Non-Traditional Isotope Laboratory Tracing subduction zone processes by using magnesium isotopes Fang-Zhen Teng Earth and Space Sciences University of Washington, Seattle Thank you, I’d like to thank prof. Li and wang for the invitation to this great workshop. Today, iam going to talk about how to use Mg isotopes trace subducted crustal materials. This research has been supported by NSF.

Winter, J. Introduction to Igneous & Metamorphic Petrology Subduction zones Winter, J. Introduction to Igneous & Metamorphic Petrology Subduction zones are one of the most important tectonic seetings in the Earth. This map shows the distribution of subction zones. There are two major types of subduction zones, one is the subduction of an oceanic crust under another oceanic curst, creating an island arc. The other is the subduction of the oceanic crust under the continental crust. It forms a continental arc. For example, subduction of the pacif plate under the north american plate created the cascadian continental arcs. By contrast, subduction of the american plate under the carribean plate forms the lesser antilles island arc. Subcutions ones are very important for understanding how the earth works. For example, here I list sever items that are relevant to my talk toay. Subuctions zone are where the juvinll cotntnets were formed. They are also where the old oceanic crust were destroyed and recycled back to the mantle. They are also where the atomsphere-hydroshpere=ltihosphere interactions occur. Formation of juvenile continents Consumption of cold oceanic crusts Atmosphere-hydrosphere-lithosphere interactions Natural hazards…

Subduction zones Inputs: Subduction: Slab: Sediments Altered basalts Abyssal peridotites Fresh mafic/ultramafic Winter, J. Intro. to Igneous & Metamorphic Petrology Island Arc Subduction: Dehydration Melting How does a subduction zone look like? Here is a crossection of a subduction zone for an island arc. The y-axis is the depth, and the x-axis is the distance to the trench. Slab: Deep mantle Subcontinents

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

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

Magnesium isotope geochemistry High abundance distribution  high-precision analysis >8% mass difference 7‰ isotope fractionation Typical error, ≤0.07‰, 2SD -1] x 103 d26Mg (‰) = [ (26Mg/24Mg)sample (26Mg/24Mg)DSM3 Positive value = enriched heavy isotopes Negative value = enriched light isotopes

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

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

Baking silicates during subduction does not fractionate Mg isotopes No Mg isotope fractionation during subduction Baking silicates during subduction does not fractionate Mg isotopes Peridotites: more complicated Li et al., 2014 G3

But extensive intra-slab Mg isotope exchange Carbonates have elevated d26Mg while silicates have lower d26Mg than their protoliths The subducted materials have even more different Mg isotopic composition than typical mantle rocks! Peridotites: more complicated Wang et al., 2014 Nature Comm.

Questions on subduction zone processes What kinds of rocks are subducting What happened during subduction The slab itself The slab and overlying lithosphere The origins of arc lavas Source materials: mantle, crust or slab? Differentiation The arc crust and the continental crust The fate of the subducted slab Shallow mantle Deep mantle Mantle heterogeneity Global elemental cycle, climate change, … 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 Arc lavas with isotopic signatures (Sr, Nd, Pb, O, Li) similar to sediments Caribbean plate American plate Peridotites: more complicated Modified from Labanieh et al., 2010 EPSL

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

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

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

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

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

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

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

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

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

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

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

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

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

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