China, Japan and the US [4], and results
from advanced experimental and computational studies
[5-8] are revolutionising our view of the formation and
general evolution of the surface and interior of the
Moon. More detailed lunar evolution models require
quantitative constraints on the physical properties of
lunar magma at high pressures (P) and temperatures
(T). For example, the plagioclase-rich crust of the
Moon exposed in the highland terranes is widely believed to have formed due to flotation of plagioclase in
a global, crystallising lunar magma ocean. However,
dynamic models of this flotation suffer from scarcity
of data on the density and viscosity of lunar melts
(which determine in part the upward velocity of rising
crystals), at lunar high pressure-temperature conditions. The accumulation of dense oxide minerals as
they crystallize from magma, likewise critically depends on their settling velocity (v) in less dense silicate
melt. The higher the settling velocity, the further the
minerals can fall before the melt solidifies, and the
more chance they have to accumulate. The viscosity
(η) of magma governs the efficiency, rate and nature of
melt transport and affects the rates of physicochemical
processes such as magma crystallization and differentiation [9,10]. It is a consequence of atomic-scale
transport processes, and therefore directly related to
the structure and thermodynamic properties of magma.
Prediction of variations in viscosity as a function of
pressure, temperature and composition is still highly
challenging. A recent empirical model [9] has been
developed for predicting the temperature-composition
dependence of magma viscosity over the range of anhydrous and hydrous terrestrial magma compositions
so far investigated. However, it remains to be seen
whether this model can be used to accurately predict
the viscosity of lunar interior magmas. In particular,
almost no data are available for the viscosity of titanium-rich magmas that are widespread on the Moon,
and the empirical nature of the model [9] precludes
extrapolation to compositions outside of the calibration
range. The effect of pressure on magma viscosity is
very poorly constrained as well, complicating extrapolation of the low-pressure model to lunar interior pressures.The aim of this experimental study is to measure
the viscosity and density of lunar magma at high pressure and temperature as a function of composition,
using in situ falling sphere viscometry and X-ray absorption techniques.
Methods: Experiments were conducted in the toroidal
Paris-Edinburgh cell at beamline ID27 at the European
Synchrotron Research Facility, Grenoble, France. This
setup is ideally suited for high-temperature experiments at all lunar interior pressures up to its core pressure at 4.7 GPa. Starting material consisted of synthetic high-titanium Apollo 14 black glass (0.26 wt%
TiO2) and low-titanium Apollo 15C green glass (16.4
wt% TiO2) [11] along with an intermediate Ti-content
Apollo 17 (74,220) orange glass (9.1 wt% TiO2).
These compositions bracket the observed Ti content
variation in Apollo surface samples and enable assessment of the effect (linear or nonlinear) of composition on lunar melt viscosity. Our experimental technique for high-P, high-T viscosity measurements of
lunar melts relies on determining the terminal velocity
of a rhenium sphere falling through molten samples
[12]. The falling sphere method is based on Stoke’s
law which relates viscosity to the terminal velocity of a
sinking sphere by the relation:
,
where η is the viscosity (Pa s),ρS-ρM the density contrast between the sinking sphere and the melt (kg/m3),
a the acceleration of typically 1 g (m/s2), r the sphere
radius (m), and v the sinking sphere velocity (m/s). CF
is the Faxen correction term which accounts for interactions between the sinking sphere and the wall of the
capsule. The density term is particularly well constrained in our experiments because of previous in situ
measurements of ρM [11] as well as in situ density
measurements performed directly after our viscosity
measurements using X-ray absorption techniques
[11,13]. Previous work [12,13] has resulted in the development of a stable assembly that can be used for
both density and viscosity measurements on the same
sample.
Results: The terminal velocity of the Re spheres was
determined from the radiographic images, taken every
20 ms during the falling period (Figure 1). Preliminary
analysis of our results indicate the viscosity coefficient
for the green glass composition to be 0.18 Pa s at conditions of 1.2 GPa and 1860 K. As illustrated in Figure
2a, this value is lower than the model predicted value
of 0.31 Pa s and lower than a previously published
experimentally derived value of 0.64 Pa s for the green
44th Lunar and Planetary Science Conference (2013)
Figure 1: Radiographic images of a falling Re
sphere in molten Apollo 15C green glass. The
Re sphere is clearly visible as a dark shadow
against the light grey sample
glass composition at 3.1 GPa and 1933 K [14]. For the
black glass composition, we obtain a viscosity of 0.14
Pa s at conditions of 1.1 GPa and 1820 K. This result is
in excellent agreement with the previously reported
experimental value of 0.19 Pa s [15] at similar P-T
conditions (1 GPa and 1733 K), but an order of magnitude lower than the model prediction [9].
a
b
1258.pdf
restrial basalts [16,17]. Such low viscosities would be
consistent with thin and extensive lunar lavas flows.
We also find significant differences between the experimentally derived values and the model predicted
values especially at high Ti contents, which underlines
the need for a systematic experimental determination
of viscosities of lunar melts under relevant conditions. Recent numerical modeling [18] indicates that plagioclase floatation on the Moon is possible despite the fact
that plagioclase only begins to crystallize after the lunar magma ocean is ~80% solidified. However, the
results of such models are sensitive to the values assumed for several interdependent physical properties
of the magmatic liquid including density and viscosity.
For example, viscosity values for the magmatic liquid
were assumed to be in the range 0.01 to 0.1 Pa s in ref.
[18]. Our intinial measurements for lunar melt viscosities lie outside this range. By looking at different bulk
compositions our data provide insight into first-order
effects of titanium on melt physical properties. The
possibility of linking viscosity variations to local melt
structure variations paves the way to comprehensive
structural models of magma viscosity. Ultimately, we
intend to use our lunar magma viscosity and density
measurements as input parameters for an integrated
model of the dynamic evolution of the lunar magma
ocean.
References: [1]Borg et al. (2011) Nature, 477, 70-72;
[2]Hauri et al. (2011) Science 333, 213; [3]Weber et
al. (2011) Science 331, 309; [4]Wieczorek et al. (2012)
Science (10.1126/science.1231530); [5]De Vries et al.
(2010) EPSL 292, 139; [6]Jutzi and Asphaugh (2011)
Nature 476, 69; [7]Zhong et al. (2000) EPSL 177, 131;
[8] Cuk and Stewert Science 338, 1047-1052;
[9]Giordano et al. (2008) Earth Planet. Sci. Lett. 271,
123; [10] Dingwell, D.B. (2006) Elements 2, 281–286.
[11]Van Kan Parker et al.(2012) Nature Geosc. 15, 8689 ; [12]Perrillat J.P. et al. (2010) High Press. Res., 30,
415-423. [13]Van Kan Parker et al. (2010) High Press.
Res. 30, 332-341; [14] Maeda et al. (2000) Japane-
se
Geosciences
Union,
abstract
PbP004;[15]Suzuki et al. (2009) Photon factory Activity
Report, 14C2/2007S2-002; [16] Sato, H. (2005) Journal of Mineralogical and Petrological Sciences, 100,
133-142 [17] Villeneuve et al. (2008) Chemical Geology, 256, 242-251 [18] Suckale et al. (2012) JGR, 117
Figure 2: a: Comparison of viscosity of Apollo 15
Green glass for a A) molten sample from this study at
1.2 GPa and 1860 K to B) Model predicted [9] and
C) from ref. [13] b: Comparison of viscosity of Apollo 14 Black glass for a A) molten sample from this
study at 1.1 GPa and 1820 K to B) Model predicted
[9] and C) from ref. [14]
Discussion and Outlook: First indications suggest
that experimentally derived viscosity coefficients for
the lunar glasses are lower than those reported for ter-