DI13C-01 INVITED
Current Status of our Understanding of the Water Content in the Mantle Transition Zone
The water content of the mantle transition zone is a key factor that controls (or reflects) the nature of materials circulation in Earth. Although the maximum amount of water in the transition zone is well constrained by the experimental data on the solubility of water in the constituent minerals, actual water content in the mantle transition zone is highly controversial, and the published results range from almost dry to nearly water-saturated transition zone. In this presentation, we will review various methods to infer the water content in the transition zone including those using (i) seismological observations and (ii) electrical conductivity. Seismological observations used to infer water content include (i) the thickness-velocity anomalies correlation, (ii) the sharpness of the 410-km boundary and (iii) the seismic wave velocities. Limitations of this approach are that the direct influence of water on seismic wave velocities is appreciable only for large water contents (1 % or more) and the physical processes to control the sharpness of the 410-km are not unique. In contrast, the physical processes by which water (hydrogen) affects the electrical conductivity is well understood and the influence of water on electrical conductivity is large even for a relatively small water content. Therefore if the electrical conductivity in the mantle transition zone is known, and temperature is constrained within a certain range, the water content in the transition zone can be estimated from the comparison of laboratory data on electrical conductivity with geophysically determined electrical conductivity in the transition zone. However, there are a few technical details that one must pay attention to in order to obtain robust results on the influence of water on electrical conductivity. (i) Electrical conductivity must be determined from the measured impedance for a broad range of frequencies to eliminate the influence of charge accumulation (at electrodes and/or grain-boundaries). (ii) In order to determine the electrical conductivity for dry sample, extreme care must be paid for sample preparation because a substantial amount of water easily goes to a sample (wadsleyite, in particular) from the pressure medium without adding any water. We have conducted new measurements on the electrical conductivity in wadsleyite under a broad range of conditions and have shown that (i) the electrical conductivity of truly dry wadsleyite is much smaller than that of nominally dry wadsleyite by Yoshino et al. (2008), (ii) the electrical conductivities determined at 0.01 Hz (most of Yoshino et al. (2008) data were collected at this frequency) are systematically lower than those determined from the impedance spectroscopy from a broad frequency range (e.g., Huang et al. (2005)), and (iii) there is little dependence of activation energy on water content in the water-rich regime whereas Yoshino et al. (2008) found systematic decrease in activation energy with water content in the water-rich regime. However, we did reproduce Yoshino et al. (2008)'s results on water-rich wadsleyite if we use only one frequency (0.01 Hz). Therefore we conclude that the cause for the discrepancy between Huang et al. (2005) and Yoshino et al. (2008) is mainly the use of inappropriate methods by Yoshino et al. (2008), and the estimate of water content by Huang et al. (2005) remains the best estimate for the Pacific transition zone (~0.1-0.2 wt %), although the water content in the transition zone beneath the Philippine sea is much larger (1 % or more).
DI13C-02 INVITED
Effects of hydration on the elastic properties of transition zone minerals
Water, dissolved as hydroxyl into the solid silicate minerals of the upper mantle, can reduce elastic wave speeds through associated defects. Efforts are underway to use new mineral physics data on how hydration affects mineral elasticity to interpret regional seismic tomography images showing potential spatial variability in mantle hydrogen content. Because the water storage capacity of olivine, wadsleyite, and ringwoodite approaches one weight percent at depths of 300-660 km, the effects of hydration on the elastic properties of transition zone minerals are needed to evaluate seismic anomalies that are not easily explained by temperature anomalies alone. We review recent and forthcoming mineral physics data on the high-pressure elastic properties of hydrous olivine, hydrous wadsleyite, and hydrous ringwoodite measured using Brillouin spectroscopy and GHz-ultrasonic interferometry. We examine regional tomography models beneath the eastern US through forward modeling of the mineral physics data that suggest a relatively high water contents compared with the surrounding mantle, which is interpreted to have derived from subducted oceanic lithosphere during Farallon subduction from west.
DI13C-03 INVITED
Cation Disorder and Hydrogen Defects in Ringwoodite and their Effects on Wave Speeds in the Earth's Transition Zone
The structure and stability of the Mg2SiO4 polymorphs play a key role in the density and impedance contrasts across the boundaries in the Earth's transition zone. Inclusion of the effects of point defects on the elasticity of spinel-structured Mg2SiO4, ringwoodite, helps reconcile mineralogical models of the transition zone with seismic observations of the impedance contrast at 520-km depth. Ringwoodite can contain significant amounts hydrogen as well as thermally activated coupled point defects between Mg- and Si-sites. This study uses density functional theory to calculate the effects of Mg-Si disorder on the elasticity of ringwoodite as well as hydrogen incorporation mechanisms in the Earth's transition zone. The energetics of Mg2SiO4 -spinel, inverse spinel, and disordered structures predict ~4% Mg-Si disorder in the mantle's transition zone, where 100 K lateral temperature variations can cause ~1.2% change in the disorder population. Lateral temperature variations in the transition zone lead to wave speed changes due to the minerals' intrinsic temperature-related elasticity changes. Independent of temperature, 1% cation disorder causes an additional decrease in c11 and c44 of 0.8% and 0.4%, respectively. In addition to cation disorder, ringwoodite can host more than 1 wt% H2O. Calculations show that the dominant hydrogen substitution mechanism is through a VMg+2H mechanism with the hydrogen located on the faces of the vacant Mg site. Calculations of the elastic constants at 1.6 wt % H2O are broadly consistent with measurements on hydrous ringwoodite. The seismic wave speeds in the transition zone are predicted to be lower for both hydrated and disordered ringwoodite when compared to normal ringwoodite, but with a different character: dlnvs/dlnvp is 15% and 20% greater for anhydrous disorder than due to the effects of temperature alone or variable hydration, respectively. Because the structure and stability of the Mg2SiO4 polymorphs play a key role in the density and impedance contrasts across the boundaries in the Earth's transition zone, the effect of defects has to be taken into account along with the effects of water and iron.
DI13C-04 INVITED
Petrologic Anomalies in the Mantle Transition Zone
Traditionally, anomalies of seismic wave speeds in the deep mantle are attributed to variations in
temperature alone. Lately, a variety of evidence is accumulating for petrologic causes of seismic anomalies in
the mantle transition zone (TZ), including effects of kinetics, volatiles and composition. For instance, along
the Tonga subduction zone, where 2/3 of all deep earthquakes occur, preferred alignment of highly
anisotropic material such as metastable olivine must be present to account for over 1% of polarization
anisotropy in the TZ where outboard earthquakes that extend about 1000 km farther to the west of the
Wadati-Benioff zone (WBZ) occur. High P- and S-wave speeds (VP and VS) are conspicuously
absent in the anisotropic region. Instead, this region is flanked by a band of isotropic anomaly, characterized
by a sudden onset of modest-sized high VP and VS which gradually diminish away from the
outboard earthquakes -- precisely how a petrologic anomaly of metastable olivine is expected to be
surrounded by a pure thermal anomaly of modest amplitude. Meanwhile, similar anisotropy and lack of high
VP and VS in the sub-horizontal, leading edge of the WBZ indicates it is indeed the predecessor of
the petrologic anomaly, and its eventual aseismic (and pure-thermal) remnants have been found along other
subduction zones in the western Pacific.
A contrasting example is the absence of high VS where a large-scale anomaly of high VP was
recently recognized in the TZ beneath central Tibet. A likely cause of the discordant anomalies in VP
and VS is water in nominally anhydrous polymorphs of olivine. Prior to thickening by continent collision,
the Tibetan lithospheric mantle was part of a mantle wedge which has been hydrated during past episodes of
subduction. The aseismic nature of the Tibetan anomaly is consistent with the fact that a small amount of
water is likely to eliminate any metastable olivine. Furthermore, convective removal of thickened sub-
continental lithospheric mantle is potentially a new pathway for water to enter the TZ.
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