Water in the Mantle ?

Can be stored:

1) Hydrous minerals- only stable shallow

2) Dissolved in normally anhydrous minerals3) A hydrous silica-rich fluid4) Water-rich melt

Hirshmann et al [2005]

Storage Capacity of Mantle Minerals

Olivine Pyroxene

Hirschmann et al [2005]

Remember 1000 ppm = 0.1 %

Water in the Mantle

A - pyroxene storage = olivine, limited garnet storage

B - maximum assumptions

C - preferred assumptions (Hirschmann et al. 2005)

What about the Transition zone?

Natural Sample of Hydrous Ringwoodite

An ultra-deep diamond from Brazil containing a tiny inclusion (40 μm) of hydrous (~ 1 wt % water) Ringwoodite (γ–spinel)

Pearson et al., 2014

Water in the mantle – summary cartoon

How does water get into the mantle?

Estimated Water Budget Model for Hydrating the Upper Mantle

Hacker [2008] Billen [2009]

Upper Mantle Serpentinization: Central America

Sub-Moho seismic velocity in subducting Cocos plate

H2O distributed in mantle

van Avendonk et al. [2011]

Nicaragua Costa Rica

Maximum depth of extensional earthquakesLefeldt et al. [2009]

Water and Mantle Melting

Water and mid-ocean ridge melting

Asimov &Langmuir, 2003

Deeper Melting?

If Transition zone is hydrous:-Melting should occur at upwelling-Will melt be buoyant?-Melt may pool at 410 km disc.

Using EM to find water and melt in the mantle

Arc/backarc systems: Sampling the mantle with distance from the

slab – variable water content

Kelley et al. [2006]

After Tivey et al. [2012]

• Earthquakes in the slab provide a source array allowing us to image the melting regions beneath arc and backarc spreading center

• Two very different melting mechanisms – decompression and flux melting. Study the interaction between arc and backarc magmatic systems in the mantle

Broadband Ocean Bottom Seismographs (OBS) from US OBSIP facility

Lamont-Doherty Scripps

Woods Hole

Lau Arc/Backarc Mantle Structure

• Rayleigh wave phase velocity inversion using 2-plane wave method (Forsyth & Li, 2005) • Has better depth and inferior lateral resolution compared to body wave tomography• Exceptionally low velocities (3.5 km/s) beneath Central Lau S. C. at 40-60 km depth• Tonga arc is not separately resolved, anomaly is dominated by the backarc

Wei et al., 2015

Are the anomalies due to water?

• Two lines of evidence that we are not imaging water content• Capacity for storing water in mantle minerals very limited at depths ~ 100 km water < 0.01 to 0.02 wt % [Hirschmann, 2006] - would lower Qs from 80 to 60 and give only a 2% decrease in Vs [Karato, 2003] - in the hot mantle wedge larger water contents will form an aqueous melt• The largest anomalies are observed in spreading centers with lower water contents

Karato, 2003

Effect of water on attenuation Water in the Mantle Source

Kelley et al [2006]

Backarc Geochemistry and Mantle Tomography

• Images show great variation in the anomaly amplitude in the melting region (~50 km depth)• Largest anomaly is in the Northeast Lau Basin (with no spreading center) and CLSC• Anomaly beneath Northeast Lau basin could be a result of melt trapped beneath lithosphere?• Much smaller anomaly beneath the Valu Fa Ridge, with large water input from the slab• Seismic anomaly difference is too large to be caused by temperature differences• Anomaly amplitude is inversely correlated with water in the melt, suggesting decreased melt porosity for aqueous melts

Wei et al., 2015Temperatures and final equilibrationdepths from Si + Mg thermobarometry(Lee et al., 2009) provided by T. Plank

How does water affect melt porosity?

Node

Tubule

• Assume the magnitude of the seismic anomalies controlled by melt porosity

• Results suggest melt porosity is reduced in regions of high water content

• If melt moves by equilibrium porous flow: melt flux ∝ d2ϕn/μ • For similar melt flux, porosity decreases with increasing grain size or decreasing melt flux

• Water increases grain growth rates (Karato, 1989) and decreases melt viscosity, both of which will decrease melt porosity

• Essentially water increases the efficiency of melt extraction, reducing porosity

Melt Porosity

Zhu et al[2011]

Melt Flux ∝ d2ϕn/μWhere: d = grain size

ϕ = porosity μ = melt viscosity n = between 2-3