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

Home , Amphibolite, Pyroxenite

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!!(

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!