Lunar and Planetary Science XLVIII (2017)
2390.pdf
PRESERVATION OF PRIMORDIAL CHEMICAL SIGNATURES IN EARTH’S MANTLE BY
PRESSURE INDUCED FREEZING AFTER A GIANT IMPACT. S. J. Lock1, S. T. Stewart2 and S.
Mukhopadhyay2, 1Harvard University, Department of Earth and Planetary Sciences ([email protected]);
2
University of California Davis, Department of Earth and Planetary Sciences.
Introduction: The late stages of planet formation
are marked by collisions between protoplanets. These
giant impacts are highly energetic events, and substantial portions of the silicate mantles melt or vaporize
[1]. In addition, planets can acquire significant angular
momentum (AM) via one or more giant impacts
[2,3,4]. The last giant impact experienced by Earth is
thought to have formed the Moon and marks the end of
the main stage of terrestrial accretion.
Despite the cataclysmic nature of giant impacts, recent studies have suggested that the Earth's lower mantle records chemical signatures that predate the formation of the Moon. Xenon and tungsten isotopic data
show that mantle plumes sample a reservoir that
formed within the first few 10’s of Myr of the solar
system [e.g., 5,6]. This primordial signature must have
survived the Moon-forming giant impact without being
homogenized with the rest of the terrestrial mantle.
Due to heterogeneous energy deposition and varying shear deformation, it is likely that portions of the
mantle did not mix completely during the impact [7],
although the extent of mixing is debated [8]. If a portion of the mantle was not homogenized during the
impact event, it must also survive the subsequent cooling and evolution of the post-impact planet. Most postimpact studies have focused on the evolution after the
silicate vapor is fully condensed and the body has
formed a magma ocean. However, the period of time
between the impact and the magma ocean is critical for
the survival of primordial reservoirs as Earth’s structure evolves rapidly. Here, we examine the structure
and evolution of post-impact bodies and discuss the
implications for the preservation of primordial chemical signatures in Earth’s mantle.
Structure of post-impact bodies: Most studies of
post-impact evolution start from the canonical Moonforming giant impact [9]. However, events similar to
the canonical impact represent only a small fraction of
giant impacts that occur during accretion [10]. [11]
showed that there is a range of possible post-impact
structures with widely varying thermal and rotational
states. In particular, for a rotating planetary body, there
is a thermal limit beyond which the rotational velocity
at the equator equals the Keplerian orbital velocity.
Beyond this corotation limit, the body does not resemble a conventional planet and instead forms a synestia,
a structure with a corotating inner region connected to
a disk-like outer region. Post-impact bodies above the
corotation limit form highly extended structures (Fig.
1). [11] showed that typical rocky planets are substantially vaporized multiple times during accretion and
that the formation of synestias is common.
Post-impact states are highly thermally stratified
due to heterogeneous energy deposition and buoyancy
driven re-equilibration. Typically, the lowermost silicate layer has much lower entropy than the rest of the
structure. The lower entropy mantle may be partially
solid [7] or near the liquidus [8]. The high entropy
portion of the corotating structure grades from liquid to
super-critical fluid to vapor with decreasing pressure.
After a giant impact, the pressure at a given location in the mantle can be 10’s of GPa lower than in the
present-day Earth. Figure 2 shows the pressure at the
core mantle boundary (A) and mid-mantle (50% by
mass) (B) from a suite of approximately Earth-mass
post-impact structures. Impacts were calculated using a
smoothed particle hydrodynamics code in the same
manner as [12], and the impact suite covers the range
of events expected in the giant impact stage of planet
formation [10,13]. The difference in pressure is largest
for hot, rapidly rotating bodies. Impacts similar to the
canonical Moon-forming impact produce structures
with pressures similar to the present-day Earth because
the degree of vaporization is relatively low and the AM
is the same as the present-day Earth-Moon system. The
pressure in the mantle of highly vaporized, rapidly
rotating post-impact bodies is lower for two reasons.
First, the larger centrifugal force acts against gravity to
reduce the pressure gradient. Second, material further
from the center of mass experiences a weaker gravitational force.
Cooling of post-impact structures: Post-impact
structures cool rapidly by radiation on timescales of at
least 10’s to 1000’s yrs. The loss of thermal energy
causes significant changes to the structure of the body.
The outer layers of the structure condense and contract
as the vapor pressure is reduced and the mantle eventually becomes a magma ocean. As the structure becomes more compact, the pressure in the mantle increases. For initially hot, rapidly rotating structures,
the core mantle boundary pressure can increase by 10’s
of GPa due to cooling alone, before removal of AM.
The increase in pressure during cooling can induce
freezing of a portion of the mantle that was initially
close to the liquidus or between the liquidus and solidus. Since the pressure increase is rapid, the compression is nearly isentropic. As a result, the solid crystal
fraction of certain layers can rapidly increase. Mantle
layers that were originally majority liquid could be
Lunar and Planetary Science XLVIII (2017)
References: [1] Melosh H.J. (1990) Origin of the Earth, 69
[2] Agnor C. et al. (1999) Icarus 142, 219 [3] Kokubo E. & Genda
H. (2010) ApJ,714, 21 [4] Rufu R. et al. (2017, available online)
Nature Geoscience [5] Mukhopadhyay, S. (2012) Nature 486, 101
[6] Rizo H. et al. (2016) Science 352 809 [7] Stewart S.T. et al.
(2015) LPSC 46, 2263 [8] Nakajima M. & Stevenson D.J. (2015)
EPSL 427, 286 [9] Canup, R.M. (2004) Icarus, 168, 433 [10] Stewart S.T. & Leinhardt Z. (2012) ApJ 751, 32 [11] Lock S.J. & Stewart
S.T. (submitted) JGR [12] Ćuk, M. & S.T. Stewart (2012) Science
338, 1047 [13] Quintana E. et al. (2016) ApJ 821, 126 [14] Solomatov V. & Stevenson D.J. (1993) JGR 98, 5391 [15] Gonnermann H.
& Mukhopadhyay S. (2009) Nature 459, 560 [16] Fischer R. et al.
90
30
70
50
20
30
Pressure [GPa]
Distance from midplane [106 m]
(2015) GCA 167, 177 [17] Siebert J. et al. (2013) Science 339, 1194
10
0
-10
10
1
1E-1
1E-2
-20
-30
1E-3
1E-4
0
10
20
30
40
50
60
1E-5
Radius [106 m]
Figure 1: Silicate pressures in a plane perpendicular to
the spin axis in an example impact-generated synestia
[11]. The pressures near the core are lower than for the
present-day Earth but are higher at larger radii.
160
Pressure [GPa]
140
B
CMB
120
100
80
60
40
50
45
A
Mid-mantle by mass
40
Pressure [GPa]
pushed beyond the critical crystal fraction [c.f. 14]
above which viscosity increases significantly.
Primordial chemical signatures: Pressureinduced freezing of a portion of the mantle has significant implications for the preservation of chemical signatures that were generated prior to a giant impact.
Assuming that complete homogenization is not
achieved during the event, a pre-impact chemical signature may be present in the lower-entropy region of
the mantle immediately after the event [7]. The pressure increase upon cooling is faster than melt can percolate, and partial melt would be trapped in a high viscosity layer. The high-viscosity layer is dynamically
decoupled from the fluid above it [14], effectively isolating it from the rest of the mantle. Thus, a pre-impact
chemical signature could be trapped and not be homogenized with the rest of the mantle during freezing
after a giant impact. Subsequently, 4.5 Gyr of convection has not completely mixed the mantle [15].
Moderately siderophile elements: The variation
in pressure in the mantle of post-impact bodies also has
implications for interpreting tracers of the pressuretemperature conditions of core formation. The relative
abundance of moderately siderophile elements has
been used to argue for metal-silicate equilibration in
the growing Earth at moderate pressures and near the
liquidus [e.g., 16,17]. The moderate equilibration pressures are seemingly in conflict with metal ponding at
the bottom of deep magma oceans expected to be produced by giant impacts. However, the lower pressures
in the mantle of hot, extended post-impact structures
(Fig. 1, 2) could account for the inferred pressures of
metal-silicate equilibration.
Conclusions: If the Moon-forming impact produced a highly vaporized, rapidly rotating body, then
the pressures in the mantle could be 10’s of GPa lower
than in the present-day Earth. As the Earth cooled on a
timescale of 10’s to 1000’s years, the mantle pressure
could increase by 10’s of GPa. As a result, pressureinduced freezing of a portion of the mantle could trap
pre-impact chemical signatures in a high viscosity layer. This layer would be dynamically decoupled from
the fluid above it, preventing homogenization with the
rest of the mantle during freezing and aiding in the
survival of primordial chemical signatures.
2390.pdf
35
30
25
20
15
10
106
107
Specific impact energy, QS [J kg
108
1
]
Figure 2: Pressure at the core mantle boundary (A)
and in the middle of the mantle by mass (B) in SPH
post-impact structures for planets of 0.9-1.1 Earth
masses as a function of a modified specific impact
energy [7,13]. Black squares are canonical Moonforming impacts. Gray bands show the range of pressures for cold 0.9-1.1 Earth mass planets with presentday Earth-Moon AM.