Download Formation of Barriers to Melt Ascent at the Base of the Ionian

Lunar and Planetary Science XLVIII (2017)
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FORMATION OF BARRIERS TO MELT ASCENT AT THE BASE OF THE IONIAN LITHOSPHERE.
J. Schools1 and L. G. J. Montési1, 1Department of Geology, University of Maryland, College Park
([email protected]; [email protected]).
Introduction: Melt from the presumed magma
ocean in Io’s interior reaches the surface at well documented paterae and hotspots. To do so, melt needs to
cross the thermal lithosphere of Io, even though, as it
loses heat, it may stall inside the lithosphere. We model here the crystallization sequence of melts as they
rise through the lithosphere of Io and determine under
what conditions a permeability barrier may form. The
barrier is generally deep near the base of the lithosphere, although it may become unstable, so that melt
may be focused and form a heat pipe through the lithosphere and a central volcanic edifice at the surface.
The resurfacing rate of Io also plays a significant role
in barrier formation. A feedback between depth of the
barrier and resurfacing may lead to an equilibrium
condition between these two quantities.
Permeability Barriers: Permeability barriers
probably form near the base of planetary lithospheres
as magma ascends towards the surface [1].
In the hot mantle beneath the lithosphere, melt likely resides in a porous network along solid grain edges.
The relatively high permeability of this network lets
melt travel upwards due to buoyancy. When this melt
enters the colder lithosphere it begins to crystallize. If
the crystallization happens a high rate, then the newly
formed crystals can clog the pore space, reducing its
permeability to essentially zero [2]. This area of zero
permeability is the permeability barrier.
Subsequent melt continues to rise through the lithosphere, accumulating underneath the barrier. The pressure in the accumulation zone increases and the solid
matrix expands (i.e., decompacts) in response to the
pressure increase [1,3,4]. The resulting high-porosity
layer beneath the permeability barrier is called the decompaction channel.
The concept of a permeability barrier and the underlying decompaction channel was introduced to explain the observation that magma is produced over a
wide area (hundreds of kilometers) under mid-ocean
ridges but only reaches the surface within 2 km of the
ridge axis [1]. Melt must be focused to the ridge and
the permeability barrier provides a way to explain that
focusing.
Barrier Formation in Io: We use the MELTS
[5,6] thermodynamic calculator with the alphaMELTS
front-end interface [7] to determine the composition of
melt and its crystallization sequence as it ascends
through the Ionian lithosphere.
Starting with an bulk silicate composition appropriate for Io [8], we calculate the aggregate melt composition generated by fractional melting upon decompresssion along a mantle adiabat to a depth representing the base of the thermal lithosphere. The mantle
potential temperature and lithospheric thickness are
varied to create a continuum of possible melt percentages and compositions. The crystallization sequence of
that melt is determined along a pressure-temperature
path representing the geothermal gradient of the lithosphere from the termination of the melting calculation
to the surface, as influenced by the burial rate of the
Ionian surface [9]:
𝑇 = 𝑇! + ∆𝑇
𝑒
!
!
𝑒
−1
!
!
where ∆𝑇 is the temperature difference between the
base of the lithosphere and the surface, 𝑇! is the surface temperature, d is the lithosphere thickness, and
𝑙 = 𝛼/𝑣, with 𝛼 the thermal diffusivity and 𝑣 the burial
rate.
In our nominal model we assume a resurfacing rate
of 1 cm/yr. The crystallizing mineral phases and crystallization rates are recorded. This method is largely
the same as in our previous work simulating permeability barrier formation in the Martian lithosphere [10],
albeit with parameters appropriate for Io and the additional effect of resurfacing on the temperature profile.
A permeability barrier will form where the compaction length 𝛿! [3] is larger than the critical compaction length 𝛿!∗ [2]:
𝛿! ≥ 𝛿!∗ where 𝛿!∗ =
𝑑𝑇 𝑑𝑓
𝑑𝑧 𝑑𝑇
!!
and 𝛿! =
4
𝑘! 𝜁 + 𝜂
3
𝜇
The compaction length depends on melt porosity φ and
grain size d through the matrix permeability k , whereas the matrix shear viscosity 𝜂 and bulk viscosity
𝜁~ 𝜂/𝜙 depend on strain rate 𝜀 and temperature. µ is
the melt viscosity. In our reference model, φ=1%,
d=3mm, and 𝜀=10-15 s-1.
θ
Lunar and Planetary Science XLVIII (2017)
2723.pdf
orders of magnitude to 10-10-10-9 s-1, the permeability
barrier formation level may be raised by less than 1 km
to be associated with the crystallization peak of clinopyroxene or nepheline. Reducing the resurfacing rate
also rises the permeability layer. An extreme minimum
of 0.02 cm/yr can raise the barrier up to 10 km in lithospheres thicker than 30 km.
Implications: Melt clearly reaches the surface of Io.
However in our models a permeability barrier always
forms near the base of the lithosphere, blocking melt
ascent. Heat released by melt crystallization may increase the temperature at the base of the lithosphere,
allowing melt to rise past the nominal permeability
barrier level. Melt would be focused to any location
where the barrier is slightly elevated, release more heat
from crystallization, and elevate the barrier further.
Due to this potential melt focusing process, shallow, rising barriers are more likely to be breached,
possibly forming the heat pipes expected on Io. In an
initial system with no resurfacing, a relatively shallow,
weaker barrier may form. This barrier may be
breached, increasing the resurfacing rate. As the lithosphere cools due to resurfacing, the barrier becomes
deeper. This may reduce the resurfacing rate, creating
an equilibrium. Mantle potential temperature does not
appear to effect this process, at least within expected
Ionian values.
A secondary effect due to strain rate may create a
positive feedback loop. As Io resurfaces, it generates
large tectonic stresses resulting in its thrust-driven,
large, non-volcanic, mountain ranges. Areas of tectonic
activity deep in the lithosphere, and therefore laterally
distant from the mountains themselves, would have an
increased strain rate, which, if large enough, would
raise the local permeability barrier. Although the this
effect may be small, it may be sufficient to initiate melt
focusing, breaching the barrier, and increasing the resurfacing. The thrust themselves, if they penetrate deep
enough in the Ionian lithosphere, may also tap melt
otherwise trapped by a permeability barrier.
Figure 1: Crystallization sequence of melt rising in
a 25 km thick Ionian lithosphere overlying a mantle
with a potential temperature of 1350ºC: a) Mass of
solid and liquid phases as a percentage of intial liquid mass. b) Crstallization rate of mineral phases
expressed as percentage over change in depth. c)
Bulk crystallization rate of all mineral phases and
barrier locations based on compaction lengthscales.
In the nominal model permeability barriers always
form at the base of the lithosphere while olivine crystallizes (Fig 1). If the strain rate is raised by several
References:
[1] Sparks D.W. and Parmentier E.M. (1991)
EPSL, 105, 368-377. [2] Korenaga J. and Kelemen
P.B. (1997) JGR, 102, 27729-27749. [3] McKenzie D.
(1984) J. Petrol., 25, 713-765. [4] Spiegelman, M.,
Phil. Trans. R. Soc. Lon., 342, 23-31. [5] Ghiorso M.S.
and Sack R.O. (1995)
CMP, 119, 197-212.
[6] Asimow P.D. and Ghiorso M.S. (1998) Am. Mineral., 83, 1127-1132. [7] Smith P.M. and Asimow P.D.
(2005) G3, 6, Q02004. [8] Keszthelyi L. and McEwen
A. (1997) Icarus, 130, 437-448.[9] O’Reilly T.C. and
Davies G.D. (1981) GRL, 8, 313-316. [10] Schools J.
and Montési L.G.J. (2016) LPSC, 47, 2080.