Fluid Processes in Subduction ZonesHydrous Minerals andDehydration Reactions

Simon M. PeacockDept. of Earth and Ocean Sciences

University of British Columbia

Effects of H2O on Subduction Zones

• H2O and hydrous minerals weaken plate interface permitting subduction and plate tectonics to occur

• H2O lowers melting temperature of mantle, generates explosive arc magmas

• Fluids released by slab dehydration promote brittle behavior and may trigger earthquakes

• Hydration structure, rheological structure, and thermal structure of subduction zones are strongly coupled

Selected Characteristics of Volcanic Arcs

• Depth to underlying Benioff zone is ~100 – 200 km• Depth to underlying Benioff zone beneath volcanic front is 124±

38 km (Gill, 1981)– suggestive of P-dependent melting or slab dehydration rxn– or may be necessary depth for slab in order to be overlain by

hot asthenosphere such that addition of H2O triggers melting• Several places where active arc is missing (or feeble)

– Peru, central Chile, SW Japan• T eruptions – 1050-1100°C• T mantle equil – commonly 1300°C+ for basalts

Geochemistry of Arc Magmas

Compared to MORBs, arc magmas are:• Fractionated

• basalts andesites dacites rhyolites

• Wet• Enriched in many trace elements, such as Rb,

that appear to be derived from the subducting slab

H2O Content of Arc Magmas

• Significantly more H2O than MORBs, OIBs

• Explosive eruptions (more H2O degassing, higher SiO2 content)

• Hornblende (amphibole) is common phenocryst

• H2O dramatically lowers melting T of rocks, mantle by 100s

of degrees. key to arc magma genesis

H2O measurements of volcanic glasses • For submarine basaltic glasses, we

can measure H2O contents directly because ocean pressure prevents H2O from exsolving.

• For subaerially erupted glasses, measure glass inclusions.

Stern (2002)

MORB glasses < 0.5 wt % H2O

Back-arc basin glasses 0 – 2.5 wt % (ave = 1.1 wt %) H2O

Arc glass inclusions 0 – 6 wt % (ave = 3.4 wt%) H2O

Arc MagmasDistinctive Trace Element Characteristics

• Arc basalts, as compared to MORBs, are – Enriched in large-ion lithophile elements (LILE)

K, Rb, Cs, Sr, Ba, and Pb, U, B, Be

– Depleted in high field strength elements (HFSE)Y, Zr, Hf, HREE, Nb, Ta (controversial niobium anomaly: Ti-phase

in source?, prev. melting event?)

• H2O-rich fluids will preferentially contain more mobile LILE – derived from slab

• Be and Th are relatively enriched in arc basalts, but believed to be relatively insoluble in H2O-rich fluids suggests sediment melting

Zandt (2002)

Variably hydratedoceanic crust andmantle

Hydration of Oceanic Crust & Mantle

Hydrothermal circulation at mid-ocean ridges• Black smokers• Adds H2O, CO2 to crust• Basalt, gabbro hi T, lo P minerals (amph, chl, epi)• Believed to be limited to oceanic crust, but could go deeper at

slow-spreading ridges• Sea-floor weathering

• Low T alteration

Likely additional hydration at trench - outer-rise, including serpentinization of uppermost mantle

Global Subduction H2O Fluxes

Input fluxes: [1012 kg/yr]

Pore H2O Sediments 1 Expelled by

Oceanic crust 0.1 porosity collapse

Chemically bound H2OSediments 0.1 Expelled byOceanic crust 1 to 2 metamorphicOceanic mantle 0.1 to 1 dehydration rxns

Output flux to arc magmas: 0.1 to 0.4 (5-20% of input flux)

Mass of hydrosphere = 1.4 x 1021 kg

• Oceanic crust: basalts = 1-3 wt % H2O gabbros = 0.5-1 wt % H2O

• High-pressure metabasalts contain more bound H2O than altered basalts recovered in DSDP/ODP drill core.

Peacock (2004)

Hyndman and Peacock (1999)

Porosity collapse: Sediment compaction Basalts ~ 200-400 °C

Slab dehydration reactions Temperature-dependent

Thermal structure of a subduction zone determines where H2O is released from slab and where hydrous minerals may be stable in the overlying plate.

Thermal structure of subduction zones

• Subducting slab drags down isotherms• Inverted isotherms beneath mantle wedge• Mantle-wedge isotherms parallel to flow lines

• Tinterface ~ 0.5 Tmantle

Cool vs. Warm Subduction Zones

after Peacock and Wang (1999)

Hyndman and Peacock (1999)

In warm subduction zones, H2O is liberated from the slab by metamorphic dehydration reactions and possibly by the collapse of porosity in the upper crust.

The amount of H2O released is predicted to be small:0.1 x 10-3 m3 / (m2 yr) =100 milliliters of H2O per m2 column per year

Metamorphic facies

– diagnostic mineral assemblage indicative of region in P-T space (particularly used in mafic systems)

Facies boundaries – complex reactions

Dehydration of subducting oceanic crust

• Metabasalt dehydration reactions are generally continuous reactions, which are smeared out in P-T space

• Metabasalt eclogite rxns release large amounts of H2O, increase density, and increase seismic velocity

blueschist eclogite

Thermal models: Peacock and Wang (1999) Currie et al. (2000) Peacock et al. (2002)

Crust subducted in warm subduction zones passes through greenschist epidote bluseschist/greenschist eclogite facies.

And transformation of metabasalt to eclogite (garnet + cpx) occurs at ~50 km.

Bostock et al. (2002)

Cascadia (central Oregon)

• Serpentinized forearc mantle• Metabasalt --> eclogite

For a given P, T, and composition (X), what is the mineralogy of the rock?

• P = g z• T = thermal models• X = bulk composition, including H2O

content - fully hydrated or anhydrous?

P

T

coesite

quartz

qtz = coes

Grxn = 0

dP/dT = S/VClapeyron slope

Higher pressure higher Higher temperature higher S

Phase diagram for SiO2

~2.5 GPa

P

T

serpentineolivine +orthopyroxene +H2O

Dehydration Reactions

dP/dT = S/V = + / - = negativebecause H2O is compressible

dP/dT = S/V = + / + = positive

Dehydration rxns:

In nature, many reactions are “smeared out” in P-T space because of:• Solid solutions – particularly in mafic rocks

(e.g., Fe-Mg)• Variable fluid compositions• Kinetics

Dehydration rxns (cont.):

Phase diagrams can be based on :• Experiments (sluggish at T < 600°C)• Thermodynamic calculations (must know

all phases, solid solutions)• Field-based petrologic studies

(observations + P-T-ometry)• Combinations of all 3

Schmidt + Poli (1998)

Experimentally determined phase relations in H2O saturated MORB

Schmidt + Poli (1998)

Maximum H2O contents bound in hydrous minerals in MORB

Schmidt + Poli (1998)Phase diagram for H2O-saturatedperidotite and maximum H2O content

Stern (2002) based on Schmidt and Poli (1998)

Important points of Schmidt & Poli (1998)

• Numerous hydrous minerals stable in subduction zones. Amphibole is not the key

• Dehydration of slab is ~continuous due to smeared out P-T rxns + isotherm / isobaric structure

• Volcanic front controlled by wedge isotherm (not a specific dehydration rxn)

• Serpentine phase A may transport H2O to great depth in cool s.z.

• Distribution + amount of H2O incoming lithosphere is critical unknown

Hacker et al. (2003)

Phase Diagram for Metabasalt

Based on field observations and thermo calculations

Facies boundaries are broader than depicted

Different parts of subducted crust intersect facies boundaries at different places

T

P

Hacker et al. (2003)

Hacker et al. (2003, JGR)

Hacker et al. (2003, JGR)

Hacker et al. (2003, JGR)

oceaniccrust

oceanicmantle

trenchmid-ocean ridge

crust

mantle?

outer rise

approximatescale

0 50 km

50km

0

1

2

3

4

5

wedge

(1) Updip fluid flow along faults to seafloor/surface(2) Incorporated into forearc crust (hydration)(3) Incorporated into forearc mantle (hydration)(4) Incorporated into arc magmas(5) Subducted past volcanic arc

Where does H2O expelled from the subducting slab go?

Zandt (2002)

Mg2SiO4 + MgSiO3 + 2H2O = Mg3Si2O5(OH)4

olivine pyroxene fluid serpentine(13 wt% H2O)

Example of retrograde metamorphic reaction:Serpentinization of the forearc mantle wedge

Serpentine mud volcanoes inMariana forearc (Fryer et al.)

• Active serpentine mud volcanoes (Mariana forearc)

• Hydrated ultramafic hanging walls of paleosubduction zones

• Low seismic velocities, high Poisson’s ratios observed in forearc mantle (Alaska, Aleutians, Andes, Cascadia, Costa Rica, Izu-Bonin, Japan, Mariana)

Evidence for Serpentinized Forearc Mantle

Consequences of serpentinized forearc mantle

• “Weak” serpentinite may control downdip limit of seismogenic zone and reduce mechanical coupling b/w slab and wedge

• Buoyancy will tend to isolate forearc wedge from corner flow

• Heating of hydrated forearc mantle (e.g., ridge subduction, post-subduction) will release significant amounts of H2O

Zandt (2002)

Bostock et al. (2002, Nature)

Cascadia (central Oregon)

Isoviscous vs. Olivine Rheology

van Keken, Kiefer, and Peacock (2002)

Abers et al. (2006)

Mantle Wedge and Interface Rheology

Isoviscous wedge

Non-Newtonian wedge

Non-NewtonianCold nose (0% coupling)

Non-NewtonianCold nose (10% coupling)

Ranero et al. (2005)

Zandt (2002)

The End