GEO 5/6690 Geodynamics 24 Nov 2014

© A.R. Lowry 2014Read for Mon 1 Dec: T&S 410-427

Last Time: Lithosphere & Plate Tectonics

Given a rigid lithosphere traveling at some velocity u0 over a low-/constant-viscosity asthenosphere with thickness h, Couette flow predicts: Lowest viscosity in the Earth occurs between ~100 to 400 km depths (on average) but Gyr+ aged structures appear to be preserved under some continental regions to 200-300 km! So why not sheared away or removed by R-T instabilities like under oceans?

Implies some combination of more buoyant/stronger lithosphere

Tectosphere hypothesis: Depletion of basalt leaves a more buoyant residuum mantle (fertile lherzolite peridotite) because melt consumes pyroxene & garnet (& increases vP!)

€

u = u0 1−z

h

⎛

⎝ ⎜

⎞

⎠ ⎟

Water Solubility in Aluminous Orthopyroxene andthe Origin of Earth’s Asthenosphere (2007)

Katrin Mierdel, Hans Keppler, Joseph R. Smyth,

Falko Langenhorst

Fundamental question:

• Which definition(s) of

lithosphere &

asthenosphere are

being used in this

paper?

Water Solubility in Aluminous Orthopyroxene andthe Origin of Earth’s Asthenosphere (2007)

Katrin Mierdel, Hans Keppler, Joseph R. Smyth,

Falko Langenhorst

Fundamental questions:

• What is responsible for

“seismic LAB”?

(Possibilities include

melt, water, ???)

• How does this impact

rheology?

Partial Melt?

Seismic LAB could be explained by presence of a small fraction of partial melt as intergranular film

Water would help to facilitate (because mantle temps are below the dry melting point of peridotite but above the water saturation solidus)

mantleplumes.org/images/LithDelamFig1a_500.gif

Partial Melt?But, would low degrees of

partial melt form intergranular film?

And, why then would there be a lower boundary to the LVZ (here described as 60-220 km under oceans; 150-220 km under continents)?

mantleplumes.org/images/LithDelamFig1a_500.gif

Methods• Experiments were carried out in an

end-loaded piston-cylinder apparatus

• Mixtures of Mg(OH)2, Al(OH)3, and SiO2 were sealed in platinum-rhodium (Pt0.95Rh0.05) capsules together with about 20 weight % of liquid water

• Experiments were run at 15 to 35 kbar and 800° to 1100° C for a few days

• Perfectly clear, inclusion-free single crystals of orthopyroxene were analyzed

*Stoichiometry of the starting mixture was chosen to correspond to aluminous orthopyroxene plus small amounts of olivine and spinel or garnet

Example of an end-loaded piston-cylinder apparatus capable of producing pressures up to 5.5GPa

Data

Polarized infrared spectra were measured

Water contents were calculated from the infrared data using extinction coefficients

Chemical analyses were obtained using an electron microprobe

Results

• Water contents in the aluminous pyroxenes are high (close to 1wt% at low pressure and temperature)

• Water solubilities decrease with both pressure and temperature (opposite of olivine and Al-free enstatite)

Table 1 – water solubility in Al-saturated enstatite

• Water contents correspond to the absorbances measured parallel to the 3 crystallographic axes

• Bulk water contents are obtained by adding these values

Figure 1 – polarized infrared spectra of 2 Al-saturated crystals synthesized at 1100°C and 15kb

• Average water contents derived from bulk water contents of all samples at a given P and T

Figure 2 – Polarized infrared spectra and total water contents of Al-saturated enstatite at 800° to 1100°C and 15 to 35kb.

• the presence of foreign phases in the crystals is unlikely:

– the infrared bands are strongly polarized

– measurements were taken only on perfectly clear and optically inclusion-free crystals

Figure 3 – High-resolution tranmission electron microscopy image of an Al-saturated enstatite with 0.75 wt% water.Shows no evidence for foreign phases or planar defects.

electron microprobe analyses

• suggest most of the water is dissolved by the coupled substitution of Al+3 + H+ for Si+4 and by the substitution of Al+3 + H+ for 2Mg+2

– both substitutions appear to occur in equal abundance

• Al is distributed equally among tetrahedral and octahedral sites, irrespective of water content– both substitution mechanisms imply a molar 1:1 ratio btw Al

and H

For aluminous enstatite…• total water solubility = water solubility in Al-free enstatite

+ water solubility coupled to Al

Water solubility of Al-free enstatite:

Water solubility coupled to Al:

A = 0.01354 ppm/bar fH2O is water fugacity -ΔH1bar = -4563 J/mol ΔVsolid = 12.1 cm3/mol R is the gas constant P is the pressure T is the absolute temp

AAl = 0.42 ppm/bar0.5

ΔH1barAl = -79,685 J/mol

ΔVsolidAl = 11.3 cm3/mol

Figure 4 – Water solubility in upper-mantle minerals as a function of depth for a typical continental shield and oceanic geotherm.

Continental and oceanic low-velocity zones correlate with a sharp decrease of water solubility in aluminous enstatite

Continental and oceanic low-velocity zones correlate with a sharp decrease of water solubility in aluminous enstatite

• At a bulk water content of 800ppm, the low-velocity zone would be oversaturated with water (water activity = 1)

– Water activity around 0.1 would induce partial melting

• such water conditions are to be expected in the upper mantle

Continental and oceanic low-velocity zones correlate with a sharp decrease of water solubility in aluminous enstatite

• this provides a straightforward explanation for the seismic observation that the top of the low velocity zone is sharp whereas the lower boundary is diffuse

Continental and oceanic low-velocity zones correlate with a sharp decrease of water solubility in aluminous enstatite

• as water solubility in mantle minerals sharply increases, the fraction of partial melt in equilibrium with these minerals will also sharply decrease (at the asthenosphere-lithosphere boundary)