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Sink%Or%Swim?%% The%Role%Of%Intracrustal%Differen9a9on%In% The%Genera9on%Of%Composi9onal% Diversity%And%Crustal%Delamina9on%In%The% Archean!

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Sink%Or%Swim?%% The%Role%Of%Intracrustal%Differen9a9on%In% The%Genera9on%Of%Composi9onal% Diversity%And%Crustal%Delamina9on%In%The% Archean! Sink%or%Swim?%% The%Role%of%Intracrustal%Differen9a9on%in% the%Genera9on%of%Composi9onal% Diversity%and%Crustal%Delamina9on%in%the% Archean! Jill$VanTongeren$ Claude!Herzberg! Boris!Kaus! Tim!Johnson! Mike!Brown! Sink%or%Swim?% Primary%Crust%in% the%Archean% Higher!basalt!MgO!content!! =! !HoEer!Mantle!PotenGal!Temp! HoEer!Mantle!PotenGal!Temp! !=! Higher!Crustal!Thickness! Data!from!Herzberg%et%al.%%2010%JPET;!1%!mantle! melt!=!1!km!of!crustal!thickness!aBer!Herzberg%and% Rudnick%(2011%Lithos);!Figures!from%Johnson%et%al.,% 2014%Nature%Geoscience% Sink%or%Swim?% 35%–%40%km%thick%primary%crust?% What!mineral!phases!and!lithologies!would!be! present!within!the!crustal!column?!! !!!Is!there!any!record!of!these!lithologies!in! !!!the!preserved!record!today?! ! Would!this!crust!be!dynamically!stable?! !!Will!it!subduct?!!Will!it!delaminate?! ! Sink%or%Swim?% 35%–%40%km%thick%primary%crust?% Johnson%et%al.%(2014)%Nature%Geoscience% Sink%or%Swim?% Primary%Crust%Thru%Time% Stable!oceanic!crust!lithologies! predicted!to!be!present!near!the!ridgeO axis!(1000°C)!in!the!pseudosecGons!of! Johnson%et%al.%(2014)%% Sink%or%Swim?% Density%Stable?%% Johnson%et%al.%2014%Nat%Geo% Sink%or%Swim?% Dynamically%Stable?% Tp!=!1600!!(~2.5O3.0!Ga)!! Tp!=!1450!!(<!~!1.5!Ga)! Finite!Element!Code!of!Boris!Kaus!–!includes!a!wet!temperatureOdependent!viscosity!! with!olivine!diffusion!&!dislocaGon!creep!in!the!mantle!lithologies!and!a!diabase!rheology!! for!the!crustal!secGons.! Sink%or%Swim?% Caveats%and%Updates% Johnson!et!al.!AssumpGons:! #1:!Homogeneous!crustal! comp!equal!to!the!primary! magma!comp!calculated!in! Herzberg!et!al.!(2010)! Problems:! #1:!Nearly!no!primary!magmas! are!erupted!to!the!surface! without!undergoing!some! degree!of!fracGonaGon!at!depth! Sink%or%Swim?% Caveats%and%Updates% Johnson!et!al.!AssumpGons:! Problems:! #2:!!Water!saturated,!fully! #2:!!How!do!you!produce!a! hydrated!crust!at!depth! fully!hydrated!crust!at!35!km! depth?!!Where!is!the!water! coming!from?! Sink%or%Swim?% Intracrustal%Differen9a9on% 2!ways!to!have!a!chemically!straGfied!crust!:! IniGally!homogeneous! Primary!magma!frac.onates$ crust!undergoes!par.al$ at$depth!into!a!more! mel.ng$at$depth$ primiGve!lower!crust!and! evolved!upper!crust$ The!viability!of!either!parGal!melGng!or!fracGonal!crystallizaGon!is!dependent!on!the! accre%onary!mechanism!of!the!crust! Sink%or%Swim?% Super-Penrose Stacked Lavas Intracrustal%Differen9a9on% LAVAS SHEETED DIKES Sheeted Sills Gabbro Glacier LAYERED PARTIAL!MELTING!MODEL!(amphibole!breakdown)! GABBROS MANTLE Time Basalt + Olivine Basalt + Olivine Basalt + Olivine Basalt + Olivine Basalt + Olivine Greenschist Greenschist Greenschist Greenschist Amphibolite Amphibolite Amphibolite Amphibolite Serpentinite Serpentinite Serpentinite Serpentinite Amphibolite Amphibolite Amphibolite Pyroxene Granulite Pyroxene Granulite Pyroxene Granulite Olivine Olivine Olivine Garnet Amphibolite TTG TTG Garnet Granulite + Olivine Garnet Pyroxenite Moho Moho Moho High MgO Harzburgite Garnet Pyroxenite Primary Magma Eclogite Figure!courtesy!of!C.!Herzberg! Sink%or%Swim?% Intracrustal%Differen9a9on% Composi.ons:$ 12!km!of!basalt! Basalt!from!the!2.7!Ga! Wawa!greenstone!belt,! Superior!Province! (Polat%et%al.,%1998)! Lower!crustal!cumulate! 24!km!of!cumulate! composiGon!calculated!by! mass!balance!assuming!a! 1:2!raGo!of! basalt:cumulate! Modern!Day! Primary!magma!soluGon!for!Wawa! basalt!from!PRIMELT3!(Herzberg%and% Primary!Magma:!47!wt%!SiO2;!19.5!wt%!MgO! Asimow,%in%prep)! Sink%or%Swim?% Updated%Phase%Equilibria%&%Densi9es% Stacked!Lavas! Super!Penrose! 0 0 0 0 5 5 Amphibolite Amphibolite 10 10 10 10 15 15 20 20 20 Hornblende 20 metapyroxenite Crustal Thickness (km) Crustal 25 Thickness (km) Crustal 25 depleted lithospheric mantle densitydepleted 30 30 30 30 2 pyroxene gabbro Pyroxenite lithospheric mantle densitydepleted 35 35 40 40 3000 3200 3400 3000 3200 3400 DENSITY (kg/m3) DENSITY (kg/m3) Stable!lithologies!along!a!30°C/km!geotherm!in!Perple_X! Sink%or%Swim?% Updated%Stabili9es% Stacked!Lavas!! Super!Penrose! “Wet”!rheology! “Dry”!rheology! Sink%or%Swim?% Conclusions% • Primary!crustal!thicknesses!in!the!Archean!were! likely!greater!than!in!the!modern!day! • Intracrustal!differenGaGon!and!the!mechanism!of! accreGon!of!the!crust!have!a!large!role!on!the! density!and!phase!stability!of!the!primary!crust! • The!role!of!water!on!the!viability!of!tectonic! styles!cannot!be!overstated! • We!need!more!experiments!on!amphibole! stability!in!high!MgO!basalts!and!picrites!.
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