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, Na2O, TiO2 and K2O 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.
Na2O=2.8
FeO=9
6.00
Series1
5.00
Na2O (wt%)
4.00
3.00
2.00
1.00
.00
6.00
7.00
8.00
9.00
FeO (wt%)
10.00
11.00
12.00
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