Basalt themobarometers and source tracers 408/508 Lecture101

What do basalt PT-meters do? Ideally they determine an average of the p and t of melting; (which is most often polybaric) In some instances, they can fingerprint depth of last magma chamber before melts reached the surface; More sophisticated that the Fe0_Na2O approach

Applications to tectonics Determine extension factor of the lithosphere Determine source origin (shallow B&R tupe extension, plume, MORB-like) using potential temperature; Other specialized applications (will talk about some later in the class)

Options Lee et al, 2009 Putirka, 2008 The LPK version as a starting point Of course, MELTS for forward modeling

Basics All of these programs take a basaltic composition and add olivine until they reach equilibrium with a Fo90 (or something like that) mantle;

Parametrization Melting is linear as a function of depth; Source is only peridotite; Shape of melting domain is triangular; no extra wings to scavenge traces; Based on McKenzie and Bickle (1988); Langmuir et al. (1992) and Wang et al. (2002).

Assumptions Ti is used as a perfectly incompatible element; Fe and Na will constrain the depth where melting starts and the length of melting column respectively; Thickness of melt column is also calculated (e.g. for MORB it should be 6 km);

Comparing against data Plot the major elements of your set against MgO (Harker type diagrams); Find the FeO, Na 2 O, TiO 2 and K 2 O corresponding to the most primitive composition; Those are the values to compare against the forward model; Works for any adiabatic melting assuming that only peridotite is the source. You can mess with fertility (% cpx source), amount of MgO, Na2O, K2O, FeO in source.

Na 2 O=2.8 FeO=9

Best match Start at 23 kbar Stop at 15 kbar 8 kbar column of melt, stops exactly at crust - mantle boundary (about 50 km under the Puna); Predicts 2.5 km of basalt accumulated in the crust; average melting 7%; Is this any good?

Hits solidus at around 1450 C

Extension factor LPK determines final LAB depth Get initial LAB depth from unextended region nearby (literature) Calculate magnitude of extension INDEPENDENTLY of surface (structure) data!

HW7 Use Blondes et al major element data for the Papoose flows only to determine the FeO and Na2O corresponding to the most primitive MgO; Use LPK model to determine the melt starting pressure, ending pressure, melt thickness and average F

Lee et al parametrization Needs to have the basalt be sourced in an olivine and opx rich mantle; Could be pyroxenite melt but one that had to equilibrate upon passing through an olivine rich mantle; Temperature is obtained by the distribution of Mg and Fe between olivine and melt, assuming a certain composition of mantle olivine; Pressure is most sensitive to silica activity expressed as the difference between “free” silica concentration and silica that goes into other cations.

Peridotite or pyroxenite melts? Important for many subduction related magmas Elsewhere too; extension magmatism Pyroxenite and peridotite melts can give basaltic composition They form at different depths Most pyroxenites and eclogites are more melt fertile than peridotite

Transition metals It has been recently shown that first row transitional metals and equivalent – Zn, Mn, Co, can distinguish between peridotites and pyroxenites; This is an important step before applying thermobarometers presented here, which rely on equilibration with olivine-rich assemblages in the mantle

Murray et al., 2015