NonTraditional Isotope Laboratory Tracing subduction zone processes by

Non-Traditional Isotope Laboratory Tracing subduction zone processes by using magnesium isotopes Fang-Zhen Teng Earth and Space Sciences University of Washington, Seattle

Subduction zones Winter, J. Introduction to Igneous & Metamorphic Petrology q q Formation of juvenile continents Consumption of cold oceanic crusts Atmosphere-hydrosphere-lithosphere interactions Natural hazards…

Subduction zones Island Arc Inputs: § Sediments § Altered basalts § Abyssal peridotites § Fresh mafic/ultramafic Subduction: § Dehydration § Melting Slab: Winter, J. Intro. to Igneous & Metamorphic Petrology § Deep mantle § Subcontinents

Subduction zones Island Arc Outputs: Arc lavas § Fluid/melt/sediment mantle wedge § Partial melting primitive arc magma § Magma ascent crystallization § Ascent to arc crustal assimilation or AFC Winter, J. Intro. to Igneous & Metamorphic Petrology § Eruption to surface seafloor alteration or continental weathering

Questions on subduction zone processes l What kinds of rocks are subducting l What happened during subduction l The slab itself l The slab and overlying lithosphere l The origins of arc lavas l Source materials: mantle, crust or slab? l Differentiation l The arc crust and the continental crust l The fate of the subducted slab l Shallow mantle l Deep mantle l Mantle heterogeneity l Global elemental cycle, climate change, … l Earthquake, volcano eruption etc. .

$Magnesium isotope geochemistry High abundance distribution high-precision analysis >8% mass difference 7‰ isotope fractionation$

Magnesium isotope geochemistry High abundance distribution high-precision analysis >8% mass difference 7‰ isotope fractionation Typical error, ≤ 0. 07‰, 2 SD d 26 Mg (‰) = [ (26 Mg/24 Mg)sample (26 Mg/24 Mg)DSM 3 -1] x 103 Positive value = enriched heavy isotopes Negative value = enriched light isotopes

Questions on subduction zone processes l What kinds of rocks are subducting l What happened during subduction l The slab itself l The slab and overlying lithosphere l The origins of arc lavas l Source materials: mantle, crust or slab? l Differentiation l The arc crust and the continental crust l The fate of the subducted slab l Shallow mantle l Deep mantle l Mantle heterogeneity l Global elemental cycle, climate change, … l Earthquake, volcano eruption etc. .

The subducting slab Rock types • Pelagic sediments • Altered basalts • Abyssal peridotites • Fresh basalts • Gabbros • Peridotites White and Klein 2014 TOG

The subducting slab is heterogeneous! Altered oceanic basalts Marine sediments MORB + mantle Low-T water-rock interactions produce large Mg isotopic variation in the subducting slab Hu et al. , 2017; Huang et al. submitte

Questions on subduction zone processes l What kinds of rocks are subducting l What happened during subduction l The slab itself l The slab and overlying lithosphere l The origins of arc lavas l Source materials: mantle, crust or slab? l Differentiation l The arc crust and the continental crust l The fate of the subducted slab l Shallow mantle l Deep mantle l Mantle heterogeneity l Global elemental cycle, climate change, … l Earthquake, volcano eruption etc. .

$No Mg isotope fractionation during subduction Baking silicates during subduction does not fractionate Mg$

No Mg isotope fractionation during subduction Baking silicates during subduction does not fractionate Mg isotopes Li et al. , 2014 G 3

But extensive intra-slab Mg isotope exchange Carbonates have elevated d 26 Mg while silicates have lower d 26 Mg than their protoliths The subducted materials have even more different Mg isotopic composition than typical mantle rocks! Wang et al. , 2014 Nature Comm.

Questions on subduction zone processes l What kinds of rocks are subducting l What happened during subduction l The slab itself l The slab and overlying lithosphere l The origins of arc lavas l Source materials: mantle, crust or slab? l Differentiation l The arc crust and the continental crust l The fate of the subducted slab l Shallow mantle l Deep mantle l Mantle heterogeneity l Global elemental cycle, climate change, … l Earthquake, volcano eruption etc. .

Origins of Lesser Antilles Volcanic Arc lavas Some Facts § American plate subducting below Caribbean plate § Orinoco River delivering detrital sediments to the Atlantic ocean § Large amount of subducting sediments e. g. , DSDP sites 144/543 Caribbean plate American plate § Arc lavas with isotopic signatures (Sr, Nd, Pb, O, Li) similar to sediments Modified from Labanieh et al. , 2010 EPSL

Lesser Antilles Volcanic Arc lavas vs. subducting sediments: Sr-Nd isotopes Winter, J. Introduction to Igneous & Metamorphic Petrology

The debates on Lesser Antilles arc lavas Where and how the sediment was incorporated into the lavas? Ø Mantle wedge: sediment or its melt was added in the mantle wedge Ø Arc crust: sediment was added in the arc crust through mixing or AFC processes Ø Mantle wedge: through fluids derived from the subducted slab in the mantle wedge Ø Let’s see how Mg isotopes can help…

Samples: Lesser Antilles Volcanic Arc Martinique Island lavas and subducting sediments Teng, Hu, Chauvel, 2016, PNAS

Samples: Lesser Antilles Volcanic Arc Martinique Island: representative of Lesser Antilles lavas Teng, Hu, Chauvel, 2016, PNAS

Results: Lesser Antilles Volcanic Arc Subducting sediments: -0. 76 to 0. 52, Mean= -0. 10 Martinique Arc lavas: -0. 25 to -0. 10, Mean= -0. 18 Mantle: -0. 25 ± 0. 04 Teng, Hu, Chauvel, 2016, PNAS

The debates on Lesser Antilles arc lavas Where and how the sediment was incorporated into the lavas? Ø Mantle wedge: sediment or its melt was added in the mantle wedge [source mixing = SM]. Not for Mg Ø Arc crust: sediment was added in the arc crust through mixing (CM) or AFC processes. Not for Mg Ø Mantle wedge: fluids derived from the subducted slab were added in the mantle wedge. Yes for Mg

Lesser Antilles Volcanic Arc: Martinique Island Teng, Hu, Chauvel, 2016, PNAS

Lesser Antilles Volcanic Arc: Martinique Island Mg is likely added through fluids, not by sediments Teng, Hu, Chauvel, 2016, PNAS

Questions on subduction zone processes l What kinds of rocks are subducting l What happened during subduction l The slab itself l The slab and overlying lithosphere l The origins of arc lavas l Source materials: mantle, crust or slab? l Differentiation l The arc crust and the continental crust l The fate of the subducted slab l Shallow mantle l Deep mantle l Mantle heterogeneity l Global elemental cycle, climate change, … l Earthquake, volcano eruption etc. .

Making cratons by subducted slabs Subducted slabs stacked below continents and form the lithosphere Lee et al. 2013 AREPS

Evidence from cratonic eclogites l Highly heterogeneous Mg isotopic composition l Consistent with an origin of subducted slab l Not consistent with other hypothesis Wang et al. 2012 EPSL; 2015 Geology

Summary l Large isotopic variation at low temperature Subducting slab is different from the mantle l Subduction further magnifies the difference l Fluids are important for arc magmatism l Subducted slabs form continental lithosphere

Geonutrino and subduction processes Island Arc l l l Key elements: K, U and Th Fluid-mobile at low T Incompatible during melting Fluid-mobile at high T, P? Distribution in the continent, arc crust, and lithosphere?

Geonutrino and subduction zone processes l What kinds of rocks are subducting l What happened during subduction l The slab itself l The slab and overlying lithosphere l The origins of arc lavas l Source materials: mantle, crust or slab? l Differentiation l The arc crust and the continental crust l The fate of the subducted slab l Shallow mantle l Deep mantle l Mantle heterogeneity l Global elemental cycle, climate change, … l Earthquake, volcano eruption etc. .