Where are these melts generated in the mantle wedge?

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Tamsyn Banks
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2) Insights into mantle melting in the presence of H 2 O. New experimental constraints. 3) What factors control melt production in subduction zones?

1 Melt generation processes in subduction zones T.L. Grove, C.B. Till, N. Chatterjee, E. Medard, S.W. Parman New experiments on H2O-saturated melting of mantle peridotite - The role of H2O in mantle wedge melting processes 1) Melting from top to bottom in the wedge. Field and experimental evidence. 2) Insights into mantle melting in the presence of H 2 O. New experimental constraints. 3) What factors control melt production in subduction zones? 4) Estimating the composition of fluid-rich component added to subduction zone magmas Topic 1: Chemical transport processes from slab to wedge. Field and experimental evidence from Mt. Shasta region, USA. Lavas are high-h 2 O mantle melts with a significant component added from the subducted slab. Where are these melts generated in the mantle wedge? Why are there variations in the H 2 O contents of magmas in the arc and back arc? Mt. Shasta, N. Calif. looking W from Med. Lake Shasta produced ~ 500 km 3 magma in ~250,000 years.

Major elements and H 2 O Wet, primitive andesites are in equilibrium with mantle residues = melts of depleted mantle 65 60 55 50 Sargents Misery Shastina Hotlum BA PMA HAOT 0.50 0.55 0.60 0.

2 Major elements and H 2 O Wet, primitive andesites are in equilibrium with mantle residues = melts of depleted mantle Sargents Misery Shastina Hotlum BA PMA HAOT Mg# O 6 H PMA BA HAOT Mg# Mantle melts in Two ways in subduction zones: Dry, adiabatic decompression Hydrous, porous flow flux melting

3 Estimates of Pre-eruptive H 2 O! H 2 O solubility in silicate melts is P-dependent and goes to ~ 0 at P = 1 bar.! So, H 2 O is often lost as a gas phase! Pre-eruptive H 2 O contents are obtained using: Thermodynamic models of mineral/melt equilibria. Effect of H 2 O on freezing path or melt composition produced during fractional crystallization. Direct measurement of H 2 O in melt inclusions. H 2 O-contents of arc magmas seem to be too high to result from any batch melting process of any potential H 2 O-bearing mantle source. Direct measurement of H 2 O in Shasta melt inclusions (Anderson, 1979). New experimental evidence changes this.

4 Sisson & Grove (1993) Estimation of pre-eruptive H 2 O content Cinder Cone Basaltic Andesite and S-1

5 Primitive BA (S-1) and PMA (S-17) Hydrous melts saturated with a harzburgite residue at top of mantle wedge > 25 % melting Opx in GPa Comp A 4.5 wt. % H2O Cpx in Oliv in 1200 Liquid Opx + Cpx in H 2 O content Oliv in P in GPa Shasta Adak Setouchi Andean MexVolBelt MPa 85-41c 200 MPa TH Fractional Crystallization CA 1 Mantle Melting SiO Shasta H 2 O-rich lavas have high SiO 2 and low FeO*, similar to adakites and Japanese sanukitoids: characteristics inherited from low-p mantle melting.

1400 1350 i n T o C 1300 1250 1200 79-35g 82-72f Liquid Ol + Pl+Liq Ol + Pl +Cpx + Liq Melting Experiments on Primitive Medicine Lake HAOT 0 0.5 1 1.

6 i n T o C g 82-72f Liquid Ol + Pl+Liq Ol + Pl +Cpx + Liq Melting Experiments on Primitive Medicine Lake HAOT P in GPa Cpx + Pl + Sp + Liq Cpx+Sp + Liq Ol + Pl + Sp + Cpx Opx + Liq Direct determination of liquidus mineral assemblage at high pressure and temperature = mantle conditions Dry HAOT magmas record melting as convecting mantle is drawn into the wedge * = vent * * * * * * * *

7 ) h m t k p ( e D Mount Shasta 200 Trend of fractional crystallization depth Slab Medicine Lake 250 Distance from trench (km) Crust Trend of mantle melting depth Shallow, hot mantle melting beneath the Cascades Inferred from Pressure of multiple saturation. T = o C. Elkins-Tanton et al. (2001)

constraints on subduction zone melting processes. Can the slab and the wedge BOTH melt?

8 T estimates of the shallow mantle wedge from Kelemen et al. (2003) includes thermal models and petrologic estimates Topic 2: New experimental constraints on subduction zone melting processes. Can the slab and the wedge BOTH melt? Can we understand the high pre-eruptive H 2 O contents of arc magmas? Mt. Shasta,. Calif. looking South from US 97.

Experimental data from Grove et al. (2006) EPSL 249: 74-89 Shows that hydrous phases are stable on the vapor-saturated mantle solidus.

9 Experimental data from Grove et al. (2006) EPSL 249: Shows that hydrous phases are stable on the vapor-saturated mantle solidus. We will use this data to develop a model for melting in the mantle wedge. Work in Progress by Till et al. (2008) is extending H 2 O-Saturated melting Stable Everywhere: Olivine Clinopyroxene Orthopyroxene

10 Piston Cylinder Experiments hot 600!m cool super-solidus (melted) texturally zoned 4 GPa 600!m hot cool sub-solidus (unmelted) homogeneous 2 GPa Piston Cylinder Experiment 820 C, 4 GPa, 168 hrs hot melt cool ol 200!m cpx 20!m opx Zoning due to melting reaction: Cpx + Opx + Al-phase " Ol + Melt garnet 20!m

11 Chlorite stable on wet solidus GPa Piston Cylinder Experiment 850 C, 2.0 GPa Piston Cylinder Experiment 840 C, 3.2 GPa chlorite chlorite 20!m 20!m Chlorite contains 12 wt % H 2 O Clinochlore: (Mg 5 Al)(AlSi 3 )O 10 (OH) 8

Medard & Grove (2006), Fumigali & Poli (2005), Pawley (2003) Chlorite breakdown

12 Chlorite on the vapor - saturated solidus a way to transport H 2 O deep into the wedge Also, Ilmenite, Rutile & Ti-clinohumite are stable. Medard & Grove (2006), Fumigali & Poli (2005), Pawley (2003) Chlorite breakdown crosses the wet solidus above 3.5 GPa. Does this cut off wet melting? Black is Martian mantle Grey is Earth s mantle

13 Where is water stored in the wedge? Hydrous phases in the mantle wedge & subducted slab. Chlorite provides a source of H 2 O for wet arc melting that is above the slab. Produced by fluid released from the slab at shallow depths. H 2 O is stored even when the slab is too hot. Chlorite also stable below the slab-wedge interface in the cool core of the slab. Where is water stored in the wedge? Hydrous phases in the mantle wedge & subducted slab. Chlorite provides a source of H 2 O for wet arc melting that is above the slab. Produced by fluid released from the slab at shallow depths. H 2 O is stored even when the slab is too hot. Chlorite also stable below the slab-wedge interface in the cool core of the slab.

Where is water stored in the wedge? Chlorite breaks down as pressure increases. Thus, the H 2 O supply through chlorite breakdown will stop at slab depths of 120 150 km.

The melting model: Melting paths calculated wherever vapor-saturated melting could occur no assumptions about melt connectivity Mibe et al. (1999).

14 Where is water stored in the wedge? Chlorite breaks down as pressure increases. Thus, the H 2 O supply through chlorite breakdown will stop at slab depths of km. Chlorite is stable below the slab-wedge interface in the cool core of the slab and this could supply H 2 O. The melting model: Melting paths calculated wherever vapor-saturated melting could occur no assumptions about melt connectivity Mibe et al. (1999). Buoyant hydrous melts leave the base of wedge and ascend into the overlying mantle by porous flow. Melt volume equilibrates with mantle at each step both thermally and chemically reactive porous flow Assumptions: initial critical melt fraction F crit = 2.5 wt. % values range from < 0.1 (Kohlstedt, 1992) to 8 % Fujii et al. (1986) Chlorite peridotite, Shikoku, Japan f.o.v.=2mm

15 Thermal model of Kelemen et al. (2003) P3,T3 solidus P2,T2 P1,T1 4 3 ~28 % H2O P1, T1 P2, T2 A 2 P3, T3 1 Vapor-sat. solidus Vertical Melting paths in wedge T o C T distribution with depth determines melting processes in wedge

16 T T2 =wall rock at P2 xt als + liq liq!t2 Phase boundaries at P1 liq + vap Melt ascends into hotter, shallower part of wedge. Melt reacts with and dissolves mantle, lowering H 2 O in melt. T1 H2O supply from slab xtals + vap Initial H2O-sat. melt at P1 Peridotite XH2O Reactive porous flow melting or Flux-Melting T P2 T2 =wall rock at P2 xt als + liq liq!f2!t2 Phase boundaries at P1 liq + vap Melt reacts with and dissolves mantle, lowering H 2 O in melt at coming into equilibrium with mantle at P2, T2. T1 H 2 O supply from slab xtals + vap Initial H2O-sat. melt at P1 Peridotite XH2O

$melt at P1 Melt ascends again into hotter, shallower wedge at T3, P3. Melt reacts with and dissolves mantle, lowering H 2 O in melt even more and increasing melt fraction.$

17 T T3 =wall rock at P3 T2 T1!F3 xtals + liq Phase boundaries at P3!T3 liq xt als + vap!f2!t2 P2 Phase boundaries at P1 liq + vap Initial H2O-sat. melt at P1 Melt ascends again into hotter, shallower wedge at T3, P3. Melt reacts with and dissolves mantle, lowering H 2 O in melt even more and increasing melt fraction.. Peridotite XH2O Flux melting process = melt extent increases and H 2 O in melt decreases melting process controlled by phase equilibria Maximum melt % occurs in a thin layer in hot core of the wedge Highest melt fraction achieved in a very thin layer. Most of wedge contains < 5% melt (double arrows). Chlorite is transported down into the wedge ABOVE the slab and gives up its H 2 O at solidus.

18 Melt at top has lowest H 2 O Melt H 2 O content continuously decreases as melt ascends and reacts w/ hotter mantle Thin layer of H 2 O-rich melt pinches out. When mantle peridotite is hydrated it contains 13 % chlorite. Bulk H 2 O of solid is 2 wt. %. 17 t) l% et Mw ( n i H 2O wt% H 2 O 0.15 wt% H 2 O Batch Melting 1.5 GPa 1.00 wt% H 2 O Melt Fraction Bulk perid. = 2 wt. % H 2 O. H 2 O content of melt could exceed 10 wt. %. High H 2 O contents are possible.

Hydrous flux melting explains the shallow final equilibration depth of arc magmas AND provides a mechanism for creating SiO 2 rich crust through arc magmatic inputs.

19 Hydrous flux melting explains the shallow final equilibration depth of arc magmas AND provides a mechanism for creating SiO 2 rich crust through arc magmatic inputs. Stable chlorite in the mantle wedge allows for high H 2 O content in arc magmas. At the same time subducted sediment and basalt can melt and transfer key trace element signatures to melts of mantle wedge. Chlorite peridotite, Shikoku, Japan f.o.v.=2mm What controls the location of volcanoes in subduction zones? From England et al Average depth to slab beneath an arc volcano is ~ 108 km.

From England et al. 2004. Distribution of arc volcanoes and intermediate depth seismicity. Perhaps these earthquakes show the zone of subducted serpentinite - chlorite mantle. England et al. (2004) and Syracuse and Abers (2006) explore the global variations of depth to slab below volcanic fronts.

20 From England et al Distribution of arc volcanoes and intermediate depth seismicity. Perhaps these earthquakes show the zone of subducted serpentinite - chlorite mantle. England et al. (2004) and Syracuse and Abers (2006) explore the global variations of depth to slab below volcanic fronts. England et al. find a systematic variation of depth to the slab Attributed to the product of convergence rate (V) and angle of descent of slab (sin!) We develop a model to test importance of down dip velocity and convergence rate on temperature structure We find that sin! has the dominant control. As dip angle increases hotter mantle is drawn to shallower depths in the nose of the wedge. Petrologic controls are temperature structure and supply of H 2 O from chlorite breakdown

the mantle wedge and the vaporsaturated peridotite

21 Primary petrologic controls on hydrous melt generation: 1) Temperature Pressure distribution in the mantle wedge and the vaporsaturated peridotite solidus. 2) Breakdown of hydrous minerals at the base of mantle wedge.

22 Comparison of numerical model results and observations of the distance between the arc front and trench vs. sin (dip). New phase equilibrium constraints help us understand subduction zone melting They also raise some interesting challenges processes of slab wedge chemical exchange Influence of hydrous phases on rheology S76 Shastina summit from Mt. Shasta, N. Calif.

23 dry v A > 5v S 6% H 2 O Porou s flow V (cm/yr) Melt ascends into overlying mantle wedge rapidly (< 25,000 years) 6 e m i t t n e c s A 5 ) ( m k r v A > 5v S Stokes flow V (cm/yr) Diapiric ascent of crystal + hydrous melt For ascent to be sufficiently rapid, diapir will cool wedge t TE >t A Thermal equilibration time

24 Old & New Expts. Why the difference? melting kinetics Olivine melting rate is slower than that of pyroxene -Olivine also melts at a lower Temperature by about 200 o C - In the short run time expts pyroxene melted first Melting behavior of peridotite vs. sediment and basalt in presence of excess H 2 O. The solidi converge as pressure increases.

25 Estimating the chemical composition of the fluidrich component. We will model this by assuming 2 components: 1) a silicate melt from a harzburgite residue (wedge) 2) a fluid-rich component from the subducted lithosphere (slab). Use mass balance. Calculate elemental contribution from mantle melting Use H 2 O content of lava to estimate the composition of the H 2 O-rich component. -35 km -60 km Crust V. 200 MPa to 800 MPa frac. cryst IV. opx = oliv +melt III. H 2 O H 2 O decreases melt % Increases sat. melting beg ins Mantle Wedge So, what medium transfers elements from hot, young subducted lithosphere? Convection Is it a melt?? fluid?? A Looks most like a low degree melt of sediment/morb eclogite. -80 km II. oliv + fluid or melt = opx 1200 oc Fluid not a good match km I. H 2 O-rich component from melting or dehydration 800 o C H2O-sat. solidus Mantle wedge / slab melt interaction improves model.

26 Shasta lava trace elements in hydrous PMA & BA Model estimates contributions from Mantle Wedge & Subducted Lithosphere. MORB-normalized trace element abundances typical of arc magmas Enrichments in LILE Depletions in HFSE and the less incompatible TE

27 Signatures of both: MORB (High La/Sm) and Sediment Components Fo 93.6 olivine Hi Fo content in PMA indicates high extents of mantle melting. F = 0.19 to 0.30 in model.

28 Mass Balance Model C fluid =( C lava X melt C melt )/(X fluid ) Substitute batch melting equation for C melt F is fraction of mantle melt and D is bulk distribution coefficient C 0 element abundance in mantle source " is a correction for other elements in fluid C fluid = (C lava -(1- X H2O /")C 0 /[F+D(1-F)])/(X H2O / ") Fluid component 85-1a, & Estimated fluid component = gray ec lc p/ m a S Lavas Silicate melt component PMA Ce Nd Sm Gd Dy Er Yb La Ce Pr Nd 82-94a Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Lavas = solid black Silicate melt of mantle = open square. Note the dichotomy in La/Sm N and Dy/Yb N

29 Estimated Slab Contribution a fl fl fl 82-94a fl fl Mt. Shasta primitive BA and PMA 0 Rb Ba Th U Nb Ta K La Ce Pb Sr P Nd Zr Hf Sm Eu Ti Ti Dy Y Yb Major element characteristics of the fluid-rich Mt. Shasta component Na 2 O = 25 to 33 wt.% of the fluid K 2 O = 5 to 13 wt. % of fluid SiO 2 = 0 wt. % H 2 O = 54 to 70 wt. % Similar to finding of Stolper & Newman (1994). So, what is it? A melt or a fluid?

Any H 2 O rich slab fluid/melt is likely to interact with the

Franz (2000) Newton and Manning (2003) Olivine + SiO 2 (fluid) =

between wedge & subduction added component in Kaapvaal

The result is Distilled Essence of Slab Melt.

Fluid in lavas Estimated fluidrich component (black circles) Least

30 Any H 2 O rich slab fluid/melt is likely to interact with the wedge SiO 2 solubility in an H 2 O-rich fluid will be low -Zhang & Franz (2000) Newton and Manning (2003) Olivine + SiO 2 (fluid) = orthopyroxene Bell et al (2005) characterize chemical interaction between wedge & subduction added component in Kaapvaal harzburgites. Metasomatic reaction is: 1.25 Oliv +1 liquid = 1.0 Opx Gar Phlog Let s further react the slab melt with the wedge. The result is Distilled Essence of Slab Melt. H 2 O-rich component a fluid? No Models from Expts. Fluid in lavas Estimated fluidrich component (black circles) Least similar to a hydrous fluid saturated with eclogitic residue Slfl = slab fluid Ds from Ayers, Brenan, Kogiso, Stalder, etc. Wdfl =wedge fluid Kesel (2005) = fluid inmorb at 4 GPa

31 H 2 O-rich component a silicate melt? Much closer. Gt + Cpx in residue Estimated fluidrich component (black circles) Most similar to a mix of hydrous low degree melt of eclogitic residue n-morb (Hofmann) and Sediment (Ben Otham) eclogite melt Ds from Green et al. (2000) But the eclogite melt model of MORB & Sediment are not perfect fits. Misfits: Highly incompatible elements &HFSE & Fluid mobile

Brown symbols show effect of wedge peridotite + slab melt interaction

highly incompatible elements -better HFSE -worse Fluid mobile - better

Is it a melt? Is it a fluid? Are minerals also involved?

melt of sediment or MORB eclogite looks OK - sort of Match is closer

32 Brown symbols show effect of wedge peridotite + slab melt interaction at base of wedge using reaction inferred by Bell et al. (2005). highly incompatible elements -better HFSE -worse Fluid mobile - better So, what medium transfers elements from subducted lithosphere? Is it a melt? Is it a fluid? Are minerals also involved? Yes, No, Yes IF the melt is REALLY H 2 O-rich and MODIFED A low degree melt of sediment or MORB eclogite looks OK - sort of Match is closer than it is for H 2 O But there are mismatches in HFSE, highly incompatible elements and fluid mobile.

- low FeO*/MgO is controlled by the reaction relation oliv + liq! opx.

Mantle melting trend to high-sio 2 - low FeO*/MgO is controlled by the reaction relation oliv + liq! opx. 4 3 2 1 Ol+Opx +Cpx+Sp +liq 79-35g 82-66 87S35 85-44 85-41c -ol +opx Mantle melting/reaction 0